Only specific baud rates allowed, but chip supports any.

If you look in serial_api.c you can see that the nRF51822 code only allows a finite set of baud rates:

static const int acceptedSpeeds[17][2] = {
    {1200, UART_BAUDRATE_BAUDRATE_Baud1200},
    {2400, UART_BAUDRATE_BAUDRATE_Baud2400},
    {4800, UART_BAUDRATE_BAUDRATE_Baud4800},
    {9600, UART_BAUDRATE_BAUDRATE_Baud9600},
    {14400, UART_BAUDRATE_BAUDRATE_Baud14400},
    {19200, UART_BAUDRATE_BAUDRATE_Baud19200},
    {28800, UART_BAUDRATE_BAUDRATE_Baud28800},
    {31250, (0x00800000UL) /* 31250 baud */},
    {38400, UART_BAUDRATE_BAUDRATE_Baud38400},
    {57600, UART_BAUDRATE_BAUDRATE_Baud57600},
    {76800, UART_BAUDRATE_BAUDRATE_Baud76800},
    {115200, UART_BAUDRATE_BAUDRATE_Baud115200},
    {230400, UART_BAUDRATE_BAUDRATE_Baud230400},
    {250000, UART_BAUDRATE_BAUDRATE_Baud250000},
    {460800, UART_BAUDRATE_BAUDRATE_Baud460800},
    {921600, UART_BAUDRATE_BAUDRATE_Baud921600},
    {1000000, UART_BAUDRATE_BAUDRATE_Baud1M}
};

In fact the baud rate can be set to anything. Instead of using fixed constants, a formula should be used instead.

Old code:

// serial_baud
// set the baud rate, taking in to account the current SystemFrequency
void serial_baud(serial_t *obj, int baudrate)
{
    if (baudrate<=1200) {
        obj->uart->BAUDRATE = UART_BAUDRATE_BAUDRATE_Baud1200;
        return;
    }

    for (int i = 1; i<17; i++) {
        if (baudrate<acceptedSpeeds[i][0]) {
            obj->uart->BAUDRATE = acceptedSpeeds[i - 1][1];
            return;
        }
    }
    obj->uart->BAUDRATE = UART_BAUDRATE_BAUDRATE_Baud1M;
}

Better code:

// serial_baud
// set the baud rate, taking in to account the current SystemFrequency
void serial_baud(serial_t *obj, int baudrate)
{
    // See https://devzone.nordicsemi.com/question/1181/uart-baudrate-register-values/#reply-1194

    // The correct value is baudrate * 2^32 / 16e6 = baudrate * 2^22 / 15625,
    // but that doesn't fit into 32 bits, and Cortex M0 has no hardware
    // divide, so instead I will do this possibly-over-optimised magic instead!
    // I have verified that this produces exactly the same values as the header,
    // except for 921600 (which is wrong in the header).

    uint32_t br = (baudrate << 8) + (baudrate << 4) - (baudrate << 1) - baudrate
            - (baudrate >> 1) - (baudrate >> 4) - (baudrate >> 9) - (baudrate >> 14)
            - (baudrate >> 16) - (baudrate >> 17) - (baudrate >> 18) - (baudrate >> 19);

    // Round it to 20 bits (see link above).
    br = (br + 0x800) & 0xFFFFF000;

    // Limit it to the valid range.
    if (br < 0x1000)
        br = 0x1000;
    if (br > 0x10000000)
        br = 0x10000000;

    obj->uart->BAUDRATE = br;
}

I'll do a pull request for this.

ARMmbed / mbed-hal-nrf51822-mcu

Only specific baud rates allowed, but chip supports any. #40