Merge pull request #1047 from marcuschangarm/master

Fixed interrupt handler in serial_api.c for the Nordic NRF51

Author: 0xc0170

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

@@ -76,7 +76,7 @@ void serial_init(serial_t *obj, PinName tx, PinName rx) {
     obj->uart->TXD = 0;
 
     obj->index = 0;
-    
+
     obj->uart->PSELRTS = RTS_PIN_NUMBER;
     obj->uart->PSELTXD = tx; //TX_PIN_NUMBER;
     obj->uart->PSELCTS = CTS_PIN_NUMBER;
@@ -162,14 +162,20 @@ extern "C" {
 #endif
 void UART0_IRQHandler()
 {
-    uint32_t irtype = 0;
-
-    if((NRF_UART0->INTENSET & 0x80) && NRF_UART0->EVENTS_TXDRDY) {
-        irtype = 1;
-    } else if((NRF_UART0->INTENSET & 0x04) && NRF_UART0->EVENTS_RXDRDY) {
-        irtype = 2;
+    if((NRF_UART0->INTENSET & UART_INTENSET_TXDRDY_Msk) && NRF_UART0->EVENTS_TXDRDY)
+    {
+        uart_irq(1, 0);
+
+        /*  Explicitly clear TX flag to prevent interrupt from firing
+            immediately after returning from ISR. This ensures that the
+            last interrupt in a transmission sequence is correcly handled.
+        */
+        NRF_UART0->EVENTS_TXDRDY = 0;
+    }
+    else if((NRF_UART0->INTENSET & UART_INTENSET_RXDRDY_Msk) && NRF_UART0->EVENTS_RXDRDY)
+    {
+        uart_irq(2, 0);
     }
-    uart_irq(irtype, 0);
 }
 
 #ifdef __cplusplus
@@ -239,11 +245,24 @@ int serial_getc(serial_t *obj)
 
 void serial_putc(serial_t *obj, int c)
 {
-    while (!serial_writable(obj)) {
+    /*  In interrupt mode, send character immediately. Otherwise, block until
+        UART is ready to receive next character before sending.
+
+        The TXDRDY flag is cleared in interrupt handler to ensure that it is
+        cleared even if there are no more characters to send.
+    */
+    if (NRF_UART0->INTENSET & UART_INTENSET_TXDRDY_Msk)
+    {
+        obj->uart->TXD = (uint8_t)c;
     }
+    else
+    {
+        while (!serial_writable(obj)) {
+        }
 
-    obj->uart->EVENTS_TXDRDY = 0;
-    obj->uart->TXD = (uint8_t)c;
+        obj->uart->EVENTS_TXDRDY = 0;
+        obj->uart->TXD = (uint8_t)c;
+    }
 }
 
 int serial_readable(serial_t *obj)