nRF52840 UART TX pin pulled high (and kept high) by receiving device

RE: nRF52840 UART TX pin pulled high (and kept high) by receiving device

awneil — Fri, 15 Feb 2019 17:34:38 GMT

[quote userid="77069" url="~/f/nordic-q-a/43729/nrf52840-uart-tx-pin-pulled-high-and-kept-high-by-receiving-device/171463"]we can see the data on the scope before the ST boots, then stepping through the ST code line by line will eventually pull the line high when the ST UART is enabled[/quote]

That would still be consistent with you having TX & Rx transposed at the STM32:

Before the UART is configured, the pin will be an input (Hi-Z);
After the UART is configured, its Tx pin will become an output - which would force the line state.

RE: nRF52840 UART TX pin pulled high (and kept high) by receiving device

Trent Palmer — Fri, 15 Feb 2019 16:45:46 GMT

I'm very confident in the connections being correct; we can see the data on the scope before the ST boots, then stepping through the ST code line by line will eventually pull the line high when the ST UART is enabled. The obvious solution to me is to modify the ST UART but there appears to be little to no configuration available on the ST UART RX side of things.

Although we are now thinking the dev kit may be part or all of the problem as the ST engineer has reported success when removing it from the equation. I will update when I know more.

RE: nRF52840 UART TX pin pulled high (and kept high) by receiving device

awneil — Thu, 14 Feb 2019 23:26:40 GMT

Are you sure you have Tx & Rx the correct way around?

The output from the nRF must go to the input of the STM, and vice versa.

Concentrate on the the input and output functions - not the "Tx" and "Rx" names ...

Is the STM on a Dev Kit? If so, check that there is nothing else on the Dev Kit using those pins ...