Merge pull request #1474 from c1728p9/uart_null_byte

Remove byte sent at 9600 baud on init on NRF51-DK

Author: sg-

libraries/mbed/targets/hal/TARGET_NORDIC/TARGET_MCU_NRF51822/serial_api.c

@@ -39,6 +39,7 @@ static const int acceptedSpeeds[17][2] = {
     {28800, UART_BAUDRATE_BAUDRATE_Baud28800},
     {31250, (0x00800000UL) /* 31250 baud */},
     {38400, UART_BAUDRATE_BAUDRATE_Baud38400},
+    {56000, (0x00E51000UL) /* 56000 baud */},
     {57600, UART_BAUDRATE_BAUDRATE_Baud57600},
     {76800, UART_BAUDRATE_BAUDRATE_Baud76800},
     {115200, UART_BAUDRATE_BAUDRATE_Baud115200},
@@ -77,7 +78,10 @@ void serial_init(serial_t *obj, PinName tx, PinName rx) {
     obj->uart->EVENTS_RXDRDY = 0;
     // dummy write needed or TXDRDY trails write rather than leads write.
     //  pins are disconnected so nothing is physically transmitted on the wire
+    obj->uart->PSELTXD = 0xFFFFFFFF;
+    obj->uart->EVENTS_TXDRDY = 0;
     obj->uart->TXD = 0;
+    while (obj->uart->EVENTS_TXDRDY != 1);
 
     obj->index = 0;