UART TX pin pulls down during reset

RE: UART TX pin pulls down during reset

Kenneth — Fri, 30 Aug 2019 17:37:43 GMT

When reset pin is pushed the pin behavior is undefined, it may be output or floating. It is not much that can be done about this other than ensure that the pin reset is pushed for a short period of time as possible, for instance >0.2us is sufficient to ensure a hardware reset.

RE: UART TX pin pulls down during reset

Hauser — Fri, 30 Aug 2019 07:20:14 GMT

Same behaviour here on nRF52840 DK and our own hardware using nRF52840-CKAA-D0.

If RESET is LOW, the UART TX line is also LOW, independent of if an external pull-up is attached or not. This leads to UART framing errors on the attached host.

Any suggestions?

RE: UART TX pin pulls down during reset

Kenneth — Thu, 25 Apr 2019 18:08:15 GMT

Hello,

The nRF52840 Preview-DK is no longer supported or recommended to develop on, it contains early engineering samples with missing features, issues and not production tested. You should get a nRF52840 DK with the production grade samples on as soon as possible (the PDK have not been available for quite some time, so just order an nRF52840 DK from a Nordic distributor and you should get a DK).

In general I recommend pulling the nRESET pin low directly if you want to measure timing, in this case pull P0.18 directly to ground:
https://infocenter.nordicsemi.com/topic/ps_nrf52840/pin.html?cp=3_0_0_6_0

All that said, during (pin) reset there is no guaranteed operation/behaviour. If you want to do a hardware pin reset you would only need to pull it low for >0.2us to ensure a proper reset.

Best regards,
Kenneth

RE: UART TX pin pulls down during reset

ivank4 — Wed, 24 Apr 2019 10:07:49 GMT

Forgot to mention I used a 10k resistor to pull up TX line.

RE: UART TX pin pulls down during reset

Jeff4BLE — Wed, 24 Apr 2019 04:36:03 GMT

The nRF5x GPIO lines are undefined during reset. The solution is to use an external pull-up or pull-down resister.