Sometimes I am a bit of an idiot. I've later figured out that I can just wire the power through the switch itself to achieve the same goal. So I only need the watchdog timer without external interrupts.
But for anyone interested... Here is the code for Watchdog sleep timer that wakes the CPU every second. It's a bit of a combination of various sources. Though if I remember correctly it is mostly from some Sparkfun project.
#include <avr/sleep.h>
#include <avr/interrupt.h>
volatile int desiredSleepTime = 30;
volatile int currentSleepTime = 0;
void setup() {
//Power down various bits of hardware to lower power usage
set_sleep_mode(SLEEP_MODE_PWR_DOWN); //Power down everything, wake up from WDT
sleep_enable();
}
void loop() {
watchdogSleep1s();
// do stuff with sleepTime
}
void watchdogSleep1s() {
ADCSRA &= ~(1<<ADEN); //Disable ADC, saves ~230uA
setup_watchdog(6); //Setup watchdog to go off after 1sec
sleep_mode(); //Go to sleep! Wake up 1sec later and check water
ADCSRA |= (1<<ADEN); //Enable ADC
}
//This runs each time the watch dog wakes us up from sleep
ISR(WDT_vect) {
currentSleepTime++;
}
//Sets the watchdog timer to wake us up, but not reset
//0=16ms, 1=32ms, 2=64ms, 3=128ms, 4=250ms, 5=500ms
//6=1sec, 7=2sec, 8=4sec, 9=8sec
//From: http://interface.khm.de/index.php/lab/experiments/sleep_watchdog_battery/
void setup_watchdog(int timerPrescaler) {
if (timerPrescaler > 9 ) timerPrescaler = 9; //Limit incoming amount to legal settings
byte bb = timerPrescaler & 7;
if (timerPrescaler > 7) bb |= (1<<5); //Set the special 5th bit if necessary
//This order of commands is important and cannot be combined
MCUSR &= ~(1<<WDRF); //Clear the watch dog reset
WDTCR |= (1<<WDCE) | (1<<WDE); //Set WD_change enable, set WD enable
WDTCR = bb; //Set new watchdog timeout value
WDTCR |= _BV(WDIE); //Set the interrupt enable, this will keep unit from resetting after each int
}