UART BAUDRATE register values

[Bengt Florin]

What is the formula for calculation of values for the UART BAUDRATE register from bit rates?

[Håkon Alseth]

Hi Bengt,

note: this formula is simplified and may differ from the actual baudrate measurement

The formula is: Baudrate = desired baudrate * 2^32 / 16000000

Example: Baudrate of 31250 should then be 8388608 decimal = 0x800000.

Note that you will have to round the number afterwards: rounded_value = (value + 0x800) & 0xFFFFF000

Since the baudrate generator will be sourced by the 16 M (Either RC or XOSC, depending on your configuration), then your error rate will be ~equal to the overall drift of the system clock and the accuracy of the baudrate generator towards your target baudrate.

Other way around: Check out this define in nrf51_bitfields.h line 5702:

UART_BAUDRATE_BAUDRATE_Baud115200 (0x01D7E000UL) /*!< 115200 baud. */

The actual baudrate set with the above define is:

baudrate_reg_val * 16M / 2^32 = 115203.86 baud.

Best regards Håkon

[Bengt Florin]

BTW, why the heavy rounding? 'nrf51_bitfields.h' have a value for 921600bps that is 0x0EBEDFA4UL. By rounding this gets 0x0EBEE000. Does this imply the the baud rate counter is not 32-bit but mere 20-bit?