/*
 * Microcontroller "piano" demo using AVR timers.
 *
 * Similar to a piano, each key (button) produces a different tone, from
 * Middle-C, up to High-C.  Each tone is at a frequency specified in the code
 * below. When a button is pressed, the corresponding tone is generated on
 * a piezo speaker and its associated LED is toggled.
 *
 * The eight LEDs are on PORTA, and eight switches on PORTC.
 * NB: both the switches and the LEDs on the STK500 are active-low.
 * 
 * This program prescales the timer input frequency to 1MHz (1 million cycles/sec)
 * by prescaling the 8MHz clock by 8 to produce the 1MHz timer input.
 *
 * The timer is configured to automagically toggle (that is, to change state
 * from 0 to 1, or 1 to 0) an output pin when the timer counter reaches a
 * designated value set in the Output Compare Register (OCR) that is a function
 * of the desired tone frequency.
 *
 * For Timer1 (the 16-bit timer on the ATmega16), there are two possible output
 * channels, one each for OCR1A and OCR1B.  In this program we only use OCR1B
 * (that is, Timer1 ChannelB).  The timer is configured to toggle a
 * particular (fixed) output pin, namely PD4 (shorthand for PORTD pin 4). 
 *
 * By changing the magnitude of the timer's TOP value, the time that it takes
 * for the timer to count up to its maximum value varies, producing different
 * output squarewave periods/frequencies.
 *
 * Last modified 2010-09-14 17:42
 */

#include <avr/io.h>

#if (F_CPU == 8000000)
#   define TIMER_HZ ((uint32_t)1000000) // timer clk speed after prescaling
#else
#   error "MCU clock speed must be 8MHz for this program to work correctly."
#endif

/*
 * One-time initialization code.
 *
 * Initialize Timer1 for Waveform Generation (COM1B1), prescaled by 8 (CS11).
 *
 * For detailed information on the Timer Control registers for Timer1, see p110
 * of the ATmega16 datasheet.  WGMs are documented on p112.
 */
void init()
{
	TCCR1A |= _BV(COM1B1);	// toggle OCR1A when Timer1 reaches value in OCR1
	TCCR1B |= _BV(CS11);    // prescale the timer clock by 8
	
	TCCR1A |= _BV(WGM10);   // Waveform Generation Mode 9, OCR1A as TOP,
	TCCR1B |= _BV(WGM13);   //   update OCR1A when we reach BOTTOM value

	DDRA  = 0xFF;           // all eight LED pins are outputs
	PORTA = 0xFF;           //   initially set high (off)

	DDRC  = 0x00;           // all eight switch pins are inputs
	PORTC = 0xFF;           // turn on the pullup resistors

	DDRD = _BV(PD4);	      // OC1B pin (output for waveform) an output
}

/*
 * Use input from switches (piano keys) to determine the tone frequency to play.
 * Use the frequency value to determine OCR1A (TOP) which sets the period of the
 * waveform, while OCR1B determines the duty cycle (50%).  This is done by
 * setting OCR1B to be half of OCR1A.
 *
 * Q: How can the handling of PORTA be simplified in the code below?
 */
int main()
{
    uint8_t  switches;
    uint16_t freq;

    init();

    while (1) {
        switches = PINC;

        switch (switches) {
            case 0b11111110:
                freq  = 262;
                PORTA = 0b11111110;
                break;
            case 0b11111101:
                freq  = 294;
                PORTA = 0b11111101;
                break;
            case 0b11111011:
                freq  = 330;
                PORTA = 0b11111011;
                break;
            case 0b11110111:
                freq  = 349;
                PORTA = 0b11110111;
                break;
            case 0b11101111:
                freq  = 392;
                PORTA = 0b11101111;
                break;
            case 0b11011111:
                freq  = 440;
                PORTA = 0b11011111;
                break;
            case 0b10111111:
                freq  = 494;
                PORTA = 0b10111111;
                break;
            case 0b01111111:
                freq  = 523;
                PORTA = 0b01111111;
                break;
            default:
                freq  =  0;
                PORTA = ~0;
                break;
        }

        OCR1A = (TIMER_HZ/freq + 1)/2;
        OCR1B = OCR1A/2;
    }	
}