HW -reset/swdio pin nrf51422 challenges

[Flemming Jensen]

Hi,

We have an application where we have very very slow vcc risetime and therefore we have an external watchdog reset circuit connected to the -reset/swdio pin in order to successfully reset the cpu.

We see that if we program the device using fx nrfjprog.exe and finish up with a nrfjprog.exe -r command - we are not able to reset the cpu externally anymore...

If we reset using -p (pinreset) we can often reset external.

Sometimes we are locked out from resetting. In this case we need to completely discharge the VCC caps and start again.

What is going on with the reset pin??

BR Flemming

[Kenneth]

Hi, calling nrfjprog with '-r' means a system reset, where the device is still in debug mode. Calling with '-p' means a pin reset, where the device will exit debug mode. Typically while in debug mode the pin reset is disabled, to handle this the application can write to the RESET register in the POWER peripheral to enable pin reset in debug mode.

[Flemming Jensen]

So we need to redesign our hardware then :(

What we see is that the CPU occasionally doesn't start and does not react on external reset from our watchdog circuit.

If we have slow risetime of VCC we might end up with a CPU not starting and the CPU might be in debug_mode thus not reacting on our external reset signal...

So we can only think of redesigning our pcb to have a POWER_GOOD signal and then switch the CPU on.

[Kenneth]

The POR require a rise time of less than 100ms from 0V to VDD to ensure it will start. I assume you are slower than that here?

[Flemming Jensen]

Yes we are slower because we get power from a generator that might be running quite slow.

So i guess we need a different power on circuitry

[Kenneth]

In such case you will need to ensure that the rise time is within our specification yes. Maybe you can consider an voltage detector (e.g. Torex XC61F-Series) to control a transistor that can turn ON the power to the nRF51 when the supply voltage has stabilized.