2. System and memory overview

2.1 System architecture

The STM32F318x8 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
  • – ADC1

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

Figure 1. System architecture

2.1.1 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 up to 16 Kbytes.

2.1.2 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 (16 Kbytes).

2.1.3 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 (16 Kbytes), the AHB to APB1/APB2 bridges, the AHB IO port and the ADC.

2.1.4 S3: 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.

2.1.5 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 bus) and seven slaves (FLITF, SRAM, AHB2GPIO and AHB2APB1/2 bridges, and ADC).

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: Memory organization on page 40 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 the 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.

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

All the memory map areas that are 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.

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

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

2.3 Embedded SRAM

STM32F3xx devices feature up to 16 Kbytes of static SRAM. It can be accessed as bytes, halfwords (16 bits) or full words (32 bits). Up to 16 Kbytes of SRAM can be addressed at maximum system clock frequency without wait state, and can be accessed by both CPU and DMA.

2.4 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. Please, refer to Section 3: 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.

2.5 Boot configuration

In the STM32F3xx, 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 2. 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).

2.5.1 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:

• • USART1 (PA9/PA10) or USART2 (PA2/PA3) on STM32F301xx devices, and
• • USART1 (PA9/PA10), USART2 (PA2/PA3), I2C1 (PB6/PB7) or I2C3 (PA8/PB5) on STM32F318xx devices.