Introduction to ATmega ADC

ADC (Analog to Digital converter) is the most widely used device in embedded systems which is designed especially for data acquisition. In the AVR ATmega series normally 10-bit ADC is inbuilt in the controller.

Let us see how to use the ADC of AVR ATmega16 / ATmega32.

ATmega16/32 supports eight ADC channels, which means we can connect eight analog inputs at a time. ADC channel 0 to channel 7 are present on PORTA. i.e. Pin no.33 to 40.

The controller has 10 bit ADC, which means we will get digital output 0 to 1023.

i.e. When the input is 0V, the digital output will be 0V & when input is 5V (and Vref=5V), we will get the highest digital output corresponding to 1023 steps, which is 5V.

So controller ADC has 1023 steps and

• Step size with Vref=5V :  5/1023 = 4.88 mV. 
• Step size with Vref=2.56 :  2.56/1023 = 2.5 mV.

So Digital data output will be Dout = Vin / step size.

ATmega16/32 ADC Features

• It is 10-bit ADC
• Converted output binary data is held in two special functions 8-bit register ADCL (result Low) and ADCH (result in High).
• ADC gives 10-bit output, so (ADCH: ADCL) only 10-bits are useful out of 16-bits.
• We have options to use this 10-bits as upper bits or lower bits.
• We also have three options for Vref. 1. AVcc (analog Vcc), 2. Internal 2.56 v3. External Aref. Pin.
• The total conversion time depends on crystal frequency and ADPS0: 2 (frequency devisor)
• If you decided to use AVcc or Vref pin as ADC voltage reference, you can make it more stable and increase the precision of ADC by connecting a capacitor between that pin and GND.

ATmega16/32 ADC Registers

In AVR ADC, we need to understand four main register -

1. ADCH: Holds digital converted data higher byte
2. ADCL: Holds digital converted data lower byte
3. ADMUX: ADC Multiplexer selection register
4. ADCSRA: ADC Control and status register

ADCH: ADCL register

First, two-register holds the digital converted data, which is 10-bit.

ADMUX Register

Bit 7: 6 – REFS1 : 0: Reference Selection Bits 

Reference voltage selection for ADC

Bit 5 – ADLAR: ADC Left Adjust Result

Use 10-bits output as upper bits or lower bits in ADCH & ADCL.

Bits 4 : 0 – MUX4 : 0: Analog Channel and Gain Selection Bits

We can select input channel ADC0 to ADC7 by using these bits. These bits are also used to select comparator (inbuilt in AVR) inputs with various gain. We will cover these comparator operations in another part.

Selecting a channel is very easy, just put the channel number in MUX4 : 0.

Suppose you are connecting the input to ADC channel 2 then put 00010 in MUX4 : 0.

Suppose you are connecting the input to ADC channel 5 then put 00101 in MUX4 : 0.

ADCSRA Register:

• Bit 7 – ADEN: ADC Enable

Writing one to this bit enables the ADC. By writing it to zero, the ADC is turned off. Turning the ADC off while a conversion is in progress, will terminate this conversion.

•  Bit 6 – ADSC: ADC Start Conversion

Writing one to this bit starts the conversion.

•  Bit 5 – ADATE: ADC Auto Trigger Enable

Writing one to this bit, results in Auto Triggering of the ADC is enabled.

•  Bit 4 – ADIF: ADC Interrupt Flag

This bit is set when an ADC conversion completes and the Data Registers are updated.

•  Bit 3 – ADIE: ADC Interrupt Enable

Writing one to this bit, the ADC Conversion Complete Interrupt is activated.

• Bits 2 : 0 – ADPS2 : 0: ADC Prescaler Select Bits

These bits determine the division factor between the XTAL frequency and the input clock to the ADC

We can select any divisor and set frequency Fosc/2, Fosc/4, etc. for ADC, But in AVR, ADC requires an input clock frequency less than 200KHz for max. accuracy. So we have to always take care of not exceeding ADC frequency more than 200KHz.

Suppose your clock frequency of AVR is 8MHz, then we must have to use devisor 64 or 128. Because it gives 8MHz/64 = 125KHz, which is lesser than 200KHz.

ATmega16/32 ADC Interfacing Diagram

Here we are displaying ADC channel 0 values on the 16x2 LCD using ATmega16/32 Microcontroller

Steps to Program ATmega16/32 ADC

1. Make the ADC channel pin as an input.
2. Set ADC enable bit in ADCSRA, select the conversion speed using ADPS2 : 0. For example, we will select devisor 128.
3. Select ADC reference voltage using REFS1: REFS0 in ADMUX register, for example, we will use AVcc as a reference voltage.
4. Select the ADC input channel using MUX4 : 0 in ADMUX, for example, we will use channel 0.
5. So our value in register ADCSRA = 0x87 and ADMUX = 0x40.
6. Start conversion by setting bit ADSC in ADCSRA. E.g.ADCSRA |= (1<<ADSC);
7. Wait for conversion to complete by polling ADIF bit in ADCSRA register.
8. After the ADIF bit gone high, read ADCL and ADCH register to get digital output.
9. Notice that read ADCL before ADCH; otherwise result will not be valid.

ATmega16/32 ADC Code

#define F_CPU 8000000UL 
#include <avr/io.h>
#include <util/delay.h>
#include <stdlib.h>
#include "LCD_16x2_H.h"

void ADC_Init()
{
	DDRA=0x0;			/* Make ADC port as input */
	ADCSRA = 0x87;			/* Enable ADC, fr/128  */
	ADMUX = 0x40;			/* Vref: Avcc, ADC channel: 0 */
	
}

int ADC_Read(char channel)
{
	int Ain,AinLow;
	
	ADMUX=ADMUX|(channel & 0x0f);	/* Set input channel to read */

	ADCSRA |= (1<<ADSC);		/* Start conversion */
	while((ADCSRA&(1<<ADIF))==0);	/* Monitor end of conversion interrupt */
	
	_delay_us(10);
	AinLow = (int)ADCL;		/* Read lower byte*/
	Ain = (int)ADCH*256;		/* Read higher 2 bits and 
					Multiply with weight */
	Ain = Ain + AinLow;				
	return(Ain);			/* Return digital value*/
}

 
int main()
{
	char String[5];
	int value;

	ADC_Init();
	LCD_Init();			/* Initialization of LCD */
	LCD_String("ADC value");	/* Write string on 1st line of LCD */

	while(1)
	{
	
		LCD_Command(0xc4);	/* LCD16x2 cursor position */
		value=ADC_Read(0);	/* Read ADC channel 0 */
		itoa(value,String,10);	/* Integer to string conversion */ 
		LCD_String(String);						
		LCD_String("  ");			
	}
	return 0;
}

Video