ble_app_uart can not start advertising

[rezaei]

Hi everyone,

I implement nrf5_SDK_17.0.2 ble_app_uart example, but when I connect both GND wire and TX wire of the external device to my nRF52840 DK first, my nRF52840 DK cannot start advertising even the external device is off. So I have to turn my nRF52840 DK on first, and then I connect the external device.
The most important problem is that when I press the reset button on my nRF52840 DK I need to reconnect the common GND or TX wire, otherwise my nRF52840 DK cannot start advertising.
Note: the external device's logic level is +5V so I have to use a lever shifter.

Thanks.

[hmolesworth]

Classic phantom power issue; the pull-up is powering the external device from nRF52 VCC via 2k pull-up into external device input pin via external device schottky diode protection to external device internal power Vcc.

We know this because the Rx pin is dragged down to 1.45 volts by the external device power rail loading.

When the external device is off RX Pin voltage is +0.72V. In this case the nRF DK cannot start advertising.
If you add a 2kOhm pull-up resister RX pin voltage is +1.45V

In general two devices with different power rails sharing a common ground must be correctly isolated if one is powered up at a different time to the other.

Test this theory by removing any connection between Rx and the external device, keep Rx pullup in place,  power up both devices, then connect the Rx line to the external device. It should work fine.