RE: CPU nRF52840 not responsive after Brownout

lanwer — Thu, 25 Jul 2024 09:13:42 GMT

Hi there, thanks for your feedback,

and thanks for the tip concerning the capacitive coupling power-on. I did not notice it myself.

I am wondering, where is the 10mSec is mentioned?

[quote userid="65515" url="~/f/nordic-q-a/113411/cpu-nrf52840-not-responsive-after-brownout/495519"] The key is the specification of the required maximum 10mSec rise-time, above which the power-on reset is unreliable[/quote]

The solution from our side will be similar to crowbar: probably using an external voltage detector to disconnect the supply voltage when it gets around 1,8V or 1,9V. However, I am still interested in an official response/confirmation to understand what is going on.

RE: CPU nRF52840 not responsive after Brownout

hmolesworth — Wed, 24 Jul 2024 23:08:40 GMT

Not an official answer, but related to issues with power-on reset I have seen. The key is the specification of the required maximum 10mSec rise-time, above which the power-on reset is unreliable. Slow VDD rise-times cause many tickets here on failing resets.

"Note: The on-chip power-on reset circuitry may not function properly for rise times longer than the specified maximum."

Here is the (potential) issue, the power-on reset is capacitively coupled to VDD so is not a simple voltage threshold:

The problem specification: "tR_VDD Supply rise time (0 V to 1.7 V) 60 ms" 0V to 1.7V is the key; 1.62V to 2V rise-time requires much less than 10mSec and indeed may never work reliably as the input gate only "jumps" by what could be less than the power-on gate input threshold when using 1.62 to 2V.

Solution: Old-fashioned crowbar. A falling VDD below the brown-out reset - ie chip held in reset - drops the VDD current to close to nothing and so the VDD can stay high for a very very long time, especially with a Supercap, and so restoration of battery power will now not bring the chip out of reset. The crowbar works by clamping VDD to GND until VDD discharges; some regulators have this built-in to discharge output capacitance when they shut down. If not using such a regulator the nRF52840 can supply a crowbar by using the Brownout interrupt to turn on a latching discharge clamp which will turn off when VDD drops to something more friendly, maybe 300mV or less, allowing a power-on reset when the battery is connected. Clearly the crowbar is only used when VDD is so low than it is known a reset will be required - perhaps set POFCON.THRESHOLD to 1.8V to generate a POFWARN and generate a crowbar via PPI and EVENTS_POFWARN

weird-issue-observed-with-nrf52832

input-protection

if-vdd-does-not-go-to-0v-when-powered-down