Clear SoftwareSerial rx delay if no interrupt register is found

Before enabling interupts, begin would see if the given receive pin
actually has an associated PCINT register. If not, the interrupts would
not be enabled.

Now, the same check is done, but when no register is available, the rx
parameters are not loaded at all (which in turn prevents the interrupt
from being enabled). This allows all code to use the same "is rx
enabled" (which will be added next).

Author: matthijskooijman

libraries/SoftwareSerial/SoftwareSerial.cpp

@@ -385,9 +385,13 @@ void SoftwareSerial::begin(long speed)
     long baud = pgm_read_dword(&table[i].baud);
     if (baud == speed)
     {
-      _rx_delay_centering = pgm_read_word(&table[i].rx_delay_centering);
-      _rx_delay_intrabit = pgm_read_word(&table[i].rx_delay_intrabit);
-      _rx_delay_stopbit = pgm_read_word(&table[i].rx_delay_stopbit);
+      if (digitalPinToPCICR(_receivePin))
+      {
+        // Only setup rx when we have a valid PCINT for this pin
+        _rx_delay_centering = pgm_read_word(&table[i].rx_delay_centering);
+        _rx_delay_intrabit = pgm_read_word(&table[i].rx_delay_intrabit);
+        _rx_delay_stopbit = pgm_read_word(&table[i].rx_delay_stopbit);
+      }
       _tx_delay = pgm_read_word(&table[i].tx_delay);
       break;
     }
@@ -396,11 +400,8 @@ void SoftwareSerial::begin(long speed)
   // Set up RX interrupts, but only if we have a valid RX baud rate
   if (_rx_delay_stopbit)
   {
-    if (digitalPinToPCICR(_receivePin))
-    {
-      *digitalPinToPCICR(_receivePin) |= _BV(digitalPinToPCICRbit(_receivePin));
-      *digitalPinToPCMSK(_receivePin) |= _BV(digitalPinToPCMSKbit(_receivePin));
-    }
+    *digitalPinToPCICR(_receivePin) |= _BV(digitalPinToPCICRbit(_receivePin));
+    *digitalPinToPCMSK(_receivePin) |= _BV(digitalPinToPCMSKbit(_receivePin));
     tunedDelay(_tx_delay); // if we were low this establishes the end
   }