Embedded Flash memory interface
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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.
3.5.2 Adaptive
real-time
memory
accelerator (ART Accelerator™)
The proprietary Adaptive real-time (ART) memory accelerator is optimized for STM32
industry-standard ARM
®
Cortex
®
-M4 with FPU processors. It balances the inherent
performance advantage of the ARM
®
Cortex
®
-M4 with FPU 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, 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 180 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.
shows the execution of sequential 32-bit instructions with and without prefetch
when 3 WSs are needed to access the Flash memory.