nRF52832 Supply rise time

[a.wallwitz]

Dear Folks,

I have a question regarding the supply rise time of the nRF52-Series.

According to the Product Specification Chapter 6 Table 8 the Supply rise time (0 V to 1.7 V) has to be at max. 60ms.

I have a circuit that can be powered via battery and external power supply. (Both with different voltages for charging the battery etc.)

To prevent a Power-On-Reset I'm thinking about using a RC element to let the supply voltage rise smoothly over a determined period of time.

In fact this would also increase to initial supply rise time from 0V to 1.7V. (In fact my measured time till 1.7V is about 250ms, starting from instant 1.2V)

The nRF52 is starting and functioning as it should. My question is if there are any concerns I should have or are there any known problems be increasing the supply rise time? (Several brown-out resets till it's up and running?)

Thank you in advance for your help.

Best Regards, Andrej

[butch]

Why would a smoothed rise time "prevent POR?" I suppose you mean a filter that has the undesired side effect of slowing the rise time? Is your external power supply noisy? In my experience with slow rise times from a solar cell power supply, a slow rise time can definitely prevent an mcu from resetting properly.

[a.wallwitz]

18.6.1 of the product specification states: "A step increase in supply voltage of 300 mV or more, with rise time of 300 ms or less, within the valid supply range, may result in a system reset." Which I want to prevent while switching from battery (2.0V) to external power supply (3.5V).

So this is not a side effect ;)

Due to the fact that I'm running of battery until the external PS is being used, will limit the bottom limit of Vcc to 2.0V. So I will not rise from 0V when I'm already running. The only problem I see is the initial startup of the device.

[butch]

OK, now I understand. Just as a matter of interest, do you think this is a flaw in the product (in its voltage monitor circuitry), i.e. in your experience is it typical of other mcu's to reset on steps increases in the valid supply range? Also, would an external voltage monitor chip deal with step increases without resetting?