Serial.print messing with Timer Interrupts

Projects Programming
7

Argh - I think we are getting closer. In HardwareSerial.cpp it says

void serialEventRun(void)
{
#ifdef serialEvent_implemented
  if (Serial.available()) serialEvent();
#endif
#ifdef serialEvent1_implemented
  if (Serial1.available()) serialEvent1();
#endif
#ifdef serialEvent2_implemented
  if (Serial2.available()) serialEvent2();
#endif
#ifdef serialEvent3_implemented
  if (Serial3.available()) serialEvent3();
#endif
}


#if !defined(USART0_UDRE_vect) && defined(USART1_UDRE_vect)
// do nothing - on the 32u4 the first USART is USART1
#else
#if !defined(UART0_UDRE_vect) && !defined(UART_UDRE_vect) && !defined(USART0_UDRE_vect) && !defined(USART_UDRE_vect)
  #error "Don't know what the Data Register Empty vector is called for the first UART"
#else
#if defined(UART0_UDRE_vect)
ISR(UART0_UDRE_vect)
#elif defined(UART_UDRE_vect)
ISR(UART_UDRE_vect)
#elif defined(USART0_UDRE_vect)
ISR(USART0_UDRE_vect)
#elif defined(USART_UDRE_vect)
ISR(USART_UDRE_vect)
#endif
{
  if (tx_buffer.head == tx_buffer.tail) {
	// Buffer empty, so disable interrupts
#if defined(UCSR0B)
    cbi(UCSR0B, UDRIE0);
#else
    cbi(UCSRB, UDRIE);
#endif
  }
  else {
    // There is more data in the output buffer. Send the next byte
    unsigned char c = tx_buffer.buffer[tx_buffer.tail];
    tx_buffer.tail = (tx_buffer.tail + 1) % SERIAL_BUFFER_SIZE;
	
  #if defined(UDR0)
    UDR0 = c;
  #elif defined(UDR)
    UDR = c;
  #else
    #error UDR not defined
  #endif
  }
}
#endif
#endif

#ifdef USART1_UDRE_vect
ISR(USART1_UDRE_vect)
{
  if (tx_buffer1.head == tx_buffer1.tail) {
	// Buffer empty, so disable interrupts
    cbi(UCSR1B, UDRIE1);
  }
  else {
    // There is more data in the output buffer. Send the next byte
    unsigned char c = tx_buffer1.buffer[tx_buffer1.tail];
    tx_buffer1.tail = (tx_buffer1.tail + 1) % SERIAL_BUFFER_SIZE;
	
    UDR1 = c;
  }
}
#endif

#ifdef USART2_UDRE_vect
ISR(USART2_UDRE_vect)
{
  if (tx_buffer2.head == tx_buffer2.tail) {
	// Buffer empty, so disable interrupts
    cbi(UCSR2B, UDRIE2);
  }
  else {
    // There is more data in the output buffer. Send the next byte
    unsigned char c = tx_buffer2.buffer[tx_buffer2.tail];
    tx_buffer2.tail = (tx_buffer2.tail + 1) % SERIAL_BUFFER_SIZE;
	
    UDR2 = c;
  }
}
#endif

#ifdef USART3_UDRE_vect
ISR(USART3_UDRE_vect)
{
  if (tx_buffer3.head == tx_buffer3.tail) {
	// Buffer empty, so disable interrupts
    cbi(UCSR3B, UDRIE3);
  }
  else {
    // There is more data in the output buffer. Send the next byte
    unsigned char c = tx_buffer3.buffer[tx_buffer3.tail];
    tx_buffer3.tail = (tx_buffer3.tail + 1) % SERIAL_BUFFER_SIZE;
	
    UDR3 = c;
  }
}
#endif

However I would have expected that this is not an issue because it returns after each character. So now my theory: my prints are to fast --> the serial buffer may sometimes be completely full --> serial interrupts will occur virtually each 87uS. Timer 2 Interrupts occur each 1000uS. Of course sometimes serial ISR may be triggered while timer 2 ISR is busy. So far so good.

This is OK. But what I do not understand: how would this make me lose 1 timer 2 tick? I just do not get it.