Measuring Your Own Supply Voltage 2
This appendix to the original article Measuring Your Own Supply Voltage shows an alternative way of measuring the supply voltage on an AVR DA-Series microcontroller, using the 'backwards' technique used on older ATtiny and ATmega chips.
Measuring VDD on an AVR128DA28 - alternative method
The 'backwards' technique is to make VDD the voltage reference for the ADC, and then measure a fixed voltage reference of 1.024V from the VREF peripheral, via the Analog Comparator's DACREF voltage divider.
Setting up the ADC
First set up the ADC as follows:
- Set the ADC's voltage reference to VDD.
- Set the Analog Comparator's shared voltage reference to 1.024V.
- Set Analog Comparator 0's DACREF value to 255 (not strictly necessary as it's the default).
- Set the ADC MUXPOS so the ADC measures DACREF0.
Here's the code to implement this:
void ADCSetup () {
VREF.ADC0REF = VREF_REFSEL_VDD_gc;
VREF.ACREF = VREF_REFSEL_1V024_gc;
AC0.DACREF = 255; // Maximum DACREF0 voltage
ADC0.MUXPOS = ADC_MUXPOS_DACREF0_gc; // Measure DACREF0
ADC0.CTRLC = ADC_PRESC_DIV64_gc; // 375kHz clock
ADC0.CTRLA = ADC_ENABLE_bm; // Single, 12-bit
}
Reading the voltage
Here's the routine to measure the supply voltage:
void MeasureVoltage () {
ADC0.COMMAND = ADC_STCONV_bm; // Start conversion
while (ADC0.COMMAND & ADC_STCONV_bm); // Wait for completion
uint16_t adc_reading = ADC0.RES; // ADC conversion result
uint16_t voltage = 41779/adc_reading;
Buffer[0] = voltage/10; Buffer[1]= voltage%10;
}
DACREF0 is 255/256 times the Analog Comparator reference, which we've set to 1.024V. So suppose we have a 12-bit reading of R. Then:
which gives:
To get VDD in tenths of a volt we therefore need to divide 41779 by the ADC reading.
The minimum voltage we can measure this way is 4177.9/4095 or 1.02V, which is below the minimum supply voltage of 1.8V.
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