Reasons that RESETREAS is zero

RE: Reasons that RESETREAS is zero

Einar Thorsrud — Thu, 24 Nov 2022 14:50:05 GMT

Hi Erwin,

The reset pin has an internal pull-up, so it is not needed to pull it up externally.

What you describe is sensible, and is a major reason for having the RESETREAS (the ability to act differently based on what triggered the reset).

If bit A is set in RESETREAS, then there has been a pin reset. You should not get this set after a power-cycle like you describe. You can test this simply with this modified hello world sample on a nRF5340 DK, built for the app core:

#include <zephyr/zephyr.h>
#include <helpers/nrfx_reset_reason.h>

void main(void)
{
	uint32_t reason;

	reason = nrfx_reset_reason_get();

	while (true)
	{
		printk("RESETREAS: 0x%08x\n", reason);
		k_sleep(K_SECONDS(2));
	}
}

If you press the reset button on the DK, you will see that bit A (the least significant bit) is set. If you power-cycle the DK, you will see that all bits are 0.

RE: Reasons that RESETREAS is zero

ErwinCes — Thu, 24 Nov 2022 13:23:44 GMT

Thanks for the fast and good answer.

Just one question for confirmation: the reset pin is only connected to the debugger connection. There is no HW that pulls it externally to high. I assume that some internal logic in the processor assured that when I simply turn on the power supply the reset bit in the register is set.

As a consequence I can implement in my firmware: if the A bit is set it was a real power cut or better a switch on of the (external) power supply. When the whole register is 0 we really had a power drop, that lead to a brown out reset. By completely switching off the power supply (for at least a second) and switching on again, I see bit A set.

Erwin

RE: Reasons that RESETREAS is zero

Einar Thorsrud — Thu, 24 Nov 2022 08:40:02 GMT

Hi Erwin,

The RESETREAS register is cleared by a power-on or a brown-out reset (see Application core reset behavior), so I would look at the supply voltage when this happens. For other reset reasons you will see the corresponding bit set in the RESETREAS (this will be the case after a reset from an exception handler).

Yes, if trying to execute all FFs you will get an exception. Cortex CPUs does not interpret programmed memory as NOPs.

Einar