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iMx7: M4 stops running on A7 reset

Question asked by Jean Pihet on Jan 17, 2017
Latest reply on May 24, 2017 by Evgeny Erlihman



I have a question related to the A7 and M4 dependency on reset.

We want to be able to keep the M4 (FreeRTOS) running if the A7 (Linux) reboots, e.g. after an update or after a crash/wdog.

Is this possible? Since the RTOS is running some critical code under FreeRTOS, it has to keep running all the time.


The issue we are experiencing is that the M4 stops running after a reset from the A7.


Here are some details about the issue:

- M4 code is loaded and started from A7 (U-Boot or Linux). The M4 code is in DDR.

- After a 'reboot' command on A7 the M4 code is untouched but the BOOTROM control registers (at 0x00180000) are changed. Also the SRC_M4RCR register value is 0xAB.

- After a press on the RESET button the M4 code and the BOOTROM control registers are untouched but the SRC_M4RCR register value is 0xAB.


The value of 0xAB in SRC_M4RCR is weird since bit 1 (SW_M4C_RST) is self-clearing.


Is there a way to prevent a reset from A7 from touching the M4 state?


Thanks a lot for looking, any feedback is welcome!



Jean Pihet