UART: manually handle IRQ if DRE + interrupts disabled or in higher priority ISR

When write is called and TX buffer is full.

cores/arduino/SERCOM.cpp

@@ -688,7 +688,7 @@ void SERCOM::initClockNVIC( void )
 
   // Setting NVIC
   NVIC_EnableIRQ(IdNvic);
-  NVIC_SetPriority (IdNvic, (1<<__NVIC_PRIO_BITS) - 1);  /* set Priority */
+  NVIC_SetPriority (IdNvic, SERCOM_NVIC_PRIORITY);  /* set Priority */
 
   //Setting clock
   GCLK->CLKCTRL.reg = GCLK_CLKCTRL_ID( clockId ) | // Generic Clock 0 (SERCOMx)

cores/arduino/SERCOM.h

@@ -21,7 +21,8 @@
 
 #include "sam.h"
 
-#define SERCOM_FREQ_REF 48000000
+#define SERCOM_FREQ_REF      48000000
+#define SERCOM_NVIC_PRIORITY ((1<<__NVIC_PRIO_BITS) - 1)
 
 typedef enum
 {

cores/arduino/Uart.cpp

@@ -154,7 +154,27 @@ size_t Uart::write(const uint8_t data)
   if (sercom->isDataRegisterEmptyUART() && txBuffer.available() == 0) {
     sercom->writeDataUART(data);
   } else {
-    while(txBuffer.isFull()); // spin lock until a spot opens up in the buffer
+    // spin lock until a spot opens up in the buffer
+    while(txBuffer.isFull()) {
+      uint8_t interruptsEnabled = ((__get_PRIMASK() & 0x1) == 0);
+
+      if (interruptsEnabled) {
+        uint32_t exceptionNumber = (SCB->ICSR & SCB_ICSR_VECTACTIVE_Msk);
+
+        if (exceptionNumber == 0 ||
+              NVIC_GetPriority((IRQn_Type)(exceptionNumber - 16)) > SERCOM_NVIC_PRIORITY) {
+          // no exception or called from an ISR with lower priority,
+          // wait for free buffer spot via IRQ
+          continue;
+        }
+      }
+
+      // interrupts are disabled or called from ISR with higher or equal priority than the SERCOM IRQ
+      // manually call the UART IRQ handler when the data register is empty
+      if (sercom->isDataRegisterEmptyUART()) {
+        IrqHandler();
+      }
+    }
 
     txBuffer.store_char(data);