nrf52840_pca10056: Spurious RTS pulse and incorrect line level with hardware flow control disabled

[carlescufi]

When running samples/hello_world with the default configuration on nrf52840_pca10056, CONFIG_UART_0_NRF_FLOW_CONTROL is disabled.

This means that the RTS line, which is an output from the nRF52840 perspective, should stay idle continuously. Instead, we see a pulse, which should not be present.
Furthermore, before and after the pulse the line is set to the low level (active) which makes the J-Link OB firmware believe that hardware flow control is enabled, as described here.

Steps to reproduce:

• Build samples/hello_world for nrf52840_pca10056
• Flash the board
• Run nrfjprog -r
• Start the protocol analyzer

Additional info:

• Segger J-Link wiki documentation
• Nordic UART specification
• Logic analyzer capture:
  

Note: this is not related to #14753.

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[carlescufi]

@anangl did we fix this at all?

[github-actions]

This issue has been marked as stale because it has been open (more than) 60 days with no activity. Remove the stale label or add a comment saying that you would like to have the label removed otherwise this issue will automatically be closed in 14 days. Note, that you can always re-open a closed issue at any time.

[anangl]

There is no bug here, actually. This is how the UART and UARTE peripherals work. As the specification says:

If flow control is not enabled, the interface will behave as if the CTS and RTS lines are kept active all the time.

The driver initially configures the GPIO pin that is to serve as the RTS line to be an output with the high (inactive) level. Shortly after that, this pin number is written into the PSEL.RTS register of the peripheral and the peripheral is enabled. From this moment the peripheral starts driving the pin, and if the hardware flow control is disabled, the pin immediately becomes active. That is why the RTS pulse appears.
I don't think it is worth trying to align with JLink OB firmware expectations regarding the RTS line, so that it could perform its auto-detection of the hardware flow control, as this would mean quite a complication of the driver code (the RTS pin would need to be disconnected from the peripheral when the flow control is not enabled). Hence, I'm closing this issue.