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

Question

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.

hmolesworth · Answer

The quoted values are not all correct so you will get much better performance if you bypass the value currently used; this table is a little dated now (some of the data sheets have a few corrections) but here are my detailed calculations: 
 typedef struct {
 uint32_t QuotedRegisterValue;
 uint32_t RequiredBaudRate;
 uint32_t QuotedActualBaudRate;
 uint32_t CalculatedRegisterValue;
 uint32_t CalculatedActualBaudRate;
 char DoesRegisterMatch[RESULT_STRING_LENGTH];
} BaudCheck_t;
// Quoted values from nRF52832 v1.4 datasheet
static BaudCheck_t BaudCheck[] = {
// -------Documentation--------- -------Calculated---------
// Register Required Actual Register Actual Ok? Index
// ========== ======== ====== ========== ====== ==== =====
 {0x0004F000, 1200, 1205, 0x0004F000, 1205, " Ok"}, // 0
 {0x0009D000, 2400, 2396, 0x0009D000, 2395, " Ok"}, // 1
 {0x0013B000, 4800, 4808, 0x0013B000, 4806, " Ok"}, // 2
 {0x00275000, 9600, 9598, 0x00275000, 9597, " Ok"}, // 3
 {0x003AF000, 14400, 14401, 0x003B0000, 14404, " -"}, // 4
 {0x004EA000, 19200, 19208, 0x004EA000, 19195, " Ok"}, // 5
 {0x0075C000, 28800, 28777, 0x0075F000, 28793, " -"}, // 6
 {0x009D0000, 38400, 38369, 0x009D5000, 38406, " -"}, // 7
 {0x00EB0000, 57600, 57554, 0x00EBF000, 57601, " -"}, // 8
 {0x013A9000, 76800, 76923, 0x013A9000, 76797, " Ok"}, // 9
 {0x01D60000, 115200, 115108, 0x01D7E000, 115203, " -"}, // 10
 {0x03B00000, 230400, 231884, 0x03AFB000, 230392, " -"}, // 11
 {0x04000000, 250000, 250000, 0x04000000, 250000, " Ok"}, // 12
 {0x07400000, 460800, 457143, 0x075F7000, 460800, " -"}, // 13
 {0x0F000000, 921600, 941176, 0x0EBEE000, 921600, " -"}, // 14
 {0x10000000, 1000000, 1000000, 0x10000000, 1000000, " Ok"}}; // 15
#define NUM_BAUD_TESTS (sizeof(BaudCheck)/sizeof(BaudCheck[0]))
 
 I posted the code I wrote required to generate this data here baudrate-register

hmolesworth · Answer

Zephyr is irrelevant; just change the Zephyr number in the table above to the value I gave you, how hard can it be? Change 0x0F000000 to 0x0EBEE000 and see if it works. The value in the Zephyr table is incorrect. 
 {0x0F000000, 921600, 941176, 0x0EBEE000, 921600, " -"}, // 14 
 #define UARTE_BAUDRATE_BAUDRATE_Baud921600 (0x0EBEE000UL) /*!< 921600 baud */