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Allow any baud rate; not just a restricted set #41

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Allow any baud rate; not just a restricted set #41

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51 changes: 21 additions & 30 deletions source/serial_api.c
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Original file line number Diff line number Diff line change
Expand Up @@ -29,25 +29,6 @@

static uint32_t serial_irq_ids[UART_NUM] = {0};
static uart_irq_handler irq_handler;
static uint32_t 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}};



void serial_init(serial_t *obj, PinName tx, PinName rx) {
UARTName uart = UART_0;
Expand Down Expand Up @@ -98,18 +79,28 @@ void serial_free(serial_t *obj)
// 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;
}
// See https://devzone.nordicsemi.com/question/1181/uart-baudrate-register-values/#reply-1194

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;
// 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);

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I would really like to have test cases for this formula, verifying that the values match the ones in the Nordic ref manual. But if you have verified manually I would say that is good enough.

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

// Limit it to the valid range.
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Where is the valid range defined? From the nRF51 ref manual any 32-bit value seems to be accepted.

if (br < 0x1000)
br = 0x1000;
if (br > 0x10000000)
br = 0x10000000;

obj->uart->BAUDRATE = br;
}

void serial_format(serial_t *obj, int data_bits, SerialParity parity, int stop_bits)
Expand Down