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nRF52832: Power OFF/Reset with UART connected

Hi,

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

  • The problem is not with the UART, the problem is that there is voltage on one of the pins of the nRF when it is powered off. You will have to remove the voltage source when the nRF is powered off to remove the problem.

  • You can add a series resistance of about 10K to limit the current.

  • Yes, in my previous message I was stressing on the pins (rather than on the UART). As you said the problem is not the UART itself, but the rx/tx pins connected to the nRF and providing 3.3V even when the nRF is unpowered. I was wondering if there is any way (e.g. a nRF internal switch) that would allow me to force the nRF to keep the 2 GPIO pins (connected to the external UART RX and TX lines) "disconnected"/floating from the nRF internal circuitry. Thank you.

  • There is no way to completely disconnect the pins. The only solution is to connect a series resistance. This will limit the amount of current that can flow (the voltage drop will be proportional to the current).

  • Thank you Ole for the suggestion, however I cannot modify my actual board (just used for prototyping). Could my actual setting (UART Rx/Tx pins held at 3.3V leaking current throughout the nRF chip) cause any HW damages to the nRF itself? For instance, the nRF specification states that Pin in/out voltage should fall in the range -0.3V to Vdd+0.3V=1.87V. But this specification is not met when the main power source is removed. In such case pin connected to UART lines get 3.3V and Vdd happens to be 1.57V, thus (1.57+0.3)V=1.87V < 3.3V! Isn't it causing a dangerous latch-up condition?

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