Fix hang on Serial overflow, PIOSerial stop bit handling (#401)

When the Serial software FIFOs overeflowed, the IRQ handler would not
read any more data from the hardware FIFOs.  This would cause the
IRQ handler to be continually called as soon as it exited since the
HW FIFOS were not empty.

Now always read every available HW FIFO entry and throw out any that
don't fit in the SW FIFO.

Also fix a too long by half stop bit timing in the PIOSerial receiver.

cores/rp2040/SerialPIO.cpp

@@ -91,7 +91,7 @@ void __not_in_flash_func(SerialPIO::_handleIRQ)() {
     if (_rx == NOPIN) {
         return;
     }
-    while ((!pio_sm_is_rx_fifo_empty(_rxPIO, _rxSM)) && ((_writer + 1) % sizeof(_queue) != _reader)) {
+    while (!pio_sm_is_rx_fifo_empty(_rxPIO, _rxSM)) {
         uint32_t decode = _rxPIO->rxf[_rxSM];
         decode >>= 32 - _rxBits;
         uint32_t val = 0;
@@ -114,9 +114,13 @@ void __not_in_flash_func(SerialPIO::_handleIRQ)() {
             }
         }
 
-        _queue[_writer] = val & ((1 << _bits) -  1);
-        asm volatile("" ::: "memory"); // Ensure the queue is written before the written count advances
-        _writer = (_writer + 1) % sizeof(_queue);
+        if ((_writer + 1) % sizeof(_queue) != _reader) {
+            _queue[_writer] = val & ((1 << _bits) -  1);
+            asm volatile("" ::: "memory"); // Ensure the queue is written before the written count advances
+            _writer = (_writer + 1) % sizeof(_queue);
+        } else {
+            // TODO: Overflow
+        }
     }
 }
 
@@ -195,7 +199,7 @@ void SerialPIO::begin(unsigned long baud, uint16_t config) {
         _writer = 0;
         _reader = 0;
 
-        _rxBits = 2 * (_bits + _stop + (_parity != UART_PARITY_NONE ? 1 : 0) + 1);
+        _rxBits = 2 * (_bits + _stop + (_parity != UART_PARITY_NONE ? 1 : 0) + 1) - 1;
         _rxPgm = _getRxProgram(_rxBits);
         int off;
         if (!_rxPgm->prepare(&_rxPIO, &_rxSM, &off)) {
@@ -213,6 +217,9 @@ void SerialPIO::begin(unsigned long baud, uint16_t config) {
         pio_sm_put_blocking(_rxPIO, _rxSM, clock_get_hz(clk_sys) / (_baud * 2) - 2);
         pio_sm_exec(_rxPIO, _rxSM, pio_encode_pull(false, false));
 
+        // Join the TX FIFO to the RX one now that we don't need it
+        _rxPIO->sm[_rxSM].shiftctrl |= 0x80000000;
+
         // Enable interrupts on rxfifo
         switch (_rxSM) {
         case 0: pio_set_irq0_source_enabled(_rxPIO, pis_sm0_rx_fifo_not_empty, true); break;

cores/rp2040/SerialUART.cpp

@@ -248,10 +248,15 @@ void arduino::serialEvent2Run(void) {
 // IRQ handler, called when FIFO > 1/4 full or when it had held unread data for >32 bit times
 void __not_in_flash_func(SerialUART::_handleIRQ)() {
     uart_get_hw(_uart)->icr |= UART_UARTICR_RTIC_BITS | UART_UARTICR_RXIC_BITS;
-    while ((uart_is_readable(_uart)) && ((_writer + 1) % sizeof(_queue) != _reader)) {
-        _queue[_writer] = uart_getc(_uart);
-        asm volatile("" ::: "memory"); // Ensure the queue is written before the written count advances
-        _writer = (_writer + 1) % sizeof(_queue);
+    while (uart_is_readable(_uart)) {
+        auto val = uart_getc(_uart);
+        if ((_writer + 1) % sizeof(_queue) != _reader) {
+            _queue[_writer] = val;
+            asm volatile("" ::: "memory"); // Ensure the queue is written before the written count advances
+            _writer = (_writer + 1) % sizeof(_queue);
+        } else {
+            // TODO: Overflow
+        }
     }
 }