nrf51822 can't reset with SWDIO/nReset pin occasionally

[SimonYao]

nrf51822's SWDIO/nReset pin is controled by a MCU GPIO output, a 1k resistor is in series. SWDCLK is connected to a pull-down resistor. And sometime (very rare) nrf51822 can't be reset by the GPIO. It cannot be reset unless powered on again. And the reson is most likely due to interference. 

But why, is the chip entered the debug mode. Do I need to attach a capacitor from SWDIO/nReset to ground. If the capacitor is attached, will it affect the firmware download.

[Kenneth]

Hello,

Can you share a schematic snippet of your circuitry connected to SWDCLK and SWDIO?

Also, can you in start of main() enable NRF_POWER->RESET=1; to make sure pin reset is enabled in debug mode?

Kenneth

[SimonYao]

Hi：

Schematic is as follows, BT_RST is controlled by a push-pull GPIO output(3.3V) of MCU. 

[Kenneth]

Hello,

My suggestions to try to narrow down the root cause is:

- Try to add NRF_POWER->RESET=1; in start of main(), check if you are able to recreate the issue.

- Try to add a 1Mohm resistor between SWDIO and GND, check if you are able to recreate the issue.

Both of the above will affect the pin reset, and if one of the above solve the issue I will be able to speculate a bit more what is the root-cause.

Kenneth