2. System and memory overview

2.1 System architecture

The main system consists of 32-bit multilayer AHB bus matrix that interconnects:

• • Seven masters:
  • – CPU1 (CPU1 Cortex ^® -M4 with FPU) core I-bus
  • – CPU1 (CPU1 Cortex ^® -M4 with FPU) core D-bus
  • – CPU1 (CPU1 Cortex ^® -M4 with FPU) core S-bus
  • – CPU2 (Cortex ^® -M0+) core S-bus
  • – DMA1
  • – Radio system
• • Ten slaves:
  • – Internal Flash memory on the CPU1 (CPU1 Cortex ^® -M4) ICode bus
  • – Internal Flash memory on CPU1 (CPU1 Cortex ^® -M4) DCode bus
  • – Internal Flash memory on CPU2 (Cortex ^® -M0+) S bus
  • – Internal SRAM1 (12 KB)
  • – Internal SRAM2a (32 KB) + SRAM2b (4 KB)
  • – AHB1 peripherals including AHB to APB bridges and APB peripherals (connected to APB1 and APB2)
  • – AHB2 peripherals
  • – AHB4 shared peripheral
  • – AHB5 including AHB to APB bridge and Radio peripherals (connected to APB3)

The bus matrix provides access from a master to a slave, enabling concurrent access and efficient operation even when several high-speed peripherals work simultaneously. This architecture is shown in Figure 1 .

Figure 1. System architecture

2.1.1 S0: CPU1 (CPU1 Cortex®-M4) I-bus

This bus connects the instruction bus of the CPU1 core to the BusMatrix. This bus is used by the core to fetch instructions. The targets of this bus are the internal Flash memory, SRAM1 (backup) and SRAM2 (backup).

2.1.2 S1: CPU1 (CPU1 Cortex®-M4) D-bus

This bus connects the data bus of the CPU1 core to the BusMatrix. This bus is used by the core for literal load and debug access. The targets of this bus are the internal Flash memory, SRAM1 (backup) and SRAM2 (backup).

2.1.3 S2: CPU1 (CPU1 Cortex®-M4) S-bus

This bus connects the system bus of the CPU1 core to the BusMatrix. This bus is used by the core to access data located in a peripheral or SRAM area. The targets of this bus are SRAM1 (backup), SRAM2 (backup), the AHB1 peripherals including the APB1 and APB2 peripherals, the AHB2 peripherals and the AHB4 peripherals.

2.1.4 S3: CPU2 (Cortex®-M0+) S-bus

This bus connects the system bus of the CPU2 core to the BusMatrix. This bus is used by the core to fetch instructions, for literal load and debug access, and access data located in a

peripheral or SRAM area. The targets of this bus are the internal Flash memory, SRAM1, SRAM2 (backup), the AHB1 peripherals including the APB1 and APB2 peripherals, the AHB2 peripherals, and the AHB4 peripherals and the AHB5 peripherals including the APB3 peripherals.

2.1.5 S4: DMA-bus

This bus connects the AHB master interface of the DMA to the BusMatrix. The targets of this bus are the SRAM1, SRAM2 (backup), the AHB1 peripherals including the APB1 and APB2 peripherals, the AHB2 peripherals and the AHB4 peripherals.

2.1.6 S5: Radio system-bus

This bus connects the AHB master interface of the Radio system to the BusMatrix. The targets of this bus is the SRAM2 (backup).

2.1.7 BusMatrix

The BusMatrix manages the access arbitration between masters. The arbitration uses a Round Robin algorithm. The BusMatrix is composed by six masters (CPU1: system bus, DCode bus, ICode bus, CPU2: system bus, DMA1-bus and Radio system-bus) and nine slaves (3 x Flash memory, SRAM1, SRAM2 (backup), AHB1 (including APB1 and APB2), AHB2, AHB4, and AHB5).

AHB/APB bridges

The two bridges AHB to APB1 and AHB to APB2 provide full synchronous connections between the AHB and the two APB buses, allowing flexible selection of the peripheral frequency.

The bridges AHB to APB3 provide an a-synchronous connections between the AHB and the APB bus, allowing flexible selection of the frequency between the AHB and peripheral.

Refer to Section 2.2: Memory organization for the address mapping of the peripherals connected to this bridge.

After each device reset, all peripheral clocks are disabled (except for the SRAM1/2 and Flash memory interface). Before using a peripheral you have to enable its clock in the RCC_AHBxENR and the RCC_APBxENR registers.

Note: When a 16- or 8-bit access is performed on an APB register, the access is transformed into a 32-bit access: the bridge duplicates the 16- or 8-bit data to feed the 32-bit vector.

2.2 Memory organization

2.2.1 Introduction

Program memory, data memory, registers and I/O ports are organized within the same linear 4-Gbyte address space.

The bytes are coded in memory in Little Endian format. The lowest numbered byte in a word is considered the word's least significant byte and the highest numbered byte the most significant.

The addressable memory space is divided into eight main blocks, of 512 Mbytes each.

2.2.2 Memory map and register boundary addresses

Figure 2. Memory map

All the memory areas not allocated to on-chip memories and peripherals are considered “Reserved”. For the detailed mapping of available memory and register areas, refer to the following table, which gives the boundary addresses of the available peripherals.

Table 1. Memory map and peripheral register boundary addresses

Table 1. Memory map and peripheral register boundary addresses (continued)

1. Bus depends upon selected CPUUn Boot area.

2.2.3 Bit banding

The CPU1 map includes two bit-band regions. These regions map each word in an alias region of memory to a bit in a bit-band region of memory. Writing to a word in the alias region has the same effect as a read-modify-write operation on the targeted bit in the bit-band region.

The AHB1, APB1, APB2 peripheral registers and the SRAM1, SRAM2a and SRAM2b are mapped to a bit-band region, hence single bit-band write and read operations are allowed. The operations are only available for CPU1 accesses, and not from other bus masters (e.g. DMA)

The peripheral bit-band alias is located from address 0x4200 0000 to 0x42FF FFFF

The SRAM bit-band alias is located from address 0x2200 0000 to 0x227F FFFF

A mapping formula shows how to reference each word in the alias region to a corresponding bit in the bit-band region. The mapping formula is:

\( \text{bit\_word\_addr} = \text{bit\_band\_base} + (\text{byte\_offset} * 32) + (\text{bit\_number} * 4) \) , where:

• – \( \text{bit\_word\_addr} \) is the address of the word in the alias memory region that maps to the targeted bit.
• – \( \text{bit\_band\_base} \) is the starting address of the alias region
• – \( \text{byte\_offset} \) is the number of the byte in the bit_band region that contains the targeted bit
• – \( \text{bit\_number} \) is the bit position (0-7) of the targeted bit

Example

The following example shows how to map bit [2] of the byte located at SRAM1 address 0x2000 0300 to the alias region.

Writing to address 0x2200 6008 has the same effect as a read-modify-write operation on bit [2] of the byte at SRAM1 address 0x2000 0300.

Reading address 0x2200 6008 returns the value 0x01 or 0x00 of bit [2] of the byte at SRAM1 address 0x2000 0300.

For more information on bit-band, refer to the Cortex®-M4 programming manual.

2.3 Boot configuration

Three different CPU1 boot modes can be selected through the BOOT0 pin or the nBOOT0 bit into the FLASH_OPTR register (if the nSWBOOT0 bit is cleared into the FLASH_OPTR register), and nBOOT1 bit in the FLASH_OPTR register, as shown in Table 2 .

Table 2. Boot modes

Table 2. Boot modes (continued)

1. 1. A flash empty check mechanism is implemented to force the boot from system flash if the first flash memory location is not programmed (0xFFFF FFFF) and if the boot selection was configured to boot from the main flash.

The values on both BOOT0 and BOOT1 are latched after a reset. It is up to the user to provide the correct value for the required boot mode.

The BOOT0 and BOOT1 are also re-sampled when exiting Standby mode. Consequently they must be kept in the required boot mode. After the startup delay, the CPU1 fetches the top-of-stack from address 0x0000 0000, then starts code execution from the boot memory at 0x0000 0004.

Depending on the selected boot mode, main flash, system flash, or SRAM1 memories are accessible as follows:

• • Boot from main flash memory: the main flash memory is aliased in the CPU1 boot memory space at address 0x0000 0000, and is accessible even from its physical address 0x0800 0000. In other words, the flash memory content can be accessed starting from address 0x0000 0000 or 0x0800 0000.
• • Boot from system flash memory: the system flash memory is aliased in the CPU1 boot memory space at address 0x0000 0000, and is also still accessible from its physical address 0x1FFF 0000.
• • Boot from SRAM: the memory is aliased in the CPU1 boot memory space at address 0x0000 0000, and is accessible even from its physical address 0x2000 0000.

Empty check

An internal empty check flag (the EMPTY bit of the FLASH access control register (FLASH_ACR)) is available for easy programming of virgin devices by the boot loader. This flag is used when BOOT0 pin is defining main flash as the target boot area. When the flag is set, the device is considered as empty, and the system memory (boot loader) is selected instead of the main flash as a boot area, to allow user to program the memory. Therefore, some of the GPIOs are reconfigured from the high-Z state. Refer to AN2606 for more details concerning the bootloader and GPIO configuration in system memory boot mode. It is possible to disable this feature by configuring the option bytes to force boot from the main flash memory (nSWBOOT0 = 0, nBOOT0 = 1).

This empty check flag is updated only during the loading of option bytes: it is set when the content of the address 0x0800 0000 is read as 0xFFFF FFFF, otherwise it is cleared. A power reset or setting the OBL_LAUNCH bit in FLASH_CR register is needed to clear this flag after programming of a virgin device, to execute user code after System reset. The EMPTY bit can be written directly by software.

CPU1 physical remap

Following CPU1 boot the application software can modify the memory map at address 0x0000 0000. This modification is performed by programming the SYSCFG memory remap register (SYSCFG_MEMRMP) in the SYSCFG controller.

The following memories can be remapped:

• • Main flash memory
• • System flash memory
• • SRAM

Embedded boot loader

The embedded boot loader is located in the system flash memory, programmed by ST during production. It is used to program the flash memory using one of the following device interfaces:

• • USART1 on pins PA9 and PA10
• • I2C1 on pins PB6 and PB7.
• • SPI1 on pins PA4, PA5, PA6 and PA7

2.4 CPU2 boot

Following a device reset the CPU2 will only boot after CPU1 has set the C2BOOT bit in the PWR control register 4 (PWR_CR4) . The C2BOOT value is retained in Standby mode and the CPU2 will boot accordingly when exit from Standby.

The CPU2 will boot from its boot reset vector as defined by the flash user option C2OPT and SBRV.

The CPU2 may boot from anywhere in user flash or SRAM1/SRAM2a/SRAM2b.

CPU2 safe boot

When, after a reset, the User options are not valid and the BOOT0 and BOOT1 select CPU1 to boot from main flash memory, the CPU2 boots from a safe boot vector in main flash memory at address 0x0804 F000.

The safe boot can be used to restore the last known user options from a copied image.

2.5 CPU2 SRAM fetch disable

CPU2 execution from SRAM can be disabled by the C2RFD bit in SYSCFG register. Disabling CPU2 execution from SRAM improves robustness of the CPU2 software.