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.

[hmolesworth]

Just for completeness, I check datasheet v1.7 for nRF52832 and get the following (Edit - updated % errors on both quoted and calculated baud rates, needs review):

//  ---------------Documentation---------------  --------Calculated----------
//  Register    Required   Actual  Error   Ok?   Register      Actual Error
//  =========== ======== ======== =======  ====  =========== ======== =======
//  0x0004F000,    1200,    1205, +0.416%,   -,  0x0004F000,    1205, +0.416%
//  0x0009D000,    2400,    2396, -0.166%,   -,  0x0009D000,    2395, -0.208%
//  0x0013B000,    4800,    4808, +0.166%,   -,  0x0013B000,    4806, +0.125%
//  0x00275000,    9600,    9598, -0.020%,   -,  0x00275000,    9597, -0.031%
//  0x003AF000,   14400,   14401, +0.006%,  No,  0x003B0000,   14404, +0.027%
//  0x004EA000,   19200,   19208, +0.041%,   -,  0x004EA000,   19195, -0.026%
//  0x0075C000,   28800,   28777, -0.079%,  No,  0x0075F000,   28793, -0.024%
//  0x009D0000,   38400,   38369, -0.080%,  No,  0x009D5000,   38406, +0.015%
//  0x00EB0000,   57600,   57554, -0.079%,  No,  0x00EBF000,   57601, +0.001%
//  0x013A9000,   76800,   76923, +0.160%,   -,  0x013A9000,   76797, -0.003%
//  0x01D60000,  115200,  115108, -0.079%,  No,  0x01D7E000,  115203, +0.002%
//  0x03B00000,  230400,  231884, +0.644%,  No,  0x03AFC000,  230407, +0.003%
//  0x04000000,  250000,  250000, +0.000%,   -,  0x04000000,  250000,       -
//  0x07400000,  460800,  457143, -0.793%,  No,  0x075F7000,  460800, +0.000%
//  0x0F000000,  921600,  941176, +2.124%,  No,  0x0EBEE000,  921600, +0.000%
//  0x10000000, 1000000, 1000000, +0.000%,   -,  0x10000000, 1000000,       -