Problem with UART TX or RX lines

RE: Problem with UART TX or RX lines

hmolesworth — Thu, 01 Aug 2024 17:05:02 GMT

Have a look at my response to an old post; the VCC of the buffer will connect to nRF52 VDD and the buffer will isolate the FTDI Tx line output from the nRF52 input pin schottky diode, you can probably ignore the Rx line, but if using the direction would be reversed. If no pull-up on the FTDI Rx line the FTDI will spew nulls to the laptop; if using an FTDI Rx pull-up to stop that then a buffer on Rx will be required. There are also twin-rail level-shifters which have two supplies, but they are bigger.

potential-risks-if-voltage-is-applied-to-a-gpio-pin-when-the-chip-is-not-powered-vdd-0-0v

RE: Problem with UART TX or RX lines

ketiljo — Thu, 01 Aug 2024 14:10:24 GMT

Sounds like the pins are damaged, either by over-voltage or ESD. This can damage a single pin only.

RE: Problem with UART TX or RX lines

Bartosz22 — Thu, 01 Aug 2024 07:59:32 GMT

So is there any good practice to design PCB with NRF52833 to avoid this problems or is there some sample schematics to do it in right way ?

RE: Problem with UART TX or RX lines

Bartosz22 — Thu, 01 Aug 2024 05:51:01 GMT

We use FT232.

RE: Problem with UART TX or RX lines

hmolesworth — Wed, 31 Jul 2024 19:21:26 GMT

Which FTDI model is being used? Should it happen to be a USB serial cable it is possible that it is internally wired with 5V Rx and Tx instead of 3V. I mention this because of a similar problem I saw caused by mixing up several FTDI interfaces (they are not always marked as 5V or 3V).

Quite apart from that consideration note that the FTDI interface must not remain connected to the nRF52833 when power is removed from or not be present on the nRF52833 as (unless there is an isolating circuit in place) the FTDI Tx output will power nRF52833 VDD via the schottky clamp diode and any pull-up on Rx may do the same.

All these scenarios will eventually damage the Rx or Tx pin with no way of predicting when. The only safe way to avoid this automatically is to ensure level-shifters are in place, ie more hardware on the product or a properly designed FTDI interface which monitors nRF52833 VDD.