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]

Likely the Rx pin doesn't have PULLUP enabled and so floats about as it picks up random electrical noise, generates a framing error which maybe stops BLE working. Test by enabling PULLUP on the Rx pin after doing the init() of the serial port. Another fix is to add an external pull-up resistor between Rx pin and VCC (say100k but any value will probably work). The internal PULLUP resistor is about 14k, which won't affect the FTDI when you do connect the FTDI

[rezaei]

Hi hmolesworth

Thank you from your reply.

I add an external pull-up resistor, which is 2K Ohm between Rx pin and +3V. So when the external device is off my nRF52840 DK can start advertising.

But, after turning the external device on if I press the reset button on my nRF52840 DK advertising cannot be started.

What do you suggest?