Blink LED example does not work

Question

I'm trying to run standard Blink LED example on Atmega8A, but my LED doesn't blink. Here is my circuit:

And here is my code:

#ifndef F_CPU
    #define 16000000UL
#endif // F_CPU

#include <avr/io.h>
#include <util/delay.h>

#define set_bit(target, num)    (   target |=   _BV(num))
#define clear_bit(target, num)  (   target &=   ~ _BV(num))
#define toggle_bit(target, num) (   target ^=   _BV(num))

#define DELAY_IN_MS 1000 /* 1 sec */


int main()
{
    set_bit(DDRD, PD7);
    set_bit(PORTD, PD7);
    while(1)
    {
        toggle_bit(PORTD, PD7);
        _delay_ms(DELAY_IN_MS);
    }

    return 0;
}

What's wrong?

Answer 1

It looks to me like you've got the Cathode of the LED wired to Vdd. That's the wrong way round, so you should turn around the LED. Or alternatively, wire the Cathode to GND instead of VDD. Also, it's poor practice not to include a resistor in the LED current path; 220 Ohms or so should be fine.

UPDATE:

It looks like you've got more problems than just that. Going by the pin-out posted by 'TMa', your Vcc,GND (p7,p8) look reversed. Also the GND on pin22 is unconnected. Usually all GNDs on an IC should be connected because they are internally connected via the substrate.

Answer 2

I fixed the problem. Thank you guys, for your attention and help:

The problem was in compilation. Here how I was compiling(Blink example does not work, verification fails):

avr-gcc -Wall blink.c -mmcu=atmega8 -DF_CPU=16000000UL -o blink.o
avr-objcopy -R .eeprom -O ihex blink.o blink.hex

Here is how I'm compiling now:

avr-gcc -Wall blink.c -mmcu=atmega8 -DF_CPU=16000000UL -o blink.elf
avr-objcopy -j .data -j .text -O ihex blink.elf blink.hex

BTW:

Code is working fine even if there is nothing connected to the second `GND`, `AREF` and `AVCC`.

Answer 3

First off, welcome to EE stack exchange!

Now, if your schematic is right, you're missing a pull-up reistor on RESET pin. Here's what usually is added:

^{simulate this circuit – Schematic created using CircuitLab}

Capacitor C1 is there to get somewhat rid of the bouncing effect when button is pressed/released.

Add a current limiting resistor for the LED (220 Ohm). LED is a Ligh-Emmiting DIODE, so without a resistor there to limit the current, the LED acts like a short circuit to GND.

Furthermore, connect both grounds(pin 8 and 22) and connect Vcc to Avcc(pin 7 and 20). Also, add a 0.1 uF capacitor from AREF to GND if it is not used for ADC reference. Beware of positive and negative terminals on the capacitor, positive should go to AREF and negative to GND.

About the code, get rid of return 0; its not wrong to have it, but embedded system dont usually return from main(). And without compiler optimization if it would return, it could cause unidentified behaviour.

Answer 4

I don't think LED is properly connected to PD7.