55. Controller area network (bxCAN)

55.1 Introduction

The Basic Extended CAN peripheral, named bxCAN , interfaces the CAN network. It supports the CAN protocols version 2.0A and B. It has been designed to manage a high number of incoming messages efficiently with a minimum CPU load. It also meets the priority requirements for transmit messages.

For safety-critical applications, the CAN controller provides all hardware functions for supporting the CAN Time Triggered Communication option.

55.2 bxCAN main features

• • Supports CAN protocol version 2.0 A, B Active
• • Bit rates up to 1 Mbit/s
• • Supports the Time Triggered Communication option

Transmission

• • Three transmit mailboxes
• • Configurable transmit priority
• • Time stamp on SOF transmission

Reception

• • Two receive FIFOs with three stages
• • Scalable filter banks:
  • – 28 filter banks shared between CAN1 and CAN2 for dual CAN
• • Identifier list feature
• • Configurable FIFO overrun
• • Time stamp on SOF reception

Time-triggered communication option

• • Disable automatic retransmission mode
• • 16-bit free running timer
• • Time stamp sent in last two data bytes

Management

• • Maskable interrupts
• • Software-efficient mailbox mapping at a unique address space

Dual CAN peripheral configuration

• • CAN1: Master bxCAN for managing the communication between a Slave bxCAN and the 512-byte SRAM
• • CAN2: Slave bxCAN, with no direct access to the SRAM.
• • The two bxCAN cells share the 512-byte SRAM (see Figure 602 )

55.3 bxCAN general description

In today CAN applications, the number of nodes in a network is increasing and often several networks are linked together via gateways. Typically the number of messages in the system (to be handled by each node) has significantly increased. In addition to the application messages, network management and diagnostic messages have been introduced.

• • An enhanced filtering mechanism is required to handle each type of message.

Furthermore, application tasks require more CPU time, therefore real-time constraints caused by message reception have to be reduced.

• • A receive FIFO scheme allows the CPU to be dedicated to application tasks for a long time period without losing messages.

The standard HLP (Higher Layer Protocol) based on standard CAN drivers requires an efficient interface to the CAN controller.

Figure 601. CAN network topology

55.3.1 CAN 2.0B active core

The bxCAN module handles the transmission and the reception of CAN messages fully autonomously. Standard identifiers (11-bit) and extended identifiers (29-bit) are fully supported by hardware.

55.3.2 Control, status and configuration registers

The application uses these registers to:

• • Configure CAN parameters, e.g. baud rate
• • Request transmissions
• • Handle receptions
• • Manage interrupts
• • Get diagnostic information

55.3.3 Tx mailboxes

Three transmit mailboxes are provided to the software for setting up messages. The transmission scheduler decides which mailbox has to be transmitted first.

55.3.4 Acceptance filters

The bxCAN provides up to 28 scalable/configurable identifier filter banks in dual CAN configuration, for selecting the incoming messages, that the software needs and discarding the others.

Receive FIFO

Two receive FIFOs are used by hardware to store the incoming messages. Three complete messages can be stored in each FIFO. The FIFOs are managed completely by hardware.

Figure 602. Dual-CAN block diagram

55.4 bxCAN operating modes

bxCAN has three main operating modes: initialization , normal and Sleep . After a hardware reset, bxCAN is in Sleep mode to reduce power consumption and an internal pull-up is active on CANTX. The software requests bxCAN to enter initialization or Sleep mode by setting the INRQ or SLEEP bits in the CAN_MCR register. Once the mode has been entered, bxCAN confirms it by setting the INAK or SLAK bits in the CAN_MSR register and the internal pull-up is disabled. When neither INAK nor SLAK are set, bxCAN is in normal mode. Before entering normal mode bxCAN always has to synchronize on the CAN bus. To synchronize, bxCAN waits until the CAN bus is idle, this means 11 consecutive recessive bits have been monitored on CANRX.

55.4.1 Initialization mode

The software initialization can be done while the hardware is in Initialization mode. To enter this mode the software sets the INRQ bit in the CAN_MCR register and waits until the hardware has confirmed the request by setting the INAK bit in the CAN_MSR register.

To leave Initialization mode, the software clears the INRQ bit. bxCAN has left Initialization mode once the INAK bit has been cleared by hardware.

While in Initialization Mode, all message transfers to and from the CAN bus are stopped and the status of the CAN bus output CANTX is recessive (high).

Entering Initialization Mode does not change any of the configuration registers.

To initialize the CAN Controller, software has to set up the Bit Timing (CAN_BTR) and CAN options (CAN_MCR) registers.

To initialize the registers associated with the CAN filter banks (mode, scale, FIFO assignment, activation and filter values), software has to set the FINIT bit (CAN_FMR). Filter initialization also can be done outside the initialization mode.

Note: When FINIT=1, CAN reception is deactivated.

The filter values also can be modified by deactivating the associated filter activation bits (in the CAN_FA1R register).

If a filter bank is not used, it is recommended to leave it non active (leave the corresponding FACT bit cleared).

55.4.2 Normal mode

Once the initialization is complete, the software must request the hardware to enter Normal mode to be able to synchronize on the CAN bus and start reception and transmission.

The request to enter Normal mode is issued by clearing the INRQ bit in the CAN_MCR register. The bxCAN enters Normal mode and is ready to take part in bus activities when it has synchronized with the data transfer on the CAN bus. This is done by waiting for the occurrence of a sequence of 11 consecutive recessive bits (Bus Idle state). The switch to Normal mode is confirmed by the hardware by clearing the INAK bit in the CAN_MSR register.

The initialization of the filter values is independent from Initialization Mode but must be done while the filter is not active (corresponding FACTx bit cleared). The filter scale and mode configuration must be configured before entering Normal Mode.

55.4.3 Sleep mode (low-power)

To reduce power consumption, bxCAN has a low-power mode called Sleep mode. This mode is entered on software request by setting the SLEEP bit in the CAN_MCR register. In this mode, the bxCAN clock is stopped, however software can still access the bxCAN mailboxes.

If software requests entry to initialization mode by setting the INRQ bit while bxCAN is in Sleep mode, it must also clear the SLEEP bit.

bxCAN can be woken up (exit Sleep mode) either by software clearing the SLEEP bit or on detection of CAN bus activity.

On CAN bus activity detection, hardware automatically performs the wakeup sequence by clearing the SLEEP bit if the AWUM bit in the CAN_MCR register is set. If the AWUM bit is cleared, software has to clear the SLEEP bit when a wakeup interrupt occurs, in order to exit from Sleep mode.

Note: If the wakeup interrupt is enabled (WKUIE bit set in CAN_IER register) a wakeup interrupt is generated on detection of CAN bus activity, even if the bxCAN automatically performs the wakeup sequence.

After the SLEEP bit has been cleared, Sleep mode is exited once bxCAN has synchronized with the CAN bus, refer to Figure 603 . The Sleep mode is exited once the SLAK bit has been cleared by hardware.

Figure 603. bxCAN operating modes

stateDiagram-v2
    direction TB
    Reset --> Sleep
    Sleep --> Normal: SLEEP.SYNC.INRQ
    Normal --> Sleep: SLEEP.ACK
    Sleep --> Initialization: SLEEP.INRQ.ACK
    Initialization --> Sleep: SLEEP.INRQ.ACK
    Normal --> Initialization: INRQ.ACK
    Initialization --> Normal: INRQ.SYNC.SLEEP

1. 1. ACK = The wait state during which hardware confirms a request by setting the INAK or SLAK bits in the CAN_MSR register.
2. 2. SYNC = The state during which bxCAN waits until the CAN bus is idle, meaning 11 consecutive recessive bits have been monitored on CANRX.

55.5 Test mode

Test mode can be selected by the SILM and LBKM bits in the CAN_BTR register. These bits must be configured while bxCAN is in Initialization mode. Once test mode has been selected, the INRQ bit in the CAN_MCR register must be reset to enter Normal mode.

55.5.1 Silent mode

The bxCAN can be put in Silent mode by setting the SILM bit in the CAN_BTR register.

In Silent mode, the bxCAN is able to receive valid data frames and valid remote frames, but sends only recessive bits on the CAN bus and cannot start a transmission. If the bxCAN has to send a dominant bit (ACK bit, overload flag, active error flag), the bit is rerouted internally so that the CAN Core monitors this dominant bit, although the CAN bus may remain in recessive state. Silent mode can be used to analyze the traffic on a CAN bus without affecting it by the transmission of dominant bits (Acknowledgment bits, Error frames).

Figure 604. bxCAN in silent mode

55.5.2 Loop back mode

The bxCAN can be set in Loop Back Mode by setting the LBKM bit in the CAN_BTR register. In Loop Back Mode, the bxCAN treats its own transmitted messages as received messages and stores them (if they pass acceptance filtering) in a Receive mailbox.

Figure 605. bxCAN in loop back mode

This mode is provided for self-test functions. To be independent of external events, the CAN Core ignores acknowledge errors (no dominant bit sampled in the acknowledge slot of a data / remote frame) in Loop Back Mode. In this mode, the bxCAN performs an internal feedback from its Tx output to its Rx input. The actual value of the CANRX input pin is disregarded by the bxCAN. The transmitted messages can be monitored on the CANTX pin.

55.5.3 Loop back combined with silent mode

It is also possible to combine Loop back mode and Silent mode by setting the LBKM and SILM bits in the CAN_BTR register. This mode can be used for a “Hot Selftest”, meaning the bxCAN can be tested like in Loop back mode but without affecting a running CAN system connected to the CANTX and CANRX pins. In this mode, the CANRX pin is disconnected from the bxCAN and the CANTX pin is held recessive.

Figure 606. bxCAN in combined mode

MS30395V2

55.6 Behavior in debug mode

When the microcontroller enters the debug mode (Cortex®-M4 core halted), the bxCAN continues to work normally or stops, depending on:

• • the DBG_CAN1_STOP bit for CAN1 or the DBG_CAN2_STOP bit for CAN2 in the DBG module for the dual mode.
• • the DBF bit in CAN_MCR. For more details, refer to Section 55.9.2: CAN control and status registers .

55.7 bxCAN functional description

55.7.1 Transmission handling

In order to transmit a message, the application must select one empty transmit mailbox, set up the identifier, the data length code (DLC) and the data before requesting the transmission by setting the corresponding TXRQ bit in the CAN_TLxR register. Once the mailbox has left empty state, the software no longer has write access to the mailbox registers. Immediately after the TXRQ bit has been set, the mailbox enters pending state and waits to become the highest priority mailbox, see Transmit Priority . As soon as the mailbox has the highest priority it is scheduled for transmission. The transmission of the message of the scheduled

mailbox starts (enter transmit state) when the CAN bus becomes idle. Once the mailbox has been successfully transmitted, it becomes empty again. The hardware indicates a successful transmission by setting the RQCP and TXOK bits in the CAN_TSR register.

If the transmission fails, the cause is indicated by the ALST bit in the CAN_TSR register in case of an Arbitration Lost, and/or the TERR bit, in case of transmission error detection.

Transmit priority

By identifier

When more than one transmit mailbox is pending, the transmission order is given by the identifier of the message stored in the mailbox. The message with the lowest identifier value has the highest priority according to the arbitration of the CAN protocol. If the identifier values are equal, the lower mailbox number is scheduled first.

By transmit request order

The transmit mailboxes can be configured as a transmit FIFO by setting the TXFP bit in the CAN_MCR register. In this mode the priority order is given by the transmit request order.

This mode is very useful for segmented transmission.

Abort

A transmission request can be aborted by the user setting the ABRQ bit in the CAN_TSR register. In pending or scheduled state, the mailbox is aborted immediately. An abort request while the mailbox is in transmit state can have two results. If the mailbox is transmitted successfully the mailbox becomes empty with the TXOK bit set in the CAN_TSR register. If the transmission fails, the mailbox becomes scheduled , the transmission is aborted and becomes empty with TXOK cleared. In all cases the mailbox becomes empty again at least at the end of the current transmission.

Non automatic retransmission mode

This mode has been implemented in order to fulfill the requirement of the Time Triggered Communication option of the CAN standard. To configure the hardware in this mode the NART bit in the CAN_MCR register must be set.

In this mode, each transmission is started only once. If the first attempt fails, due to an arbitration loss or an error, the hardware does not automatically restart the message transmission.

At the end of the first transmission attempt, the hardware considers the request as completed and sets the RQCP bit in the CAN_TSR register. The result of the transmission is indicated in the CAN_TSR register by the TXOK, ALST and TERR bits.

Figure 607. Transmit mailbox states

stateDiagram-v2
    [*] --> EMPTY_TL: EMPTY, RQCP=X, TXOK=X, TME = 1
    EMPTY_TL --> PENDING: TXRQ=1
    PENDING --> EMPTY_TL: ABRQ=1
    PENDING --> SCHEDULED: Mailbox has highest priority
    SCHEDULED --> PENDING: Mailbox does not have highest priority
    SCHEDULED --> TRANSMIT: CAN Bus = IDLE
    TRANSMIT --> SCHEDULED: Transmit failed * NART
    TRANSMIT --> EMPTY_TL: Transmit succeeded
    SCHEDULED --> EMPTY_TL: ABRQ=1
  

55.7.2 Time triggered communication mode

In this mode, the internal counter of the CAN hardware is activated and used to generate the time stamp value stored in the CAN_RDTxR/CAN_TDTxR registers, respectively (for Rx and Tx mailboxes). The internal counter is incremented each CAN bit time (refer to Section 55.7.7 ). The internal counter is captured on the sample point of the Start Of Frame bit in both reception and transmission.

55.7.3 Reception handling

For the reception of CAN messages, three mailboxes organized as a FIFO are provided. In order to save CPU load, simplify the software and guarantee data consistency, the FIFO is managed completely by hardware. The application accesses the messages stored in the FIFO through the FIFO output mailbox.

Valid message

A received message is considered as valid when it has been received correctly according to the CAN protocol (no error until the last but one bit of the EOF field) and it passed through the identifier filtering successfully, see Section 55.7.4 .

Figure 608. Receive FIFO states

stateDiagram-v2
    [*] --> EMPTY
    EMPTY --> PENDING_1 : Valid Message Received
    PENDING_1 --> EMPTY : Release Mailbox
    PENDING_1 --> PENDING_2 : Valid Message Received
    PENDING_2 --> PENDING_1 : Release Mailbox RFOM=1
    PENDING_2 --> PENDING_3 : Valid Message Received
    PENDING_3 --> PENDING_2 : Release Mailbox RFOM=1
    PENDING_3 --> OVERRUN : Valid Message Received
    OVERRUN --> PENDING_3 : Release Mailbox RFOM=1
    OVERRUN --> OVERRUN : Valid Message Received
  

FIFO management

Starting from the empty state, the first valid message received is stored in the FIFO which becomes pending_1 . The hardware signals the event setting the FMP[1:0] bits in the CAN_RFR register to the value 01b. The message is available in the FIFO output mailbox. The software reads out the mailbox content and releases it by setting the RFOM bit in the CAN_RFR register. The FIFO becomes empty again. If a new valid message has been received in the meantime, the FIFO stays in pending_1 state and the new message is available in the output mailbox.

If the application does not release the mailbox, the next valid message is stored in the FIFO which enters pending_2 state (FMP[1:0] = 10b). The storage process is repeated for the next valid message putting the FIFO into pending_3 state (FMP[1:0] = 11b). At this point, the software must release the output mailbox by setting the RFOM bit, so that a mailbox is free to store the next valid message. Otherwise the next valid message received causes a loss of message. Refer also to Section 55.7.5 .

Overrun

Once the FIFO is in pending_3 state (i.e. the three mailboxes are full) the next valid message reception leads to an overrun and a message is lost. The hardware signals the

overrun condition by setting the FOVR bit in the CAN_RFR register. Which message is lost depends on the configuration of the FIFO:

• • If the FIFO lock function is disabled (RFLM bit in the CAN_MCR register cleared) the last message stored in the FIFO is overwritten by the new incoming message. In this case the latest messages are always available to the application.
• • If the FIFO lock function is enabled (RFLM bit in the CAN_MCR register set) the most recent message is discarded and the software has the three oldest messages in the FIFO available.

Reception related interrupts

Once a message has been stored in the FIFO, the FMP[1:0] bits are updated and an interrupt request is generated if the FMPIE bit in the CAN_IER register is set.

When the FIFO becomes full (i.e. a third message is stored) the FULL bit in the CAN_RFR register is set and an interrupt is generated if the FFIE bit in the CAN_IER register is set.

On overrun condition, the FOVR bit is set and an interrupt is generated if the FOVIE bit in the CAN_IER register is set.

55.7.4 Identifier filtering

In the CAN protocol the identifier of a message is not associated with the address of a node but related to the content of the message. Consequently a transmitter broadcasts its message to all receivers. On message reception a receiver node decides - depending on the identifier value - whether the software needs the message or not. If the message is needed, it is copied into the SRAM. If not, the message must be discarded without intervention by the software.

To fulfill this requirement the bxCAN Controller provides 28 configurable and scalable filter banks (27-0) to the application, in order to receive only the messages the software needs.

This hardware filtering saves CPU resources which would be otherwise needed to perform filtering by software. Each filter bank x consists of two 32-bit registers, CAN_FxR0 and CAN_FxR1.

Scalable width

To optimize and adapt the filters to the application needs, each filter bank can be scaled independently. Depending on the filter scale a filter bank provides:

• • One 32-bit filter for the STDID[10:0], EXTID[17:0], IDE and RTR bits.
• • Two 16-bit filters for the STDID[10:0], RTR, IDE and EXTID[17:15] bits.

Refer to Figure 609 .

Furthermore, the filters can be configured in mask mode or in identifier list mode.

Mask mode

In mask mode the identifier registers are associated with mask registers specifying which bits of the identifier are handled as “must match” or as “don’t care”.

Identifier list mode

In identifier list mode, the mask registers are used as identifier registers. Thus instead of defining an identifier and a mask, two identifiers are specified, doubling the number of single

identifiers. All bits of the incoming identifier must match the bits specified in the filter registers.

Filter bank scale and mode configuration

The filter banks are configured by means of the corresponding CAN_FMR register. To configure a filter bank it must be deactivated by clearing the FACT bit in the CAN_FMR register. The filter scale is configured by means of the corresponding FSCx bit in the CAN_FS1R register, refer to Figure 609. The identifier list or identifier mask mode for the corresponding Mask/Identifier registers is configured by means of the FBMx bits in the CAN_FMR register.

To filter a group of identifiers, configure the Mask/Identifier registers in mask mode.

To select single identifiers, configure the Mask/Identifier registers in identifier list mode.

Filters not used by the application should be left deactivated.

Each filter within a filter bank is numbered (called the Filter Number ) from 0 to a maximum dependent on the mode and the scale of each of the filter banks.

Concerning the filter configuration, refer to Figure 609.

Figure 609. Filter bank scale configuration - Register organization

Filter match index

Once a message has been received in the FIFO it is available to the application. Typically, application data is copied into SRAM locations. To copy the data to the right location the application has to identify the data by means of the identifier. To avoid this, and to ease the access to the SRAM locations, the CAN controller provides a filter match index.

This index is stored in the mailbox together with the message according to the filter priority rules. Thus each received message has its associated filter match index.

The filter match index can be used in two ways:

• • Compare the filter match index with a list of expected values.
• • Use the filter match index as an index on an array to access the data destination location.

For non masked filters, the software no longer has to compare the identifier.

If the filter is masked the software reduces the comparison to the masked bits only.

The index value of the filter number does not take into account the activation state of the filter banks. In addition, two independent numbering schemes are used, one for each FIFO. Refer to Figure 610 for an example.

Figure 610. Example of filter numbering

Filter priority rules

Depending on the filter combination it may occur that an identifier passes successfully through several filters. In this case the filter match value stored in the receive mailbox is chosen according to the following priority rules:

• • A 32-bit filter takes priority over a 16-bit filter.
• • For filters of equal scale, priority is given to the Identifier List mode over the Identifier Mask mode
• • For filters of equal scale and mode, priority is given by the filter number (the lower the number, the higher the priority).

Figure 611. Filtering mechanism example

The example above shows the filtering principle of the bxCAN. On reception of a message, the identifier is compared first with the filters configured in identifier list mode. If there is a match, the message is stored in the associated FIFO and the index of the matching filter is stored in the filter match index. As shown in the example, the identifier matches with Identifier #2 thus the message content and FMI 2 is stored in the FIFO.

If there is no match, the incoming identifier is then compared with the filters configured in mask mode.

If the identifier does not match any of the identifiers configured in the filters, the message is discarded by hardware without disturbing the software.

55.7.5 Message storage

The interface between the software and the hardware for the CAN messages is implemented by means of mailboxes. A mailbox contains all information related to a message; identifier, data, control, status and time stamp information.

Transmit mailbox

The software sets up the message to be transmitted in an empty transmit mailbox. The status of the transmission is indicated by hardware in the CAN_TSR register.

Table 412. Transmit mailbox mapping

Receive mailbox

When a message has been received, it is available to the software in the FIFO output mailbox. Once the software has handled the message (e.g. read it) the software must release the FIFO output mailbox by means of the RFOM bit in the CAN_RFR register to make the next incoming message available. The filter match index is stored in the MFMI field of the CAN_RDTxR register. The 16-bit time stamp value is stored in the TIME[15:0] field of CAN_RDTxR.

Table 413. Receive mailbox mapping

Figure 612. CAN error state diagram

stateDiagram-v2
    [*] --> ERROR_ACTIVE: When 128*11 recessive bits occur
    ERROR_ACTIVE --> ERROR_PASSIVE: When TEC or REC > 127
    ERROR_PASSIVE --> ERROR_ACTIVE: When TEC and REC < 128
    ERROR_PASSIVE --> BUS_OFF: When TEC > 255
  

55.7.6 Error management

The error management as described in the CAN protocol is handled entirely by hardware using a Transmit Error Counter (TEC value, in CAN_ESR register) and a Receive Error Counter (REC value, in the CAN_ESR register), which get incremented or decremented according to the error condition. For detailed information about TEC and REC management, refer to the CAN standard.

Both of them may be read by software to determine the stability of the network. Furthermore, the CAN hardware provides detailed information on the current error status in CAN_ESR register. By means of the CAN_IER register (ERRIE bit, etc.), the software can configure the interrupt generation on error detection in a very flexible way.

Bus-Off recovery

The Bus-Off state is reached when TEC is greater than 255, this state is indicated by BOFF bit in CAN_ESR register. In Bus-Off state, the bxCAN is no longer able to transmit and receive messages.

Depending on the ABOM bit in the CAN_MCR register, bxCAN recovers from Bus-Off (become error active again) either automatically or on software request. But in both cases the bxCAN has to wait at least for the recovery sequence specified in the CAN standard (128 occurrences of 11 consecutive recessive bits monitored on CANRX).

If ABOM is set, the bxCAN starts the recovering sequence automatically after it has entered Bus-Off state.

If ABOM is cleared, the software must initiate the recovering sequence by requesting bxCAN to enter and to leave initialization mode.

Note: In initialization mode, bxCAN does not monitor the CANRX signal, therefore it cannot complete the recovery sequence. To recover, bxCAN must be in normal mode.

55.7.7 Bit timing

The bit timing logic monitors the serial bus-line and performs sampling and adjustment of the sample point by synchronizing on the start-bit edge and resynchronizing on the following edges.

Its operation may be explained simply by splitting nominal bit time into three segments as follows:

• • Synchronization segment (SYNC_SEG) : a bit change is expected to occur within this time segment. It has a fixed length of one time quantum ( \( 1 \times t_q \) ).
• • Bit segment 1 (BS1) : defines the location of the sample point. It includes the PROP_SEG and PHASE_SEG1 of the CAN standard. Its duration is programmable between 1 and 16 time quanta but may be automatically lengthened to compensate for positive phase drifts due to differences in the frequency of the various nodes of the network.
• • Bit segment 2 (BS2) : defines the location of the transmit point. It represents the PHASE_SEG2 of the CAN standard. Its duration is programmable between 1 and 8 time quanta but may also be automatically shortened to compensate for negative phase drifts.

The resynchronization jump width (SJW) defines an upper bound to the amount of lengthening or shortening of the bit segments. It is programmable between 1 and 4 time quanta.

A valid edge is defined as the first transition in a bit time from dominant to recessive bus level provided the controller itself does not send a recessive bit.

If a valid edge is detected in BS1 instead of SYNC_SEG, BS1 is extended by up to SJW so that the sample point is delayed.

Conversely, if a valid edge is detected in BS2 instead of SYNC_SEG, BS2 is shortened by up to SJW so that the transmit point is moved earlier.

As a safeguard against programming errors, the configuration of the Bit timing register (CAN_BTR) is only possible while the device is in Standby mode.

Note: For a detailed description of the CAN bit timing and resynchronization mechanism, refer to the ISO 11898 standard.

Figure 613. Bit timing

with:

where \( t_q \) refers to the Time quantum

\( t_{PCLK} \) = time period of the APB clock,

BRP[9:0], TS1[3:0] and TS2[2:0] are defined in the CAN_BTR Register.

MS31001V2

Figure 614. CAN frames

55.8 bxCAN interrupts

Four interrupt vectors are dedicated to bxCAN. Each interrupt source can be independently enabled or disabled by means of the CAN interrupt enable register (CAN_IER).

Figure 615. Event flags and interrupt generation

• • The transmit interrupt can be generated by the following events:
  • – Transmit mailbox 0 becomes empty, RQCP0 bit in the CAN_TSR register set.
  • – Transmit mailbox 1 becomes empty, RQCP1 bit in the CAN_TSR register set.
  • – Transmit mailbox 2 becomes empty, RQCP2 bit in the CAN_TSR register set.
• • The FIFO 0 interrupt can be generated by the following events:
  • – Reception of a new message, FMP0 bits in the CAN_RF0R register are not '00'.
  • – FIFO0 full condition, FULL0 bit in the CAN_RF0R register set.
  • – FIFO0 overrun condition, FOVR0 bit in the CAN_RF0R register set.
• • The FIFO 1 interrupt can be generated by the following events:
  • – Reception of a new message, FMP1 bits in the CAN_RF1R register are not '00'.
  • – FIFO1 full condition, FULL1 bit in the CAN_RF1R register set.
  • – FIFO1 overrun condition, FOVR1 bit in the CAN_RF1R register set.
• • The error and status change interrupt can be generated by the following events:
  • – Error condition, for more details on error conditions refer to the CAN Error Status register (CAN_ESR).
  • – Wakeup condition, SOF monitored on the CAN Rx signal.
  • – Entry into Sleep mode.

55.9 CAN registers

The peripheral registers have to be accessed by words (32 bits).

55.9.1 Register access protection

Erroneous access to certain configuration registers can cause the hardware to temporarily disturb the whole CAN network. Therefore the CAN_BTR register can be modified by software only while the CAN hardware is in initialization mode.

Although the transmission of incorrect data does not cause problems at the CAN network level, it can severely disturb the application. A transmit mailbox can be only modified by software while it is in empty state, refer to Figure 607: Transmit mailbox states .

The filter values can be modified either deactivating the associated filter banks or by setting the FINIT bit. Moreover, the modification of the filter configuration (scale, mode and FIFO assignment) in CAN_FMXR, CAN_FSxR and CAN_FFAR registers can only be done when the filter initialization mode is set (FINIT=1) in the CAN_FMR register.

55.9.2 CAN control and status registers

Refer to Section 1.2 for a list of abbreviations used in register descriptions.

CAN master control register (CAN_MCR)

Address offset: 0x00

Reset value: 0x0001 0002

Bits 31:17 Reserved, must be kept at reset value.

Bit 16 DBF : Debug freeze

0: CAN working during debug

1: CAN reception/transmission frozen during debug. Reception FIFOs can still be accessed/controlled normally.

Bit 15 RESET : bxCAN software master reset

0: Normal operation.

1: Force a master reset of the bxCAN -> Sleep mode activated after reset (FMP bits and CAN_MCR register are initialized to the reset values). This bit is automatically reset to 0.

Bits 14:8 Reserved, must be kept at reset value.

Bit 7 TTTCM : Time triggered communication mode

0: Time Triggered Communication mode disabled.

1: Time Triggered Communication mode enabled

Note: For more information on Time Triggered Communication mode, refer to Section 55.7.2: Time triggered communication mode .

Bit 6 ABOM : Automatic bus-off management

This bit controls the behavior of the CAN hardware on leaving the Bus-Off state.

0: The Bus-Off state is left on software request, once 128 occurrences of 11 recessive bits have been monitored and the software has first set and cleared the INRQ bit of the CAN_MCR register.

1: The Bus-Off state is left automatically by hardware once 128 occurrences of 11 recessive bits have been monitored.

For detailed information on the Bus-Off state refer to Section 55.7.6: Error management .

Bit 5 AWUM : Automatic wakeup mode

This bit controls the behavior of the CAN hardware on message reception during Sleep mode.

0: The Sleep mode is left on software request by clearing the SLEEP bit of the CAN_MCR register.

1: The Sleep mode is left automatically by hardware on CAN message detection.

The SLEEP bit of the CAN_MCR register and the SLAK bit of the CAN_MSR register are cleared by hardware.

Bit 4 NART : No automatic retransmission

0: The CAN hardware automatically retransmits the message until it has been successfully transmitted according to the CAN standard.

1: A message is transmitted only once, independently of the transmission result (successful, error or arbitration lost).

Bit 3 RFLM : Receive FIFO locked mode

0: Receive FIFO not locked on overrun. Once a receive FIFO is full the next incoming message overwrites the previous one.

1: Receive FIFO locked against overrun. Once a receive FIFO is full the next incoming message is discarded.

Bit 2 TXFP : Transmit FIFO priority

This bit controls the transmission order when several mailboxes are pending at the same time.

0: Priority driven by the identifier of the message

1: Priority driven by the request order (chronologically)

Bit 1 SLEEP : Sleep mode request

This bit is set by software to request the CAN hardware to enter the Sleep mode. Sleep mode is entered as soon as the current CAN activity (transmission or reception of a CAN frame) has been completed.

This bit is cleared by software to exit Sleep mode.

This bit is cleared by hardware when the AWUM bit is set and a SOF bit is detected on the CAN Rx signal.

This bit is set after reset - CAN starts in Sleep mode.

Bit 0 INRQ : Initialization request

The software clears this bit to switch the hardware into normal mode. Once 11 consecutive recessive bits have been monitored on the Rx signal the CAN hardware is synchronized and ready for transmission and reception. Hardware signals this event by clearing the INAK bit in the CAN_MSR register.

Software sets this bit to request the CAN hardware to enter initialization mode. Once software has set the INRQ bit, the CAN hardware waits until the current CAN activity (transmission or reception) is completed before entering the initialization mode. Hardware signals this event by setting the INAK bit in the CAN_MSR register.

CAN master status register (CAN_MSR)

Address offset: 0x04

Reset value: 0x0000 0C02

Bits 31:12 Reserved, must be kept at reset value.

Bit 11 RX : CAN Rx signal

Monitors the actual value of the CAN_RX Pin.

Bit 10 SAMP : Last sample point

The value of RX on the last sample point (current received bit value).

The CAN hardware is currently receiver.

The CAN hardware is currently transmitter.

Bits 7:5 Reserved, must be kept at reset value.

When SLKIE=1, this bit is set by hardware to signal that the bxCAN has entered Sleep Mode. When set, this bit generates a status change interrupt if the SLKIE bit in the CAN_IER register is set.

This bit is cleared by software or by hardware, when SLAK is cleared.

Note: When SLKIE=0, no polling on SLAKI is possible. In this case the SLAK bit can be polled.

This bit is set by hardware to signal that a SOF bit has been detected while the CAN hardware was in Sleep mode. Setting this bit generates a status change interrupt if the WKUIE bit in the CAN_IER register is set.

This bit is cleared by software.

This bit is set by hardware when a bit of the CAN_ESR has been set on error detection and the corresponding interrupt in the CAN_IER is enabled. Setting this bit generates a status change interrupt if the ERRRIE bit in the CAN_IER register is set.

This bit is cleared by software.

This bit is set by hardware and indicates to the software that the CAN hardware is now in Sleep mode. This bit acknowledges the Sleep mode request from the software (set SLEEP bit in CAN_MCR register).

This bit is cleared by hardware when the CAN hardware has left Sleep mode (to be synchronized on the CAN bus). To be synchronized the hardware has to monitor a sequence of 11 consecutive recessive bits on the CAN RX signal.

Note: The process of leaving Sleep mode is triggered when the SLEEP bit in the CAN_MCR register is cleared. Refer to the AWUM bit of the CAN_MCR register description for detailed information for clearing SLEEP bit

This bit is set by hardware and indicates to the software that the CAN hardware is now in initialization mode. This bit acknowledges the initialization request from the software (set INRQ bit in CAN_MCR register).

This bit is cleared by hardware when the CAN hardware has left the initialization mode (to be synchronized on the CAN bus). To be synchronized the hardware has to monitor a sequence of 11 consecutive recessive bits on the CAN RX signal.

Address offset: 0x08

Reset value: 0x1C00 0000

Bit 31 LOW2 : Lowest priority flag for mailbox 2

This bit is set by hardware when more than one mailbox are pending for transmission and mailbox 2 has the lowest priority.

Bit 30 LOW1 : Lowest priority flag for mailbox 1

This bit is set by hardware when more than one mailbox are pending for transmission and mailbox 1 has the lowest priority.

Bit 29 LOW0 : Lowest priority flag for mailbox 0

This bit is set by hardware when more than one mailbox are pending for transmission and mailbox 0 has the lowest priority.

Note: The LOW[2:0] bits are set to 0 when only one mailbox is pending.

Bit 28 TME2 : Transmit mailbox 2 empty

This bit is set by hardware when no transmit request is pending for mailbox 2.

Bit 27 TME1 : Transmit mailbox 1 empty

This bit is set by hardware when no transmit request is pending for mailbox 1.

Bit 26 TME0 : Transmit mailbox 0 empty

This bit is set by hardware when no transmit request is pending for mailbox 0.

Bits 25:24 CODE[1:0] : Mailbox code

In case at least one transmit mailbox is free, the code value is equal to the number of the next transmit mailbox free.

In case all transmit mailboxes are pending, the code value is equal to the number of the transmit mailbox with the lowest priority.

Bit 23 ABRQ2 : Abort request for mailbox 2

Set by software to abort the transmission request for the corresponding mailbox.

Cleared by hardware when the mailbox becomes empty.

Setting this bit has no effect when the mailbox is not pending for transmission.

Bits 22:20 Reserved, must be kept at reset value.

Bit 19 TERR2 : Transmission error of mailbox 2

This bit is set when the previous TX failed due to an error.

Bit 18 ALST2 : Arbitration lost for mailbox 2

This bit is set when the previous TX failed due to an arbitration lost.

Bit 17 TXOK2 : Transmission OK of mailbox 2

The hardware updates this bit after each transmission attempt.

0: The previous transmission failed

1: The previous transmission was successful

This bit is set by hardware when the transmission request on mailbox 2 has been completed successfully. Refer to Figure 607 .

Bit 16 RQCP2 : Request completed mailbox2

Set by hardware when the last request (transmit or abort) has been performed.

Cleared by software writing a “1” or by hardware on transmission request (TXRQ2 set in CAN_TMID2R register).

Clearing this bit clears all the status bits (TXOK2, ALST2 and TERR2) for Mailbox 2.

Bit 15 ABRQ1 : Abort request for mailbox 1

Set by software to abort the transmission request for the corresponding mailbox.

Cleared by hardware when the mailbox becomes empty.

Setting this bit has no effect when the mailbox is not pending for transmission.

Bits 14:12 Reserved, must be kept at reset value.

Bit 11 TERR1 : Transmission error of mailbox1

This bit is set when the previous TX failed due to an error.

Bit 10 ALST1 : Arbitration lost for mailbox1

This bit is set when the previous TX failed due to an arbitration lost.

Bit 9 TXOK1 : Transmission OK of mailbox1

The hardware updates this bit after each transmission attempt.

0: The previous transmission failed

1: The previous transmission was successful

This bit is set by hardware when the transmission request on mailbox 1 has been completed successfully. Refer to Figure 607 .

Bit 8 RQCP1 : Request completed mailbox1

Set by hardware when the last request (transmit or abort) has been performed.

Cleared by software writing a “1” or by hardware on transmission request (TXRQ1 set in CAN_TI1R register).

Clearing this bit clears all the status bits (TXOK1, ALST1 and TERR1) for Mailbox 1.

Bit 7 ABRQ0 : Abort request for mailbox0

Set by software to abort the transmission request for the corresponding mailbox.

Cleared by hardware when the mailbox becomes empty.

Setting this bit has no effect when the mailbox is not pending for transmission.

Bits 6:4 Reserved, must be kept at reset value.

Bit 3 TERR0 : Transmission error of mailbox0

This bit is set when the previous TX failed due to an error.

Bit 2 ALST0 : Arbitration lost for mailbox0

This bit is set when the previous TX failed due to an arbitration lost.

Bit 1 TXOK0 : Transmission OK of mailbox0

The hardware updates this bit after each transmission attempt.

0: The previous transmission failed

1: The previous transmission was successful

This bit is set by hardware when the transmission request on mailbox 1 has been completed successfully. Refer to Figure 607 .

Bit 0 RQCP0 : Request completed mailbox0

Set by hardware when the last request (transmit or abort) has been performed.

Cleared by software writing a “1” or by hardware on transmission request (TXRQ0 set in CAN_TI0R register).

Clearing this bit clears all the status bits (TXOK0, ALST0 and TERR0) for Mailbox 0.

Address offset: 0x0C

Reset value: 0x0000 0000

Bits 31:6 Reserved, must be kept at reset value.

Bit 5 RFOM0 : Release FIFO 0 output mailbox

Set by software to release the output mailbox of the FIFO. The output mailbox can only be released when at least one message is pending in the FIFO. Setting this bit when the FIFO is empty has no effect. If at least two messages are pending in the FIFO, the software has to release the output mailbox to access the next message.

Cleared by hardware when the output mailbox has been released.

Bit 4 FOVR0 : FIFO 0 overrun

This bit is set by hardware when a new message has been received and passed the filter while the FIFO was full.

This bit is cleared by software.

Bit 3 FULL0 : FIFO 0 full

Set by hardware when three messages are stored in the FIFO.

This bit is cleared by software.

Bit 2 Reserved, must be kept at reset value.

Bits 1:0 FMP0[1:0] : FIFO 0 message pending

These bits indicate how many messages are pending in the receive FIFO.

FMP is increased each time the hardware stores a new message in to the FIFO. FMP is decreased each time the software releases the output mailbox by setting the RFOM0 bit.

Address offset: 0x10

Reset value: 0x0000 0000

Bits 31:6 Reserved, must be kept at reset value.

Bit 5 RFOM1 : Release FIFO 1 output mailbox

Set by software to release the output mailbox of the FIFO. The output mailbox can only be released when at least one message is pending in the FIFO. Setting this bit when the FIFO is empty has no effect. If at least two messages are pending in the FIFO, the software has to release the output mailbox to access the next message.

Cleared by hardware when the output mailbox has been released.

Bit 4 FOVR1 : FIFO 1 overrun

This bit is set by hardware when a new message has been received and passed the filter while the FIFO was full.

This bit is cleared by software.

Bit 3 FULL1 : FIFO 1 full

Set by hardware when three messages are stored in the FIFO.

This bit is cleared by software.

Bit 2 Reserved, must be kept at reset value.

Bits 1:0 FMP1[1:0] : FIFO 1 message pending

These bits indicate how many messages are pending in the receive FIFO1.

FMP1 is increased each time the hardware stores a new message in to the FIFO1. FMP is decreased each time the software releases the output mailbox by setting the RFOM1 bit.

CAN interrupt enable register (CAN_IER)

Address offset: 0x14

Reset value: 0x0000 0000

Bits 31:18 Reserved, must be kept at reset value.

Bit 17 SLKIE : Sleep interrupt enable

0: No interrupt when SLKIE bit is set.

1: Interrupt generated when SLKIE bit is set.

Bit 16 WKUIE : Wakeup interrupt enable

0: No interrupt when WKUIE bit is set.

1: Interrupt generated when WKUIE bit is set.

Bit 15 ERRIE : Error interrupt enable

0: No interrupt is generated when an error condition is pending in the CAN_ESR.

1: An interrupt is generated when an error condition is pending in the CAN_ESR.

Bits 14:12 Reserved, must be kept at reset value.

Bit 11 LECIÉ : Last error code interrupt enable

0: ERRI bit is not set when the error code in LEC[2:0] is set by hardware on error detection.

1: ERRI bit is set when the error code in LEC[2:0] is set by hardware on error detection.

Bit 10 BOFIE : Bus-off interrupt enable

0: ERRI bit is not set when BOFF is set.

1: ERRI bit is set when BOFF is set.

Bit 9 EPVIE : Error passive interrupt enable

0: ERRI bit is not set when EPVF is set.

1: ERRI bit is set when EPVF is set.

Bit 8 EWGIE : Error warning interrupt enable

0: ERRI bit is not set when EWGF is set.

1: ERRI bit is set when EWGF is set.

Bit 7 Reserved, must be kept at reset value.

Bit 6 FOVIE1 : FIFO overrun interrupt enable

0: No interrupt when FOVR is set.

1: Interrupt generation when FOVR is set.

Bit 5 FFIE1 : FIFO full interrupt enable

0: No interrupt when FULL bit is set.

1: Interrupt generated when FULL bit is set.

Bit 4 FMPIE1 : FIFO message pending interrupt enable

0: No interrupt generated when state of FMP[1:0] bits are not 00b.

1: Interrupt generated when state of FMP[1:0] bits are not 00b.

Bit 3 FOVIE0 : FIFO overrun interrupt enable

0: No interrupt when FOVR bit is set.

1: Interrupt generated when FOVR bit is set.

Bit 2 FFIE0 : FIFO full interrupt enable

0: No interrupt when FULL bit is set.

1: Interrupt generated when FULL bit is set.

Bit 1 FMPIE0 : FIFO message pending interrupt enable

0: No interrupt generated when state of FMP[1:0] bits are not 00b.

1: Interrupt generated when state of FMP[1:0] bits are not 00b.

Bit 0 TMEIE : Transmit mailbox empty interrupt enable

0: No interrupt when RQCPx bit is set.

1: Interrupt generated when RQCPx bit is set.

Note: Refer to Section 55.8: bxCAN interrupts .

Address offset: 0x18

Reset value: 0x0000 0000

The implementing part of the fault confinement mechanism of the CAN protocol. In case of an error during reception, this counter is incremented by 1 or by 8 depending on the error condition as defined by the CAN standard. After every successful reception the counter is decremented by 1 or reset to 120 if its value was higher than 128. When the counter value exceeds 127, the CAN controller enters the error passive state.

The implementing part of the fault confinement mechanism of the CAN protocol.

Bits 15:7 Reserved, must be kept at reset value.

This field is set by hardware and holds a code which indicates the error condition of the last error detected on the CAN bus. If a message has been transferred (reception or transmission) without error, this field is cleared to 0.

The LEC[2:0] bits can be set to value 0b111 by software. They are updated by hardware to indicate the current communication status.

000: No error

001: Stuff error

010: Form error

011: Acknowledgment error

100: Bit recessive error

101: Bit dominant error

110: CRC error

111: Set by software

Bit 3 Reserved, must be kept at reset value.

This bit is set by hardware when it enters the bus-off state. The bus-off state is entered on TEC overflow, greater than 255, refer to Section 55.7.6: Error management .

This bit is set by hardware when the Error passive limit has been reached (Receive Error Counter or Transmit Error Counter > 127).

This bit is set by hardware when the warning limit has been reached (Receive Error Counter or Transmit Error Counter ≥ 96).

CAN bit timing register (CAN_BTR)

Address offset: 0x1C

Reset value: 0x0123 0000

This register can only be accessed by the software when the CAN hardware is in initialization mode.

Bit 31 SILM : Silent mode (debug)

0: Normal operation

1: Silent Mode

Bit 30 LBKM : Loop back mode (debug)

0: Loop Back Mode disabled

1: Loop Back Mode enabled

Bits 29:26 Reserved, must be kept at reset value.

Bits 25:24 SJW[1:0] : Resynchronization jump width

These bits define the maximum number of time quanta the CAN hardware is allowed to lengthen or shorten a bit to perform the resynchronization.

Bit 23 Reserved, must be kept at reset value.

Bits 22:20 TS2[2:0] : Time segment 2

These bits define the number of time quanta in Time Segment 2.

Bits 19:16 TS1[3:0] : Time segment 1

These bits define the number of time quanta in Time Segment 1

For more information on bit timing, refer to Section 55.7.7: Bit timing .

Bits 15:10 Reserved, must be kept at reset value.

Bits 9:0 BRP[9:0] : Baud rate prescaler

These bits define the length of a time quanta.

55.9.3 CAN mailbox registers

This chapter describes the registers of the transmit and receive mailboxes. Refer to Section 55.7.5: Message storage for detailed register mapping.

Transmit and receive mailboxes have the same registers except:

• • The FMI field in the CAN_RDTxR register.
• • A receive mailbox is always write protected.
• • A transmit mailbox is write-enabled only while empty, corresponding TME bit in the CAN_TSR register set.

There are 3 TX Mailboxes and 2 RX Mailboxes. Each RX Mailbox allows access to a 3-level depth FIFO, the access being offered only to the oldest received message in the FIFO.

Each mailbox consist of four registers.

CAN TX mailbox identifier register (CAN_TIxR) (x = 0..2)

Address offsets: 0x180, 0x190, 0x1A0

Reset value: 0xXXXX XXXX (except bit 0, TXRQ = 0)

All TX registers are write protected when the mailbox is pending transmission (TMEx reset).

This register also implements the TX request control (bit 0) - reset value 0.

Bits 31:21 STID[10:0]/EXID[28:18] : Standard identifier or extended identifier

The standard identifier or the MSBs of the extended identifier (depending on the IDE bit value).

Bit 20:3 EXID[17:0] : Extended identifier

The LSBs of the extended identifier.

Bit 2 IDE : Identifier extension

This bit defines the identifier type of message in the mailbox.

0: Standard identifier.

1: Extended identifier.

Bit 1 RTR : Remote transmission request

0: Data frame

1: Remote frame

Bit 0 TXRQ : Transmit mailbox request

Set by software to request the transmission for the corresponding mailbox.

Cleared by hardware when the mailbox becomes empty.

CAN mailbox data length control and time stamp register
(CAN_TDTxR) (x = 0..2)

All bits of this register are write protected when the mailbox is not in empty state.

Address offsets: 0x184, 0x194, 0x1A4

Reset value: 0xXXXX XXXX

Bits 31:16 TIME[15:0] : Message time stamp

This field contains the 16-bit timer value captured at the SOF transmission.

Bits 15:9 Reserved, must be kept at reset value.

Bit 8 TGT : Transmit global time

This bit is active only when the hardware is in the Time Trigger Communication mode, TTCM bit of the CAN_MCR register is set.

0: Time stamp TIME[15:0] is not sent.

1: Time stamp TIME[15:0] value is sent in the last two data bytes of the 8-byte message:

TIME[7:0] in data byte 7 and TIME[15:8] in data byte 6, replacing the data written in CAN_TDHxR[31:16] register (DATA6[7:0] and DATA7[7:0]). DLC must be programmed as 8 in order these two bytes to be sent over the CAN bus.

Bits 7:4 Reserved, must be kept at reset value.

Bits 3:0 DLC[3:0] : Data length code

This field defines the number of data bytes a data frame contains or a remote frame request.

A message can contain from 0 to 8 data bytes, depending on the value in the DLC field.

CAN mailbox data low register (CAN_TDLxR) (x = 0..2)

All bits of this register are write protected when the mailbox is not in empty state.

Address offsets: 0x188, 0x198, 0x1A8

Reset value: 0xXXXX XXXX

Bits 31:24 DATA3[7:0] : Data byte 3

Data byte 3 of the message.

Bits 23:16 DATA2[7:0] : Data byte 2

Data byte 2 of the message.

Bits 15:8 DATA1[7:0] : Data byte 1

Data byte 1 of the message.

Bits 7:0 DATA0[7:0] : Data byte 0

Data byte 0 of the message.

A message can contain from 0 to 8 data bytes and starts with byte 0.

CAN mailbox data high register (CAN_TDHxR) (x = 0..2)

All bits of this register are write protected when the mailbox is not in empty state.

Address offsets: 0x18C, 0x19C, 0x1AC

Reset value: 0xXXXX XXXX

Bits 31:24 DATA7[7:0] : Data byte 7

Data byte 7 of the message.

Note: If TGT of this message and TTCM are active, DATA7 and DATA6 are replaced by the TIME stamp value.

Bits 23:16 DATA6[7:0] : Data byte 6

Data byte 6 of the message.

Bits 15:8 DATA5[7:0] : Data byte 5

Data byte 5 of the message.

Bits 7:0 DATA4[7:0] : Data byte 4

Data byte 4 of the message.

Address offsets: 0x1B0, 0x1C0

Reset value: 0xXXXX XXXX

All RX registers are write protected.

The standard identifier or the MSBs of the extended identifier (depending on the IDE bit value).

The LSBs of the extended identifier.

This bit defines the identifier type of message in the mailbox.

0: Standard identifier.

1: Extended identifier.

0: Data frame

1: Remote frame

Bit 0 Reserved, must be kept at reset value.

Address offsets: 0x1B4, 0x1C4

Reset value: 0xXXXX XXXX

All RX registers are write protected.

Bits 31:16 TIME[15:0] : Message time stamp

This field contains the 16-bit timer value captured at the SOF detection.

Bits 15:8 FMI[7:0] : Filter match index

This register contains the index of the filter the message stored in the mailbox passed through. For more details on identifier filtering refer to Section 55.7.4: Identifier filtering .

Bits 7:4 Reserved, must be kept at reset value.

Bits 3:0 DLC[3:0] : Data length code

This field defines the number of data bytes a data frame contains (0 to 8). It is 0 in the case of a remote frame request.

CAN receive FIFO mailbox data low register (CAN_RDLxR) (x = 0..1)

All bits of this register are write protected when the mailbox is not in empty state.

Address offsets: 0x1B8, 0x1C8

Reset value: 0xXXXX XXXX

All RX registers are write protected.

Bits 31:24 DATA3[7:0] : Data Byte 3

Data byte 3 of the message.

Bits 23:16 DATA2[7:0] : Data Byte 2

Data byte 2 of the message.

Bits 15:8 DATA1[7:0] : Data Byte 1

Data byte 1 of the message.

Bits 7:0 DATA0[7:0] : Data Byte 0

Data byte 0 of the message.

A message can contain from 0 to 8 data bytes and starts with byte 0.

CAN receive FIFO mailbox data high register (CAN_RDHxR) (x = 0..1)

Address offsets: 0x1BC, 0x1CC

Reset value: 0xXXXX XXXX

All RX registers are write protected.

Bits 31:24 DATA7[7:0] : Data Byte 7

Data byte 3 of the message.

Bits 23:16 DATA6[7:0] : Data Byte 6

Data byte 2 of the message.

Bits 15:8 DATA5[7:0] : Data Byte 5

Data byte 1 of the message.

Bits 7:0 DATA4[7:0] : Data Byte 4

Data byte 0 of the message.

55.9.4 CAN filter registers

CAN filter master register (CAN_FMR)

Address offset: 0x200

Reset value: 0x2A1C 0E01

All bits of this register are set and cleared by software.

Bits 31:14 Reserved, must be kept at reset value.

Bits 13:8 CANSB[5:0] : CAN start bank

These bits are set and cleared by software. When both CAN are used, they define the start bank of each CAN interface:

000001 = 1 filter assigned to CAN1 and 27 assigned to CAN2

011011 = 27 filters assigned to CAN1 and 1 filter assigned to CAN2

• – to assign all filters to one CAN set CANSB value to zero and deactivate the non used CAN
• – to use CAN1 only: stop the clock on CAN2 and/or set the CAN_MCR.INRQ on CAN2
• – to use CAN2 only: set the CAN_MCR.INRQ on CAN1 or deactivate the interrupt register CAN_IER on CAN1

Bits 7:1 Reserved, must be kept at reset value.

1. Bit 0 FINIT : Filter initialization mode
   Initialization mode for filter banks
   0: Active filters mode.
   1: Initialization mode for the filters.

CAN filter mode register (CAN_FM1R)

Address offset: 0x204

Reset value: 0x0000 0000

This register can be written only when the filter initialization mode is set (FINIT=1) in the CAN_FMR register.

Note: Refer to Figure 609: Filter bank scale configuration - Register organization .

Bits 31: Reserved, must be kept at reset value.

1. Bits 27:0 FBMx : Filter mode
   Mode of the registers of Filter x.
   0: Two 32-bit registers of filter bank x are in Identifier Mask mode.
   1: Two 32-bit registers of filter bank x are in Identifier List mode.

CAN filter scale register (CAN_FS1R)

Address offset: 0x20C

Reset value: 0x0000 0000

This register can be written only when the filter initialization mode is set (FINIT=1) in the CAN_FMR register.

Bits 31:28 Reserved, must be kept at reset value.

1. Bits 27:0 FSCx : Filter scale configuration
   These bits define the scale configuration of Filters 27-0.
   0: Dual 16-bit scale configuration
   1: Single 32-bit scale configuration

Note: Refer to Figure 609: Filter bank scale configuration - Register organization on page 2046 .

CAN filter FIFO assignment register (CAN_FFA1R)

Address offset: 0x214

Reset value: 0x0000 0000

This register can be written only when the filter initialization mode is set (FINIT=1) in the CAN_FMR register.

Bits 31:28 Reserved, must be kept at reset value.

Bits 27:0 FFAx : Filter FIFO assignment for filter x

The message passing through this filter is stored in the specified FIFO.

0: Filter assigned to FIFO 0

1: Filter assigned to FIFO 1

CAN filter activation register (CAN_FA1R)

Address offset: 0x21C

Reset value: 0x0000 0000

Bits 31:28 Reserved, must be kept at reset value.

Bits 27:0 FACTx : Filter active

The software sets this bit to activate Filter x. To modify the Filter x registers (CAN_FxR[0:7]), the FACTx bit must be cleared or the FINIT bit of the CAN_FMR register must be set.

0: Filter x is not active

1: Filter x is active

Address offsets: 0x240 to 0x31C

Reset value: 0xXXXX XXXX

There are 28 filter banks, i= 0 to 27. Each filter bank i is composed of two 32-bit registers, CAN_FiR[2:1].

This register can only be modified when the FACTx bit of the CAN_FAxR register is cleared or when the FINIT bit of the CAN_FMR register is set.

In all configurations:

Each bit of the register specifies the level of the corresponding bit of the expected identifier.

0: Dominant bit is expected

1: Recessive bit is expected

Each bit of the register specifies whether the bit of the associated identifier register must match with the corresponding bit of the expected identifier or not.

0: Do not care, the bit is not used for the comparison

1: Must match, the bit of the incoming identifier must have the same level has specified in the corresponding identifier register of the filter.

Note: Depending on the scale and mode configuration of the filter the function of each register can differ. For the filter mapping, functions description and mask registers association, refer to Section 55.7.4: Identifier filtering .

A Mask/Identifier register in mask mode has the same bit mapping as in identifier list mode.

For the register mapping/addresses of the filter banks refer to Table 414 .

55.9.5 bxCAN register map

Refer to Section 2.2 on page 93 for the register boundary addresses. The registers from offset 0x200 to 0x31C are present only in CAN1.

Table 414. bxCAN register map and reset values

Table 414. bxCAN register map and reset values (continued)

Table 414. bxCAN register map and reset values (continued)

Table 414. bxCAN register map and reset values (continued)

Refer to Section 2.2 on page 93 for the register boundary addresses.