Functionality if VDD does not go to 0V when powered down?

Question

using the NRF52832 processor. 
 The issue we’re having is sometimes the processor won’t start up properly. 
 We think that it’s because the VDD voltage does not going all the way to 0V when power cycling, it’s resting at about 300 to 600mV. 
 Is there a minimum voltage that the regulator should be held at to power up correctly? 
 Does the 52832 need to cycle to 0V in a power down? 
 shouldn't the POR/BOR cause a system reset below 1.6v?

hmolesworth · Accepted Answer

There is indeed a problem with the VDD not going (nearly) all the way to 0V; most circuits simply do not work as designed when the voltage is very low but not quite 0v, Better to just fix the cause rather than arguing about what should and shouldn't work at specific low voltages; by this I mean often the reason VDD does not fall is due to external power leaking into VDD by way on the internal schottky diode protection devices, aka phantom power or back-drive. A classic cause is using something like an FTTDI Uart interface connected to the nRF52832 Rx input, where the Tx output on the FTTDI interface drives power through the nRF52832 pin onto VDD via the internal schottky; the current is small but the VDD consumption as VDD falls is mere 10s of nanoAmps, ie tiny so very low leakage is sufficient to prevent correct nRF52832 reset operation. How to fix this? Remove the external source leaking into the io pins, ie in the case of FTTDI unplug it. If there is something leaking power which cannot be removed, maybe GPS or some other device, then if that external drive can't be turned off a crowbar is an option. The crowbar shorts VDD when VDD falls below (say) 800mV. Some regulators have a built-in crowbar action, ie when disabled they will drop the power to 0v instead of just letting VDD decay. 
 To summarise the source of current holding up VDD needs to be identified. Maybe share the schematic if there is any doubt. There are a few posts on this kind of issue, not directly the same, eg nrf52832-io-problem

Functionality if VDD does not go to 0V when powered down?

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