//Reset and brown-out code 
//R. Traylor 11.16.09

//this code requires that the Low fuse has bits 6 and 7 set to zero
//to enable brown out detection and to set it to 4v detection level.

#define F_CPU 16000000UL     //16Mhz clock
//however, the effective F_CPU changes are the code executes

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

/********************************** tcnt0_init ********************************/
void tcnt0_init(){
//timer counter 0 setup
ASSR  |= (1<<AS0);               //ext osc TOSC running at 32.768kHz
TIMSK |= (1<<OCIE0);             //enable interrupts for compare register match
TCCR0 |= (1<<CS00) | (1<<WGM01); //CTC mode, no prescale
OCR0   = 0x03;                   //4 cycles between interrupts is 122uS
}
/******************************************************************************/

/***************************  debounce_switch  ********************************/
//check PORTD.0 switch
uint8_t debounce_switch(){
  static uint16_t state = 0; //holds present state

  state = (state << 1) | (! bit_is_clear(PIND, 0)) | 0xE000;
  if (state == 0xF000) return 1;
  return 0;
}
/******************************************************************************/

/******************************  TIMER 0 ISR  *********************************/
ISR(TIMER0_COMP_vect){ //this code is entered every 122us approx
  //if switch is pushed, clear all the reset source bits in the MCUCSR register
  if(debounce_switch() == 1){MCUCSR = 0x00;}
}
/******************************************************************************/

int main(){
uint8_t i;

//this block sets all ports to inputs with pullups on except PORTB
//which is used as an indicator of what is going on.  Comment
//and uncomment to see the difference in power consumption.
  DDRA = 0x00;  PORTA = 0xFF; //all inputs with pullups
  DDRB = 0xFF;  PORTB = 0x00; //blink lights with PORTB 
  DDRC = 0x00;  PORTC = 0xFF; //all inputs with pullups
  DDRD = 0x00;  PORTD = 0xFF; //all inputs with pullups
  DDRE = 0x00;  PORTE = 0xFF; //all inputs with pullups
  DDRF = 0x00;  PORTF = 0xFF; //all inputs with pullups
  DDRG = 0x00;  PORTG = 0xFF; //all inputs with pullups

  tcnt0_init();//initalize timer counter zero to interrupt at 122us intervals

  sei(); //interrupts enabled 

  while(1){ 
    PORTB = MCUCSR;  //display the MCUCSR register value
    _delay_ms(100);  //delay 1.0 sec
    PORTB = 0x00;                        //clear PORTB LEDs 
  }//while(1)

} //main