DocID15687 Rev 5

9/29

PM0059

Flash memory interface

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Decreasing the CPU frequency

1.

Modify the CPU clock source by writing the SW bits in the RCC_CFGR register

2.

If needed, modify the CPU clock prescaler by writing the HPRE bits in RCC_CFGR

3.

Check that the new CPU clock source or/and the new CPU clock prescaler value is/are

taken into account by reading the clock source status (SWS bits) or/and the AHB

prescaler value (HPRE bits), respectively, in the RCC_CFGR register

4.

Program the new number of wait states to the LATENCY bits in FLASH_ACR

5.

Check that the new number of wait states is used to access the Flash memory by

reading the FLASH_ACR register

Note:

A change in CPU clock configuration or wait state (WS) configuration may not be effective

straight away. To make sure that the current CPU clock frequency is the one you have

configured, you can check the AHB prescaler factor and clock source status values. To

make sure that the number of WS you have programmed is effective, you can read the

FLASH_ACR register.

2.4.2

Adaptive real-time memory accelerator (ART Accelerator™)

The proprietary Adaptive real-time (ART) memory accelerator is optimized for STM32

advantage of the ARM Cortex-M3 over Flash memory technologies, which normally requires

the processor to wait for the Flash memory at higher operating frequencies.

To release the processor full performance of 150 DMIPS, the accelerator implements an

instruction prefetch queue and branch cache which increases program execution speed

from the 128-bit Flash memory. Based on CoreMark benchmark, the performance achieved

thanks to the ART accelerator is equivalent to 0 wait state program execution from Flash

memory at a CPU frequency up to 120 MHz.

Instruction prefetch

Each Flash memory read operation provides 128 bits from either four instructions of 32 bits

or 8 instructions of 16 bits according to the program launched. So, in case of sequential

code, at least four CPU cycles are needed to execute the previous read instruction line.

Prefetch on the I-Code bus can be used to read the next sequential instruction line from the

Flash memory while the current instruction line is being requested by the CPU. Prefetch is

enabled by setting the PRFTEN bit in the FLASH_ACR register. This feature is useful if at

least one wait state is needed to access the Flash memory.

Figure 2

shows the execution of sequential 32-bit instructions with and without prefetch

when 3 WSs are needed to access the Flash memory.