main.c

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

// bit twiddling: enable_bit(s), disable_bit(s), toggle_bit(s), set_bit(s),
// basically wrappers for bitwise operations to make code more readable
#include "bitwit.h"

// USART stuff: initUSART, printString, transmitByte, receiveByte
#include "USART.h"

static uint16_t read16(void);

ISR(TIMER1_OVF_vect){
  disable_bit(DDRB, PB1);
}

int main(void){
  initUSART();

  // timer1 mode 14 (Fast PWM with top at ICR1),
  // toggle OC1A (aka PB1) and prescaler = 1
  enable_bits(TCCR1A, WGM11, COM1A1);
  enable_bits(TCCR1B, WGM13, WGM12, CS10);
  enable_bit(TIMSK1, TOIE1);
  // cycle length with prescale = 1 and CPU frequency = 1 000 000 Hz (1MHz)
  // is 20000us = 20ms, or 50Hz
  ICR1 = 20000;

  sei();

  while(1){
    printString("ready for input\r\n");
    OCR1A = read16();
    enable_bit(DDRB, PB1);
  }

  return 0;
}

static uint16_t read16(void){
  uint16_t ret = 0;

  while(1){
    char c = receiveByte();
    if(c == '\r' || c == '\n')
      break;

    if(c == 127){
      transmitByte('\b');
      ret /= 10;
    }else if(c >= '0' && c <= '9'){
      transmitByte(c);
      ret = ret * 10 + (c - '0');
    }
  }

  transmitByte('\r');
  transmitByte('\n');

  return ret;
}