Uart baud rate 512

[Fritz Kendeffy-Hermann]

Is it possibe to configure uart with baud rate of 512 on nrf52832?

I tried to configure the uart with this baud rate, but I am not able to get the uart communication working

[MagnusPersson]

Sorry about digging up an old post here.

With regards to the values in the UART_BAUDRATE register I have a hard time match the "actual rate" in the nrf52_bitfields.h with the formula given.

#define UART_BAUDRATE_BAUDRATE_Baud921600 (0x0EBED000UL) /*!< 921600 baud (actual rate: 941176) */

0x0EBED000 * 16000000 / 2^32 = 921585

Did I miscalculate or misunderstand anything?

Thx,

-Magnus

[hmolesworth]

There is a 2% error in the documented baud rate for 921600 baud:

// Quoted values from nRF52832 v1.4 datasheet - 16 correct, 10 incorrect
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
// Extra and changed values from nRF52840 Product Specification
   {0x003B0000,    14400,   14414,  0x003B0000,   14404, " Ok"},  // 16
   {0x0075F000,    28800,   28829,  0x0075F000,   28793, " Ok"},  // 17
   {0x00800000,    31250,   31250,  0x00800000,   31250, " Ok"},  // 18
   {0x009D5000,    38400,   38462,  0x003B0000,   38406, " Ok"},  // 19
   {0x00E50000,    56000,   55944,  0x00E56000,   55999, "  -"},  // 20
   {0x00EBF000,    57600,   57762,  0x00EBF000,   57601, " Ok"},  // 21
   {0x01D7E000,   115200,  115942,  0x01D7E000,  115203, " Ok"},  // 22
   {0x03AFB000,   230400,  231884,  0x03AFB000,  230392, " Ok"},  // 23
   {0x075F7000,   460800,  470588,  0x075F7000,  460800, " Ok"},  // 24
   {0x0EBED000,   921600,  941176,  0x0EBEE000,  921600, "  -"}}; // 25
#define NUM_BAUD_TESTS (sizeof(BaudCheck)/sizeof(BaudCheck[0]))

This link shows my calculations: uart-baud-rate-generator which can be used to get 512 baud

Updated for v1.7 datasheet framing-error-and-noisy-data-when-using-uarte-at-high-baud-rate

[MagnusPersson]

Thanks a lot. so if I understand your tables I have calculated correctly and the Nordic quoted numbers in the code and Product Specification differ from our understanding.

It would be interesting to get a background to the numbers in the Product Specification.

Would also be interesting to know why the register numbers for UART and UARTE differs in the Product Specification.

Thanks,

-Magnus

[Einar Thorsrud]

Hi Magnus,

MagnusPersson said:

It would be interesting to get a background to the numbers in the Product Specification.

This has to do with the way the different frequencies are generated within the nRF (derived from the HF clock), and the provided values are the closest you can get. I cannot comment on the implementation details, unfortunately.

MagnusPersson said:

Would also be interesting to know why the register numbers for UART and UARTE differs in the Product Specification.

There are some implementation differences between those peripherals (though they share a lot and in fact overlap physically), but this is not publicly documented and I cannot comment on the details.

[MagnusPersson]

OK, thanks. One more question: Does this mean that the formula suggested in this ancient post linked above to calculate the values for the BAUDRATE register and the actual baud rate is actually not valid?

Anyway, I guess the difference should be within the fault tolerance of any system using start and stop bits properly.

Thanks,

-Magnus

[Einar Thorsrud]

Hi Magnus,

That is right. The formula from the old post (describing the UART peripheral in the nRF51 series) is not entirely valid, as is also stated in the post - "this formula is simplified".