UART TX pin not pulling

[tomkawal]

I was using for some time my code with FTDI USB converter to UART.

Recently I got built new boards, all with IC top marking N52832 QFABB0 1717AQ

 and whilst any other board with earlier versions of nRF52832 does work with UART,

these new boards cannot pull 25uA of FTDI converter to GND level

I tried the code changes with testing some GPIO settings for the TX pin and also changed my TX pin from  7 to 6 and 9, with the same result, the line is pulled only less than 1V. I tried with Vdd 3.1V and 3.6V. No difference.

With other application, like sampling ADC etc, these boards work OK, like the previous builds.

[hmolesworth]

That looks suspiciously like a missing Gnd connection between FTDI and nRF52832, maybe worth double-checking that. The supply voltage indicated - close to 4.0 volts - is too high for the nRF52832 Vdd as the io pin will back-drive the entire nRF52832 if the latter's Vdd is lower (say 3,3 volts). Absolute max on any pin is 3.9 volts, recommend max 3.6 volts. However if Gnd and Vdd are indeed correct then try increasing the low drive:

NRF_GPIO_PIN_H0D1,
ie:
GPIO_PIN_CNF_DRIVE_H0D1, ///< !< High-drive '0', disconnect '1'

[tomkawal]

Thanks for your prompt reply! I will work on it in 2 weeks time, after my summer break!

[tomkawal]

Solved. Thanks for suggestions.  Some of the signals I measured before were crosstalk from Rx line. 

Somehow 1Meg external pullup did the job.
Regarding back-driving: I was thinking that with IC current consumption of several mA in my case,
the 25uA of FTDI pullup to 5V0 should be negligible.
Anyways, I connected 1Meg resistor to 3.1Vdd and with GPIO_PIN_CNF_DRIVE_H0D1
and now the UART Tx works with full swing from 0V to Vdd. 
I will update the post if there's anything specific with the pin choice (tested with port pin 9)