Power-on-reset Circuit for MBN52832 (nRF52832)

[HanCatNguyen]

I want to reset the MBN52832 right after power off then back on so that I will only need a power switch for my custom board (instead of also adding a reset button). However, when I tried to make a power-on-reset circuit with just a 1uF (or 10uF) capacitor and a 10KOhm resistor connecting like below, the chip doesn't reset when I switch the power back on. Is there something wrong with my circuit? Any suggestions on any other ways I can get the automatic reset upon powering off then back on?

[Karl Ylvisaker]

Hello,

Have you had a chance to use an oscilloscope to verify that the voltage at the pin follows the requirements in the Power On Reset documentation?

the chip doesn't reset when I switch the power back on.

Could you elaborate what you mean when you say that it does not reset? Is the device completely unresponsive after you power it up, or does it do anything else unexpected?

If you could share your schematics as a pdf directly here on DevZone through the Insert->Files options that would be very helpful as well.

Best regards,
Karl

[HanCatNguyen]

MBN52832_PowerOnReset.PDF

I connected the power-on-reset circuit (10uF Capacitor and 10K Ohm resistor) to the RESET pin - P0.21). DVDD is 3.3V. I have measured the voltage rise at P.021 when power is on, it took about 540 us for the voltage to rise from 0V to about 3V as below:

I can reset the MBN52832 if I press a push button connected to pin P0.21, but if i connected the capacitor and resistor as in the above schematic, MBN52832 doesn't reset upon powering on.

[Karl Ylvisaker]

Hello,

Thank you for the clarifications.
The nRESET pin has internal POR already, and so you do not need any external components there - could you try this again without the external circuitry and see if the device then resets as expected?
You could also take a look at how the reset button is implemented in the nRF52 DK hardware files.

Best regards,
Karl

[HanCatNguyen]

I've tried having the nRESET pin connected to nothing (float) and connecting to 3.3V, but none of them let the MCU reset upon powering on.

I'm using MCP1700T-3302E/TT for 3.3V regulator from ~ 3.7V, and it takes about 4 ms for DVDD to raise from 0V to 3.3V every time the board is powered on. Do you think that causes the reset problem? 

Is it possible that something in my code affects that? If so, how may I send you my code privately?

My reset button works, I just wonder if there's a way to reset the board every time powering on without that button so that I only need a power switch for my custom board. Thanks.

[Karl Ylvisaker]

Hello,

HanCatNguyen said:

My reset button works, I just wonder if there's a way to reset the board every time powering on without that button so that I only need a power switch for my custom board. Thanks.

Perhaps I am not understanding you correctly here - irregardless of the nReset pin discussion, is the issue here that you are not seeing your device boot up when you power it?
Have you measured the VDD to the chip when you power it on? Does this power curve also fulfill the Power-on Reset requirements as described earlier?

Best regards,
Karl

[HanCatNguyen]

Is the requirement you're referring to "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."? Then yes, my DVDD powering up satisfies that, but the chip doesn't reset. Is it supposed to automatically reset when powering on (without connecting nRESET pin to anything)?

The chip does boot up (but doesn't reset) when I turn the power back on. My code is based on the ble_app_uart example of the nRF5 SDK with added SPI communication. That example code had the same problem that the chip doesn't BLE re-advertise after powering off then back on (without resetting). My ultimate goal is to be able to BLE re-advertise after powering on without the need of pressing a RESET button. Any suggestions?

[ketiljo]

The POR will always hold the device in reset after power up. Why do think it's not reset? Do you have any pins that are high before the power is applied?

Adding a very large cap to the reset pin does not solve this, remove this.

[HanCatNguyen]

I think it doesn't reset because it didn't BLE re-advertise after powering off then back on. The LED pin for indicating BLE status (P0.17) went HIGH after powering up instead of blinking to indicate BLE advertising. I have tried without anything connected to the reset pin, and same problem. How should I fix this?

[ketiljo]

Since the LED is lit, it indicates that POR is working since the pin is set as an high output. Hva you tried to debug to see what's going on after reset? Doing another reset is not a good solution here, you need to find out why it fails after power on.