Framing error and noisy data when using UARTE at high baud rate

[moose]

Hello, in my zephyr application, I have uart1 configured as receive only using uarte at 921600 baud rate. Below is the description node:

&uart1 {
	compatible = "nordic,nrf-uarte";
	label = "data_uart";
	current-speed = <921600>;
	status = "okay";
	rx-pin = <30>;
};

 

I have a device sending a uart packet every 50ms and although I can see the right number of bytes being received, the data itself is corrupted (some bytes are correct but some are not) and I get a framing error thrown by the IRQ for every packet. I scoped the signal and it is very clean. I have also connected a terminal software (minicom) and I was able to receive the expected data while nrf52833 cannot. I do have another uart running on the same SoC (uart0) and that one is working fine but it is running at 115200 baud rate. I am not sure if I have to do anything special due to the high baud rate. I thought the DMA should help and I have allocated a timer for the byte processing. Here are my config statements:

#uart
CONFIG_SERIAL=y
CONFIG_UART_ASYNC_API=y
CONFIG_UART_0_ASYNC=y
CONFIG_UART_0_NRF_HW_ASYNC=y
CONFIG_UART_0_NRF_HW_ASYNC_TIMER=2
CONFIG_UART_0_NRF_ASYNC_LOW_POWER=y
CONFIG_UART_1_ASYNC=y
CONFIG_UART_1_NRF_HW_ASYNC=y
CONFIG_UART_1_NRF_HW_ASYNC_TIMER=4
CONFIG_CLOCK_CONTROL_NRF_K32SRC_XTAL=y 
CONFIG_CLOCK_CONTROL_NRF_K32SRC_20PPM=y

I thought maybe it is a clock issue but my understanding is the system automatically enables the HFCLK. The datasheet mentions a note about the pin drive strength for high baud rate but since I am not transmitting, I don't think this is related to my case. Please let me know if I am missing anything. Thank you.

[moose]

hmolesworth which file are you referring to? I found UARTE_BAUDRATE_BAUDRATE_Baud921600 in nrf_uarte.h located in modules/hal/nordic/nrfx/hal but I can't find the file that defines it. Also, how did you calculate the new value?

Finally, I was able to solve the problem by enabling HFXO without changing any register values. Do you still think it is related to this definition value?

[hmolesworth]

I wouldn't bother; the original post said the HFCLK was running but if HFCLK was not running that has a much greater effect.

I posted the link to my detailed code to calculate baud rate register values in my first answer. baud-rate-generator

[moose]

HFCLK is always running but the issue was solved by switching the source of HFCLK from RC oscillator to the external oscillator (HFXO).

[hmolesworth]

Quite, as we all kno'; not using the crystal causes most synch issues on the uart I misunderstood the original post assuming you meant the crystal was already being used as did kazi, oh well

[moose]

I actually didn't know it is needed but now I know. An issue has been created to update the zypher UART driver to enable HFXO automatically when needed. 

However, I am just wondering if I need to do any extra calculations and update the base register like you did. I will look into and see if that is needed. 

[hmolesworth]

Take care in turning off the HFCLK as if not correctly handled it can consume significant power when sleeping for long periods; not an issue if there is plenty of power available, but a coin cell killer. This is covered in multiple posts over the years.