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nRF52832: Power OFF/Reset with UART connected


I wanted to know if the UART pins can cause any issues if kept connected to usb (Rx/Tx pins stable to 3.3V) and directly to the GPIOs of the nRF52, while performing a soft-reset or switching OFF and then ON the device.

I have a sensor (ADXL362) connected to nRF52 via SPI and when I restart the device, all internal registers of this sensor appear to be reset to 0x00 (which should not be possible, they should have some defaults values). Thus this makes me suspect that SPI is not actually succeeding in reading those registers, but no error is shown. When, in addition to switching off the device, I also disconnect the UART connector, then the system gets reset properly and registers of the sensor are read correctly.

Can anyone help me understand what is happening?

Thank you in advance. Marco

Top Replies

  • If the RX/TX pins are 3.3V while powering off the nRF, the nRF will actually not be powered off because of ESD protection diodes on the pins, see for example this picture. When the voltage on the pin is higher than the Vdd plus the diode forward voltage drop, the diode will start to conduct current and the new Vdd will be the voltage on the input pin minus the forward voltage drop over the diode.

    I don't know why this is messing up your sensor SPI readings. You should connect a logic analyzer to see if the data transferred is correct.

  • Do you mean that some power-blocks of the nRF stay powered up, while others switch off? Because for example my firmware stops running (even if UART is still connected) and starts again only when I power the nRF up again.

  • The whole chip will stay powered if the voltage on a pin is supplying the chip. When you say that your firmware is not running, may it be because the code has entered an error condition and is stuck in the error handler?

    You can of course measure Vdd when the chip is powered off and the UART connector is still connected, to see if what I described in my last comment is the case.

  • As a test I'm just running an UART printing loop. When I power off the chip, messages stop been printed out via UART. When I power up nRF again, it starts again from the bootloader. UART Tx and Rx are connected to pins P0.30 and P0.31, and they feed the nRF with 3.3V. The nRF itself then keeps Vdd to 1.57V. 1.57V is within the absolute maximum voltage supply rating for nRF52 (-0.3V to 3.3V), however it is smaller than the minimum recommended supply voltage (1.7V). This might explain why the firmware stops running. In addition the nRF specification states as well that Pin in/out voltage should fall in the range -0.3V to Vdd+0.3V=1.87V, and this is not the case since nRF gets 3.3V in. I know I am looking at it in the opposite way, since power supply is not coming from Vdd directly, so I am not sure how such specification applies in this case.

  • Thank you Ole for your support. Your help together with this link are the answer: It turned out that my sensor was not resetting according to its specifications. This was happening because the sensor did not get powered off properly (1.57V in due to UART pins). Is there any way I could force the UART pins to "disconnect" so that any voltage on UART won't affect the system?