Reset over CTRL-AP

[Glenn Andrews]

We're developing an application using the NRF54L15. SWD will be connected to an external device that can update the firmware on the MCU if there are any issues, but we don't have room to wire up nRESET.

Can you confirm that we can do a hard reset of the MCU using just the SWD lines if the nRESET line is tied high through a pull-up resistor?

We'd want the ability to do a hard reset and start the application running (same as asserting/releasing nRESET) as well as reprogramming the MCU in case of issues with the app itself.

Are there any other considerations, hardware or software, we should be aware of?

I saw in one answer a recommendation for 1MOhm pull-downs on the SWD lines, or is that for a different family?

Thanks,

Glenn

[Vidar Berg]

Hello Glenn,

Yes, it’s possible to perform a “pinreset” via the CTRL-AP without physically asserting the pinreset input on the chip. Below are the commands I used when testing this here with J-Link Commander (JLinkExe/jlink.exe):

/* Use SWD */
SWDSelect
/* Power on debug interface by setting CSYSPWRUPREQ bits */ 
/* https://developer.arm.com/documentation/ihi0031/a/Debug-Port-Registers/Debug-Port--DP--register-descriptions/The-Control-Status-Register--CTRL-STAT */
SWDWriteDP 1 0x50000000
/* Select CTRL-AP (APSEL = 2) */
/* https://developer.arm.com/documentation/ihi0031/a/Debug-Port-Registers/Debug-Port--DP--register-descriptions/The-AP-Select-Register--SELECT */
/* https://docs.nordicsemi.com/bundle/nan_047/page/APP/nan_production_programming/nRF54L/nrf54l_select_access_port.html */
SWDWriteDP 2 0x2000000 
/* Pinreset */
SWDWriteAP 0 0x4
/* Release the RESET register value */ 
SWDWriteAP 0 0x0

(Ref. https://docs.nordicsemi.com/bundle/ps_nrf54L15/page/chapters/power-and-clock/reset/doc/reset.html#ariaid-title11) 

I saw in one answer a recommendation for 1MOhm pull-downs on the SWD lines, or is that for a different family?

This is not needed or recommended on the nRF54L. The internal pull up is sufficient.

Best regards,

Vidar

[Glenn Andrews]

That's what I thought. Thank you for the quick response!

[Glenn Andrews]

Sorry for the delay before reopening this.

I ran the commands on our nRF54L15 project board.

SWDWriteAP 0 0x0 did reset the board, but I was expecting SWDWriteAP 0 0x4 to hold the board in reset until it was released.

Is there a command that does that?

[Vidar Berg]

No problem. I realize now I didn’t take this requirement into account when I answered earlier, and unfortunately I don’t see a way to do this through the CTRL-AP. However, would it be sufficient to keep the CPU halted after the firmware is programmed? 

https://developer.arm.com/documentation/ddi0413/d/debug/debug-control/debug-halting-control-and-status-register 

[Glenn Andrews]

I think that should do it. 

To be honest I'm not sure if being held in reset is even required.

Our main goal is to be able to erase and reprogram the nRF over SWD and restart it without needing the nRESET line wired up.

If we can do that, I think anything else is a bonus.

[Vidar Berg]

Good to hear. Yes, it should be sufficient to reset the chip through the CTRL-AP