Measuring V_DD

The code snippet below shows how to initialize the ADC and how to use the V_DD as an internal input to measure the voltage which is powering the device. It also uses USART to transmit the results in such a manner that it may be graphed by the Data Visualizer. See Section Get Code Examples from GitHub for instructions on how to download the code, and Section Plotting Graph in Data Visualizer for instructions on how to set up the graph in Data Visualizer.

#define F_CPU 3333333ul

#include <avr/io.h>
#include <util/delay.h>
#include <math.h>

#define TIMEBASE_VALUE ((uint8_t) ceil(F_CPU*0.000001))
#define ADC_MAX_VALUE   ((1 << 12) - 1) /* In single-ended mode, the max value is 4095 */
#define BAUD_RATE 9600
#define BAUD_REG_VAL ((float)(64 * F_CPU / (16 * (float)BAUD_RATE)) + 0.5)

static uint16_t adc_reading;
static float voltage;

/**********************************************************************************
ADC initialization
**********************************************************************************/
void adc_init()
{
	ADC0.CTRLA = ADC_ENABLE_bm;
	ADC0.CTRLB = ADC_PRESC_DIV2_gc; /* fCLK_ADC = 3.333333/2 MHz */
	ADC0.CTRLC = ADC_REFSEL_1024MV_gc | (TIMEBASE_VALUE << ADC_TIMEBASE_gp);
	ADC0.CTRLE = 17; /* (SAMPDUR + 0.5) * fCLK_ADC = 10.5 µs sample duration */

	ADC0.MUXPOS = ADC_MUXPOS_VDDDIV10_gc; /* ADC channel VDD/10 */
	ADC0.COMMAND = ADC_MODE_SINGLE_12BIT_gc; /* Single 12-bit mode */
}

/**********************************************************************************
USART initialization
**********************************************************************************/
void usart_init()
{
	PORTB.DIRSET = PIN2_bm; /* Set TXD to output */
	USART0.CTRLB = USART_TXEN_bm; /* Enable USART transmitter */
	USART0.BAUD = BAUD_REG_VAL; /* Set baud rate */
}

/**********************************************************************************
Send float via USART to Data Visualizer
**********************************************************************************/
void USART_send_DV(float *float_ptr)
{
	uint8_t *byte_ptr = (uint8_t *) float_ptr;

	while(!(USART0.STATUS & USART_DREIF_bm));
	USART0.TXDATAL = 0x33; /* Send data stream start byte */

	for(uint8_t i = 0; i < sizeof(float); i++)
	{
		while(!(USART0.STATUS & USART_DREIF_bm));
		USART0.TXDATAL = byte_ptr[i];
	}

	while(!(USART0.STATUS & USART_DREIF_bm));
	USART0.TXDATAL = ~0x33; /* Send data stream stop byte */
}

int main(void)
{
	adc_init();
	usart_init();

	while(1)
	{
		ADC0.COMMAND |= ADC_START_IMMEDIATE_gc;     /* Start ADC conversion */
		while(!(ADC0.INTFLAGS & ADC_SAMPRDY_bm));   /* Wait until conversion is done */

		adc_reading = ADC0.SAMPLE; /* Read ADC sample, clears flag */
		/* Calculate VDD, VREF = 1.024V, 12-bit resolution.
		   Multiplied by 10 because the input channel is VDD/10. */
		voltage = (float)(adc_reading * 1.024 * 10) / ADC_MAX_VALUE;

		USART_send_DV(&voltage); /* Transmit voltage to Data Visualizer */

		_delay_ms(500);
	}
}