nRF51822 doesn't start if UART RX pin is connected

[sunil kumar]

Iam using softdivice 110 and sdk 10.0.0, when i connect UART RX pin to TX of STN1110 the processor doesn't start until I remove connection between RX of nRF51822 and TX of STN1110(TX of nRF51822 and RX of STN1110 are connected), once i remove the connection(RX of nRF51822 and TX of STN1110) the nRF51822 starts and it is not effected if i later connect them, I tried pin p0.0 and p0.8(as RX pins) same result in both case, and i cannot configure STN1110, kindly help me. It is getting stuck in SOFTDEVICE_HANDLER_INIT in ble_stack_init().

[sunil kumar]

It is working with no code change, but i added a 10K pullup resistor at RX of nRF51822, is it compulsory to connect a pullup resistor at RX of uart?

[Ole Bauck]

It is normally not compulsory with a pull-up on the UART RX pin, but looking at the datasheet of the STN1110 it is in this case:

UART_TX
UART transmit output. Open drain – requires a pull-up
to VDD or +5V (4 mA max). Pull-up value depends on
UART baud rate and the trace length (higher resistor
values can be used with lower baud rates and shorter
traces); typical value is 4.7 kΩ.

This is probably to support multiple voltage levels.