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STM32F746: Instruction Cache

Question asked by gardner.jerry on Jul 9, 2017
Latest reply on Jul 20, 2017 by STOne -32

I've got a simple test that runs under an RTOS. There are two identical tasks that blink separate LEDs using a software delay loop. The two tasks are identical except for the LED that each blinks. Both tasks have identical priorities, so the RTOS runs them in strict round robin.


If I have task1 first in the C file, then the LED associated with that task blinks at twice the rate of the LED associated with task2. If I swap the positions of the two tasks in the C file, then task2's blinks twice as fast as task1's LED. Swapping the tasks in the C file swaps their positions in FLASH.


The only thing that changes when I swap the positions of task1 and task2 in memory is the code addresses. The data and stack stay in the same place.


The only explanation for this behavior that I can think of is the relative positions of these two tasks in memory affects how the code is cached by the CPU. This is where I'm puzzled, however. According to the datasheet, only FLASH on the ITCM interface uses the ART accelerator. My code is in the FLASH on the AXIM interface.


Is there another instruction cache different from the ART accelerator? Is there any way to disable it to check whether it's the cause of the effect I'm seeing? I didn't find anything in either the datasheet or the reference manual.


This isn't a subtle effect--the software delay loop runs twice as fast in one task relative to the other.