/*
 * Lab 6 - Servo Control
 *
 * This program demonstrates how to use the keyboard connected to a PC
 * running HyperTerminal can communicate with the ATmega16 on the STK500
 * to manipulate the position of a servo.
 *
 * Last modified 2010-10-04 20:50 (ebw)
 */

#include <avr/io.h>
#include <stdio.h>
#include <avr/interrupt.h>
#include "uart.h"

#define T ((double)16.384e-3)

#define OCR0_MIN   0
#define OCR0_MAX 255

/*
 * The ISR() function below is what's called an interrupt handler.
 * You don't need to worry about it too much right now as the concept of
 * interrupts will be covered in future labs and lectures.
 *
 * For now all you need to know is that interrupt handlers are functions that
 * are called when a particular event happens "out of nowhere", as opposed to
 * functions that are called under direct control of your program.
 *
 * This particular interrupt handler gets called every time the UART in the
 * microcontroller receives a character over the serial link, i.e. when a key
 * is pressed on the keyboard on the PC in HyperTerminal.
 *
 * When either 'a' or 'z' are received as input, we increment or decrement
 * OCR0 to effect a change in the duty cycle for the PWM signal driving the
 * servo.  We ensure that OCR0 doesn't exceed its lower and upper boundaries.
 */
ISR(USART_RXC_vect)
{
    uint8_t input = UDR;

    if ((input == 'a') && (OCR0 != OCR0_MAX))
        OCR0++;
    else if ((input =='z') && (OCR0 != OCR0_MIN))
        OCR0--;
}

/*
 * This initialization function just sets-up Timer0 for:
 *  - phase Correct PWM
 *  - clearing OC0 on up-count
 *  - prescaled by 256
 * and remembering to enable the output pin (PB3) driven by the timer.
 */
void init()
{
    TCCR0 = _BV(WGM00) | _BV(COM01) | _BV(CS02);
												
    DDRB = _BV(PB3);
	
    uart_init();
}


int main()
{
    init();

    printf("ME106 Servo Demonstration Program\r\n");
    printf("Press \'a\' to increase the period, \'z\' to decrease.\r\n");

    while (1)
    {
        uint8_t ocr0 = OCR0;

        printf("Period: %f/%f\tOCR0: %u/255\r\n", T*ocr0/255, T, ocr0);
    }

    return 0;    // ain't never gonna happen
}