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ESPCLOCK2, Part 3 - Reading VCC using interrupt

In this post, I read the VCC voltage level to the ATtiny85 and save certain state values to the EEPROM if the voltage level is found to fall below a certain value. But this way of reading the VCC uses delay() and is found to interfere with I2C communication. So a better way needs to be found.

After much experimentation, I found the following code to work reliably:

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volatile byte adc_countdown = 5;
volatile bool adc_ready = false;

// adc_countdown is decremented elsewhere every second
if (adc_ready) {
  adc_ready = false;
  vcc = 1125300L / ADC; // Calculate VCC (in mV); 1125300 = 1.1*1023*1000
  adc_countdown = 5;
}
else if (!bit_is_set(ADCSRA, ADSC) && adc_countdown == 0) {
  ADMUX = bit(MUX3) | bit(MUX2); // Measure VCC using internal bandgap as reference
  ADCSRA = bit(ADEN) | bit(ADSC) | bit(ADIE) | bit(ADPS2) | bit(ADPS1) | bit(ADPS0); // Start conversion
}

ISR(ADC_vect) {
  adc_ready = true;
}

Initially, the ADC register is read in the ISR. But even that takes long enough to interfere with I2C. So a flag adc_ready is set in the ISR, and the actual reading takes place in loop().

The counter adc_countdown makes sure we don't run the ADC too frequently to conserve power (currently set to every 5 seconds).

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