AVR Lesson: Output Pins I

Goals

In this lesson, you'll program the micro-controller version of "hello world", letting you fire up your programmer, compiler, chip, LED, and test them all out. You'll learn how to setup and use pins for output, and along the way get some exposure to AVR-specific coding practice.

The Circuit

Possibly the simplest circuit you can imagine: plug an LED into pins PB4 and ground. PB4 is going to go the positive leg of the LED to source current across the LED and light it up.

If you want, you can include a current-limiting resistor inline with the LED (around 100 ohms is good). I've done it both ways -- the AVR will drive the LED at ~50mA, which will burn out the LED over the long run, but for this demo, it'll be fine.

The Code

/* Blinker Demo */

#include <avr/io.h>		/* Defines pins, ports, etc */
#define F_CPU 1000000UL	   /* Sets up the chip speed for delay.h -- 1MHz for Tiny13*/
#include <util/delay.h>		/* Functions to waste time */

#define LED PB4			/* Defines pin PB4 for the LED.  I
				 often incorporate a bunch of the circuit
				 info in the defines, which makes
				 porting the code to another chip
				 easier and reminds you of how to
				 hook it up. */

int main(void){

  DDRB = _BV(LED);		      /* Data Direction Register B:
				       writing a one to the bit
				       enables output.  More on the
				       _BV() macro in the next
				       lesson.*/
	
  while(1){			/* the main loop, from which we never return */

    PORTB = _BV(LED); 		/* Turn on the LED bit/pin in PORTB */
    _delay_ms(400);		/* wait */

    PORTB &= ~_BV(LED);		/*  Turn off the LED bit/pin in PORTB */
   _delay_ms(400);		/* wait */

  }

  return(0);			/* never reached */
}

Discussion

Even though this is a very short program, it provides a sketch of what most AVR programs look like.

At the top is a bunch of includes, defines, and etc. io.h defines all of the port and pin macros and is linked to the individual chip's io header during compilation, which allows the same code to work for different AVR chips. You pretty much always want to include io.h. The delay functions in delay.h are fairly generic, but depend on knowing the chip's CPU clock speed to get the timing right. Also note that the maximum delay you can request also depends on the chip's clock speed. Read up in delay.h for details if you're interested. Practically speaking, if you're asking it to delay for 600ms and it's only delaying for 200ms, try calling the _delay_ms() function three times, each for 200ms.

After the defines and includes we get down to the main() function. Most AVR main()'s have at least an initialization section and an infinitely-repeating loop where the bulk of the chip's work gets done.

In this example, the initialization section just sets up the LED pin for output by writing a 1 to the relevant bit in the Data Direction Register for port B. When the chip is reset or powered on, all of the input/output ports are initialized as inputs (DDRB = 0). We write a one to the DDR do set it as output. More on the bitwise math in the next lesson.

The main loop of the function then blinks the LED by alternately driving the output pin high (the supply voltage, VCC) or low (GND, 0v), and it does this forever, or until the chip is reset or powered off. (The return(0) line is just there to stop the compiler from warning about main() functions that don't return -- a requirement of GCC that isn't really relevant to the AVR, but it doesn't hurt the code either.)

Trouble?

Programmer woes:

Is the LED in the correct polarity? (Positive to pin PB4, negative to GND).

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