My Gorram Frakking Blog

With a 16 MHz clock, the full 16-bit count wraps around in about 780 µs. Since we need a PWM period of 20 ms, this is clearly too fast. Prescaling Timer/Counter 1 by dividing the sytem clock by 64 gives us a full-count period of 50 ms. By setting the upper limit of the count using ICR1 to 12500, this results in the desired 20 ms period.

Update: I’ve gotten some math wrong in the preceding paragraph, and I'm not sure what I was thinking. It seems that a full 16-bit count would take 4.096 ms at 16 MHz. Also, the comments in the code regarding prescaling values don’t all agree. Sorry about that. I hope you can still find this useful.

To get the appropriate servo pulse width between 0.9 ms and 2.1 ms, the OCR1A register is loaded with values between 225 and 525.

Circuit Set-up

Please excuse the lack of a schematic; I simply don’t have time to create one right now.

The ATmega32 is set up to run with a 16-MHz crystal oscillator. On the PDIP 40 package, the following pins are connected:

Fuse Set-up

The fuses are set for a 16 MHz external crystal oscillator.

Register Set-up

The following registers are set, in this order. Although the order isn’t strictly necessary, some subset of it might be (be specific).

Code

#include <inttypes.h>
#include <avr/io.h>
#include <avr/interrupt.h>
#include <avr/signal.h>

#define kPinInitCompleteLED         PA0
#define kDelay                      10000
#define kReverseDelay               300000

#define kLowerLimit                 300                 //  Min 225 == 0.9 ms
#define kUpperLimit                 450                 //  Max 525 == 2.1 ms

int
main()
{
    volatile long       d;
    short               dir = 1;
    
    //  Set LED output pins…
    
    DDRA = _BV(kPinInitCompleteLED);
    DDRB = 0;                                           //  Port B inputs
    
    //  Set up OCR pins (PD4, PD5) as outputs (00110000)…
    
    DDRD = _BV(PD4) | _BV(PD5);
    
    //  Set up Timer 1. Timer 1 should reset when
    //  it reaches TOP = ICR1 (WGM13:0 = 1110b). On
    //  compare match clear output, at TOP set (COM1A1:0 = 10b).
    
    TCCR1A = _BV(COM1A1) | !_BV(COM1A0)                 //  Both PWM outputs set at TOP,
                | _BV(COM1B1) | !_BV(COM1B0)            //    clear on compare match
                | !_BV(FOC1A) | !_BV(FOC1B)             //  PWM mode, can't force output
                | _BV(WGM11) | !_BV(WGM10);             //  Fast PWM, TOP = ICR1
    
    TCCR1B = !_BV(ICNC1) | !_BV(ICES1)                  //  Disable input capture noise canceler,
                                                        //    edge select to negative.
                | _BV(WGM13) | _BV(WGM12)               //  Fast PWM, TOP = ICR1
                | !_BV(CS12) | _BV(CS11) | _BV(CS10);   //  clk(i/o) / 1024
    
    //  PWM duty cycle…
    
    OCR1A = kLowerLimit;
    OCR1B = kLowerLimit;
    
    //  PWM period…
    
    ICR1 = 12500;
    
    //  Show initialization complete…
    
    PORTA = _BV(kPinInitCompleteLED);
    
    //  Loop forever steering left-to-right-to-left…
    
    OCR1A = kLowerLimit;            //    Min value == 0.9 ms
    while (1)
    {
        d = kDelay;
        while (d--);
        
        OCR1A += dir;
        
        if (OCR1A < kLowerLimit)
        {
            OCR1A = kLowerLimit;
            dir = 1;
            d = kReverseDelay;
            while (d--);
        }
        
        if (OCR1A > kUpperLimit)    //    Max value == 2.1 ms
        {
            OCR1A = kUpperLimit;
            dir = -1;
            d = kReverseDelay;
            while (d--);
        }
        
    }
    
    return 0;
}