NRF52840 does not wake up correctly after fast power cycle

[ste2108]

Hello,

We are experiencing a strange issue when quickly turning off/on the NRF52840 on our board. In order to check whether the MCU wakes up correctly, a test fw was set up which starts toggling a pin after the MCU is up. You can see the correct behaviour in the first picture (the green signal is the reset, the purple one the MCU power and the yellow one is the toggling pin). When we quickly power off/on the MCU (where quick means about half a second) we see that the MCU randomly gets stuck, so that the situation after the reset is as in pic. 2. Once in this condition, the normal functionality can be recovered with either a "slow" power cycle (off/on in about a couple of seconds) or by resetting the MCU (without powering it off). This latter operation however may or may not unfreeze the MCU in an apparently random way.

We experience MCU freezing also when several close-in-time reset signals are given to the MCU, again in a random way. The solution is the same as before.

Any idea on what might be the cause?

Thank you.

Regards,

Stefano

[Kenneth]

Even your "good" board doesn't seem to be working well, because the startup time should be in the few tens of ms according to:
https://infocenter.nordicsemi.com/topic/ps_nrf52840/power.html#unique_1752780615 

My suggestion is to take another look at the schematic and bill of materials for your board:
https://infocenter.nordicsemi.com/topic/ps_nrf52840/ref_circuitry.html 

Make sure the pin toggling is placed at the very beginning of main(), so you are sure that the problem you experience is not an infinite loop due to some init() not working as expected.

Best regards,
Kenneth