Baudrate on VCOM on nRF52840DK

[mmajchrzycki]

Hi,

I wanted to use the console on nRF52840DK in high-speed UART mode (921600bps) but it fails and I just saw the DK User Manual says:

Baud rate 921 600 is not supported through the virtual COM port.

Where is this constraint come from? What about 1M baudrate? I see it is working, but is it guaranteed?

[Turbo J]

Please clarifiy: Do you mean the Jlink COM port (USB->Jlink->NRF) or do you mean the CDC on the Nordic chip (USB->NRF chip)?

The UART on the Jlink chip has its own restrictions, AFAIK it can only do 115200 baud.

[mmajchrzycki]

Yes, I mean the VCOM over the JLink. I'm curious, because the DKs manuals only mention that 921600 bps is not supported. There is nothing like "higher than". So, what about 1M which seems to be working? Where is that limitation come from? 

[Kenneth]

The nRF52840 UART hardware supports the baudrate listed here:
https://infocenter.nordicsemi.com/topic/ps_nrf52840/uarte.html#register.BAUDRATE

As you can see, the actual baudrate when configured as 921600 differ some from the ideal, the actual is 941176 due to internal clock generation. Likely this prevent the SEGGER chip to not receive the data reliably on the nRF52840-DK.

Kenneth