Wrote code to put Arduino into Power Down sleep mode when calling a delay. Seems to work ok. Comments appreciated. I haven't learnt how do libraries yet but eventually, I'll turn it into a library and also put it in the Playground.
Features are:
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Calibration of timer: calibrate(). Watchdog timer can be 10% off and varies with temperature. This will calibrate watchdog timer with clock timer and adjusts delay so that it is closer to actual time. You can call the calibrate function every hour or so to readjust.
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Replacement for millis(). Since clock stops during sleep, I have provided estMillis() which will give you an estimated actual time. What this does is to keep the accumulated sleep time (calibrated) and adds it to millis() to give you actual time.
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Delay function sleepCPU_delay(long millis). Will sleep for the specified time. Note that sleep granularity is 16ms so that is the smallest amount of time it will sleep and it will always sleep in multiples of 16ms.
#include <avr/sleep.h>
#include <avr/wdt.h>
long timeSleep = 0; // total time due to sleep
float calibv = 0.93; // ratio of real clock with WDT clock
volatile byte isrcalled = 0; // WDT vector flag
// Internal function: Start watchdog timer
// byte psVal - Prescale mask
void WDT_On (byte psVal)
{
// prepare timed sequence first
byte ps = (psVal | (1<<WDIE)) & ~(1<<WDE);
cli();
wdt_reset();
/* Clear WDRF in MCUSR */
MCUSR &= ~(1<<WDRF);
// start timed sequence
WDTCSR |= (1<<WDCE) | (1<<WDE);
// set new watchdog timeout value
WDTCSR = ps;
sei();
}
// Internal function. Stop watchdog timer
void WDT_Off() {
cli();
wdt_reset();
/* Clear WDRF in MCUSR */
MCUSR &= ~(1<<WDRF);
/* Write logical one to WDCE and WDE */
/* Keep old prescaler setting to prevent unintentional time-out */
WDTCSR |= (1<<WDCE) | (1<<WDE);
/* Turn off WDT */
WDTCSR = 0x00;
sei();
}
// Calibrate watchdog timer with millis() timer(timer0)
void calibrate() {
// timer0 continues to run in idle sleep mode
set_sleep_mode(SLEEP_MODE_IDLE);
long tt1=millis();
doSleep(256);
long tt2=millis();
calibv = 256.0/(tt2-tt1);
}
// Estimated millis is real clock + calibrated sleep time
long estMillis() {
return millis()+timeSleep;
}
// Delay function
void sleepCPU_delay(long sleepTime) {
ADCSRA &= ~(1<<ADEN); // adc off
PRR = 0xEF; // modules off
set_sleep_mode(SLEEP_MODE_PWR_DOWN);
int trem = doSleep(sleepTime*calibv);
timeSleep += (sleepTime-trem);
PRR = 0x00; //modules on
ADCSRA |= (1<<ADEN); // adc on
}
// internal function.
int doSleep(long timeRem) {
byte WDTps = 9; // WDT Prescaler value, 9 = 8192ms
isrcalled = 0;
sleep_enable();
while(timeRem > 0) {
//work out next prescale unit to use
while ((0x10<<WDTps) > timeRem && WDTps > 0) {
WDTps--;
}
// send prescaler mask to WDT_On
WDT_On((WDTps & 0x08 ? (1<<WDP3) : 0x00) | (WDTps & 0x07));
isrcalled=0;
while (isrcalled==0) {
// turn bod off
MCUCR |= (1<<BODS) | (1<<BODSE);
MCUCR &= ~(1<<BODSE); // must be done right before sleep
sleep_cpu(); // sleep here
}
// calculate remaining time
timeRem -= (0x10<<WDTps);
}
sleep_disable();
return timeRem;
}
// wdt int service routine
ISR(WDT_vect) {
WDT_Off();
isrcalled=1;
}
I like your code, it's way above my head though, as I'm just getting into the Arduino. I see you have comments all over that code, that's very helpful for understanding it.