cpu/nrf5x: implemented UART TX only mode

cpu/nrf5x_common/periph/uart.c

@@ -1,5 +1,5 @@
 /*
- * Copyright (C) 2014-2016 Freie Universität Berlin
+ * Copyright (C) 2014-2017 Freie Universität Berlin
  *               2015 Jan Wagner <mail@jwagner.eu>
  *
  *
@@ -50,14 +50,20 @@ int uart_init(uart_t uart, uint32_t baudrate, uart_rx_cb_t rx_cb, void *arg)
    /* power on the UART device */
     NRF_UART0->POWER = 1;
 #endif
+
     /* reset configuration registers */
     NRF_UART0->CONFIG = 0;
-    /* configure RX/TX pin modes */
+
+    /* configure RX pin */
+    if (rx_cb) {
+        GPIO_BASE->DIRCLR = (1 << UART_PIN_RX);
+        NRF_UART0->PSELRXD = UART_PIN_RX;
+    }
+
+    /* configure TX pin */
     GPIO_BASE->DIRSET = (1 << UART_PIN_TX);
-    GPIO_BASE->DIRCLR = (1 << UART_PIN_RX);
-    /* configure UART pins to use */
     NRF_UART0->PSELTXD = UART_PIN_TX;
-    NRF_UART0->PSELRXD = UART_PIN_RX;
+
     /* enable HW-flow control if defined */
 #if UART_HWFLOWCTRL
     /* set pin mode for RTS and CTS pins */
@@ -127,10 +133,14 @@ int uart_init(uart_t uart, uint32_t baudrate, uart_rx_cb_t rx_cb, void *arg)
     NRF_UART0->ENABLE = UART_ENABLE_ENABLE_Enabled;
     /* enable TX and RX */
     NRF_UART0->TASKS_STARTTX = 1;
-    NRF_UART0->TASKS_STARTRX = 1;
-    /* enable global and receiving interrupt */
-    NVIC_EnableIRQ(UART_IRQN);
-    NRF_UART0->INTENSET = UART_INTENSET_RXDRDY_Msk;
+
+    if (rx_cb) {
+        NRF_UART0->TASKS_STARTRX = 1;
+        /* enable global and receiving interrupt */
+        NVIC_EnableIRQ(UART_IRQN);
+        NRF_UART0->INTENSET = UART_INTENSET_RXDRDY_Msk;
+    }
+
     return UART_OK;
 }