3. System and memory overview

3.1 System architecture

The STM32F302xB/C/D/E main system consists of:

• • Five masters:
  • – Cortex ^® -M4 core I-bus
  • – Cortex ^® -M4 core D-bus
  • – Cortex ^® -M4 core S-bus
  • – GP-DMA1 and GP-DMA2 (general-purpose DMA)
• • Six (seven in STM32F302xD/E) slaves:
  • – Internal Flash memory on the DCode
  • – Internal Flash memory on ICode
  • – Up to internal 40-Kbyte SRAM
  • – FMC in STM32F302xD/E
  • – AHB to APBx (APB1 or APB2), which connect all the APB peripherals
  • – AHB dedicated to GPIO ports
  • – ADCs 1, 2.

The STM32F302x6/8 main system consists of:

• • Four masters:
  • – Cortex ^® -M4 core I-bus
  • – Cortex ^® -M4 core D-bus
  • – Cortex ^® -M4 core S-bus
  • – DMA1 (general-purpose DMA)
• • Six slaves:
  • – Internal Flash memory on the DCode
  • – Internal Flash memory on ICode
  • – Up to 16-Kbyte internal SRAM
  • – AHB to APBx (APB1 or APB2), which connect all the APB peripherals
  • – AHB dedicated to GPIO ports
  • – ADC 1

The interconnection uses a multilayer AHB bus architecture as shown in figures 1 to 3.

Figure 1. STM32F302xB/C system architecture

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Figure 2. STM32F302x6/8 system architecture

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Figure 3. STM32F302xD/E system architecture

3.1.1 S0: I-bus

This bus connects the Instruction bus of the Cortex®-M4 core to the BusMatrix. This bus is used by the core to fetch instructions. The targets of this bus are the internal Flash memory and the SRAM.

3.1.2 S1: D-bus

This bus connects the DCode bus (literal load and debug access) of the Cortex®-M4 core to the BusMatrix. The targets of this bus are the internal Flash memory and the SRAM.

3.1.3 S2: S-bus

This bus connects the system bus of the Cortex®-M4 core to the BusMatrix. This bus is used to access data located in the peripheral or SRAM area. The targets of this bus are the SRAM, the AHB to APB1/APB2 bridges, the AHB IO port and the 2 ADCs.

3.1.4 S3, S4: DMA-bus

This bus connects the AHB master interface of the DMA to the BusMatrix which manages the access of different Masters to Flash, SRAM and peripherals.

3.1.5 BusMatrix

The BusMatrix manages the access arbitration between Masters. The arbitration uses a Round Robin algorithm. The BusMatrix is composed of five masters (CPU AHB, System bus, DCode bus, ICode bus, DMA1/2 bus) and seven slaves (FLITF, SRAM, AHB2GPIO and AHB2APB1/2 bridges, and ADCs).

AHB/APB bridges

The two AHB/APB bridges provide full synchronous connections between the AHB and the two APB buses. APB1 is limited to 36 MHz, APB2 operates at full speed (72 MHz).

Refer to Section 2.2.2: Memory map and register boundary addresses on page 48 for the address mapping of the peripherals connected to this bridge.

After each device reset, all peripheral clocks are disabled (except for the SRAM and FLITF). Before using a peripheral user has to enable its clock in the RCC_AHBENR, RCC_APB2ENR or RCC_APB1ENR register.

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.

3.2 Memory organization

3.2.1 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.

3.2.2 Memory map and register boundary addresses

Refer to the device datasheet for a comprehensive diagram of the memory map.

The following table gives the boundary addresses of the peripherals available in the devices.

Table 2. STM32F302xB/C peripheral register boundary addresses ⁽¹⁾

1. The gray color is used for reserved Flash memory addresses.

1. The gray color is used for reserved Flash memory addresses.

1. The gray color is used for reserved Flash memory addresses.

3.3 Embedded SRAM

STM32F302xB/C devices feature up to 40 Kbytes of static SRAM. It can be accessed as bytes, halfwords (16 bits) or full words (32 bits). It can be addressed at maximum system clock frequency without wait states and can be accessed by both CPU and DMA.

STM32F302x6/8 devices feature only up to 16 Kbytes of static SRAM.

STM32F302xD/E devices feature up to 64 Kbytes of static SRAM.

3.3.1 Parity check

On the STM32F302xD/E devices, for the 40-Kbyte SRAM, a parity check is implemented only on the first 16 Kbytes.

On the STM32F302xD/E devices, the parity check is implemented on the first 32 Kbytes of SRAM. On the STM32F302x6/x8 the SRAM parity check is not supported. The SRAM parity check is disabled by default. It is enabled by the user, when needed, using an option bit.

The data bus width of the SRAM supporting the parity check is 36 bits because 4 bits are available for parity check (1 bit per byte) in order to increase memory robustness, as required for instance by Class B or SIL norms.

The parity bits are computed on data and address and stored when writing into the SRAM. Then, they are automatically checked when reading. If one bit fails, an NMI is generated if the SRAM parity check is enabled. The same error can also be linked to the Break input of TIMER 1, 8, 15, 16 and 17, by setting the SRAM_PARITY_LOCK control bit in the SYSCFG configuration register 2 (SYSCFG_CFGR2). In case of parity error, the SRAM Parity Error flag (SRAM_PEF) is set in SYSCFG_CFGR2. For more details, refer to SYSCFG_CFGR2.

The BYP_ADD_PAR bit in SYSCFG_CFGR2 can be used to prevent an unwanted parity error to occur when the user programs a code in the RAM at address 0x2XXXXXXX (address in the address range 0x20000000-0x20002000) and then executes the code from RAM at boot (RAM is remapped at address 0x00).

3.4 Flash memory overview

The Flash memory is composed of two distinct physical areas:

• • The main Flash memory block. It contains the application program and user data if necessary.
• • The information block. It is composed of two parts:
  • – Option bytes for hardware and memory protection user configuration
  • – System memory which contains the proprietary boot loader code. Refer to Section 4: Embedded Flash memory for more details.

Flash memory instructions and data access are performed through the AHB bus. The prefetch block is used for instruction fetches through the ICode bus. Arbitration is performed in the Flash memory interface, and priority is given to data access on the DCode bus. It also implements the logic necessary to carry out the Flash memory operations (Program/Erase) controlled through the Flash registers.

3.5 Boot configuration

In the STM32F302xx, three different boot modes can be selected through the BOOT0 pin and nBOOT1 bit in the user option byte, as shown in the following table:

Table 5. Boot modes

The values on both BOOT0 pin and nBOOT1 bit are latched on the 4th rising edge of SYSCLK after a reset.

It is up to the user to set the nBOOT1 and BOOT0 to select the required boot mode. The BOOT0 pin and nBOOT1 bit are also resampled when exiting from Standby mode. Consequently they must be kept in the required Boot mode configuration in Standby mode. After this startup delay has elapsed, the CPU fetches the top-of-stack value from address 0x0000 0000, then starts code execution from the boot memory at 0x0000 0004. Depending on the selected boot mode, main Flash memory, system memory or SRAM is accessible as follows:

• • Boot from main Flash memory: the main Flash memory is aliased in the boot memory space (0x0000 0000), but still accessible from its original memory space (0x0800 0000). In other words, the Flash memory contents can be accessed starting from address 0x0000 0000 or 0x0800 0000.
• • Boot from system memory: the system memory is aliased in the boot memory space (0x0000 0000), but still accessible from its original memory space (0x1FFF D800).
• • Boot from the embedded SRAM: the SRAM is aliased in the boot memory space (0x0000 0000), but it is still accessible from its original memory space (0x2000 0000).

3.5.1 Embedded boot loader

The embedded boot loader is located in the system memory, programmed by ST during production. It is used to reprogram the Flash memory through the following peripherals:

• • USART1(PA9/PA10), USART2(PD5/PD6) or USB(PA11/PA12) on STM32F302xB/C devices
• • USART1(PA9/PA10), USART2(PA2/PA3) or USB(PA11/PA12) on STM32F302x6/8 and STM32F302xD/E devices.