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[on:]

I just want to toggle an LED when the USART finishes receiving, so I wrote the following for the USART_RX_vect, but the LED never changes. However, the LED does toggle if move the code from the ISR into the main loop and precede it with while ( !(UCSR0A & (1<<UDRE0)) );

Can anyone describe my error to me?

Code:

#define F_CPU 16000000UL
#define UBRR_VALUE 416 // baudrate 2400
#include <avr/io.h>
#include <avr/interrupt.h>
#include <util/delay.h>

ISR(USART_RX_vect)
{
  char data = UDR0;
  PORTB ^= (1 << PB0); // Toggle the LED
  _delay_ms(500);
}

int main()
{
  DDRB |= (1 << PB0); // Set LED as output

  // Set baud rate
  UBRR0H = (uint8_t)(UBRR_VALUE>>8);
  UBRR0L = (uint8_t)UBRR_VALUE;
  // Set frame format to 8 data bits, no parity, 1 stop bit
  UCSR0C |= (1<<UCSZ01)|(1<<UCSZ00);
  //enable reception and RC complete interrupt
  UCSR0B |= (1<<RXEN0)|(1<<RXCIE0);
  //  Enable global interrupts
  sei();
  
  while (1)
    ;
}

Thank you for your assistance.

[Reply #1 on:]

You are calling delay in an interrupt routine - nothing can happen during that delay and you'll muck up
the millis() clock completely.

Never call delay() in an interrupt routine.

[Reply #2 on:]

Thanks. That's instructive.

[Reply #3 on:]

Is there some other issue I'm failing to recognize?

(As before, if I put this code in the main loop instead of in an ISR, it executes as I expect: the LED is illuminated.)

(That was going to be my first question - is the port manipulation coded correctly? Seems you have proved it is.)

Have you proved that the interrupt handler is executing? (For example you could change a volatile global and then detect and report that change in loop.)

[Reply #4 on:]

Hardware issue. My jumper wire was not quite seated in the breadboard!

Thank you much.