NRF52 serial: Trigger TxIrq interrupt manually on enabling

It is required by Mbed HAL API to generate TxIrq interrupt when TXD register is empty (also after enabling TxIrq interrupt):
https://github.com/ARMmbed/mbed-os/blob/f73a62afbf4052b4da8c5b862ffb4708a80c1b6e/hal/serial_api.h#L144-L147

The driver uses DMA to perform uart transfer and TxIrq is generated after the transfer is finished.
While enabling TxIrq we will check if TXD reg is empty and manually trigger the interrupt.

Author: mprse

targets/TARGET_NORDIC/TARGET_NRF5x/TARGET_NRF52/serial_api.c

@@ -1008,7 +1008,7 @@ void serial_free(serial_t *obj)
         if (nordic_nrf5_uart_state[instance].usage_counter == 0) {
 
             nrf_uarte_disable(nordic_nrf5_uart_register[instance]);
-            
+
             /* Turn NRF_UARTE0_BASE or NRF_UARTE1_BASE power off and on to reset peripheral. */
             if (instance == 0) {
                 *(volatile uint32_t *)0x40002FFC = 0;
@@ -1022,7 +1022,7 @@ void serial_free(serial_t *obj)
                 *(volatile uint32_t *)0x40028FFC = 1;
             }
 #endif
-            
+
         }
     }
 }
@@ -1273,6 +1273,8 @@ void serial_irq_set(serial_t *obj, SerialIrq irq, uint32_t enable)
     struct serial_s *uart_object = obj;
 #endif
 
+    int instance = uart_object->instance;
+
     /* Convert Mbed type to Nordic IRQ mask. */
     uint32_t type = (irq == TxIrq) ? NORDIC_TX_IRQ : NORDIC_RX_IRQ;
 
@@ -1282,10 +1284,20 @@ void serial_irq_set(serial_t *obj, SerialIrq irq, uint32_t enable)
         uart_object->mask |= type;
         nordic_nrf5_serial_configure(obj);
 
+        /*  It is required by Mbed HAL API to generate TxIrq interrupt when TXD register is empty (also after enabling TxIrq interrupt).
+            Driver uses DMA to perform uart transfer and TxIrq is generated after the transfer is finished.
+            Trigger TxIrq interrupt manually on enabling the TxIrq. */
+        if (irq == TxIrq) {
+            if (nrf_uarte_event_check(nordic_nrf5_uart_register[instance], NRF_UARTE_EVENT_TXDRDY)) {
+                nordic_swi_tx_trigger(instance);
+            }
+        }
     } else {
 
         uart_object->mask &= ~type;
     }
+
+
 }
 
 /** Get character. This is a blocking call, waiting for a character