6.1 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 GUID-23C18D03-EA8D-4AD5-9FB7-951D5E2B3538.html 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);
	}
}