UART give garbage values after waking up from sleep(arduino)

Most micros support a low-power 32.768 kHz watch crystal oscillator with some kind of prescaler and timer interrupt. Set the prescaler so the timer is counting slowly and the interrupt happens at the period you desire.

Some micros also have a built-in low-power RC timer if exact timing isn't critical.

The datasheet for any low-power micro will list the power with 32.768 oscillator (and nothing else) running. It's pretty close to zero. You can do the math to see if this is acceptable, and compare it to the current drawn by the watchdog.

OK, for example on the msp430f2013, let's look at power in the datasheet.

0.5 μA is almost zero, although it is five times the true OFF mode.

For more detail, we can look inside the datasheet.
Going from LPM4 (everything off) to LPM3 (running the oscillator) is the difference between 0.5 μA and 1 μA.

Suppose the battery is CR2032 with 225 mAh capacity. Then standby in LPM4 is about 50 years and in LPM3 is about 25 years. 25 years is long enough for many applications, because the ON-current (during the measurement itself) dominates the consumption.

So I ended up by finding myself the answer on the ATTiny85 specs.

As I was trying out the code I found out that each time I used pin 3 (PCINT3) as the interrupt pin and analog reading, the code actually worked. So I started digging the a little bit more the details of each pin and I found out the following:

Port B, Bit 3 – XTAL1/CLKI/ADC3/OC1B/PCINT3 • XTAL1: Chip Clock Oscillator pin 1. Used for all chip clock sources except internal calibrateble RC oscillator. When used as a clock pin, the pin can not be used as an I/O pin. • CLKI: Clock Input from an external clock source, see “External Clock” on page 26. • ADC3: Analog to Digital Converter, Channel 3 . • OC1B: Inverted Output Compare Match output: The PB3 pin can serve as an external output for the Timer/Counter1 Compare Match B when configured as an output (DDB3 set). The OC1B pin is also the inverted output pin for the PWM mode timer function. • PCINT3: Pin Change Interrupt source 3.

Port B, Bit 2 – SCK/ADC1/T0/USCK/SCL/INT0/PCINT2 • SCK: Master Clock output, Slave Clock input pin for SPI channel. When the SPI is enabled as a Slave, this pin is configured as an input regardless of the setting of DDB2. When the SPI is enabled as a Master, the data direction of this pin is controlled by DDPB2. When the pin is forced by the SPI to be an input, the pull-up can still be controlled by the PORTB2 bit. • ADC1: Analog to Digital Converter, Channel 1 . • T0: Timer/Counter0 counter source. • USCK: Three-wire mode Universal Serial Interface Clock. • SCL: Two-wire mode Serial Clock for USI Two-wire mode. • INT0: External Interrupt source 0. • PCINT2: Pin Change Interrupt source 2.

The problem was that I assumed that the pin number was the same as the analogue channel, so I ended up by always reading something else.

Here's the working code (in the meanwhile I recoded it)

#include <avr/sleep.h>
#include <avr/interrupt.h>

const int switchPin = A1;
const int door = 3;
const int sleep = 4;

int outputValue = 0;

void setup() {

    //pinMode(switchPin, INPUT);
    //digitalWrite(switchPin, HIGH);

    pinMode(door, OUTPUT);
    digitalWrite(door, LOW);

    pinMode(sleep, OUTPUT);
    digitalWrite(sleep, HIGH);

    // Flash quick sequence so we know setup has started
    for (int k = 0; k < 10; k = k + 1) {
        if (k % 2 == 0) {
            digitalWrite(door, HIGH);
            }
        else {
            digitalWrite(door, LOW);
            }
        delay(250);
        } // for
    } // setup

void MCsleep() {

    GIMSK |= _BV(PCIE);                     // Enable Pin Change Interrupts
    PCMSK |= _BV(PCINT2);                   // Use PB2 as interrupt pin
    ADCSRA &= ~_BV(ADEN);                   // ADC off
    //digitalWrite(switchPin, LOW);
    set_sleep_mode(SLEEP_MODE_PWR_DOWN);    // replaces above statement

    sleep_enable();                         // Sets the Sleep Enable bit in the MCUCR Register (SE BIT)
    sei();                                  // Enable interrupts
    sleep_cpu();                            // sleep

    cli();                                  // Disable interrupts
    PCMSK &= ~_BV(PCINT2);                  // Turn off PB2 as interrupt pin
    sleep_disable();                        // Clear SE bit
    //digitalWrite(switchPin, HIGH);
    ADCSRA |= _BV(ADEN);                    // ADC on

    sei();                                  // Enable interrupts
    } // sleep

// Whatever the pin this IRS should always point to the PCINT0_vect, because is the only capable of
// providing the interrupt. The ISR must be provided otherwise, the MC just resets instead of resuming.
ISR(PCINT0_vect) {
    // This is called when the interrupt occurs, but I don't need to do anything in it
    }

void loop() {

  MCsleep();
  delay(20);

  outputValue = analogRead(switchPin);

  if(outputValue > 200){
    digitalWrite(sleep, LOW);
    delay(1000);
    digitalWrite(door, HIGH);
    delay(2000);
    digitalWrite(sleep, HIGH);
  }
  else{
    digitalWrite(sleep, LOW);
    delay(1000);
    digitalWrite(door, LOW);
    delay(2000);
    digitalWrite(sleep, HIGH);
  }
}