ATMega644P Analog to Digital Converter (ADC) Example

Overview

The Analog to Digital Converter (ADC) convert analog voltage value to a digital representation that the micro-processor could understand. Most of modern micro-controller has at least one built-in ADC module that can be use by software configuration. The most common ADC resolution is between 8-bit and 12-bit with an acceptable conversion rate.

 

The resolution give more precise digital data for the micro-controller. However its reference voltage and lower clock also effect the precision of the analog input value. Some 32-bit micro-controller especially the STM32 micro-controller series has an ADC module with the resolution of 12-bit with the conversion rate up to 2MSPS. Some 16-bit dsPIC micro-controller series has an ADC module with the resolution of 10-bit and 1~2MSPS conversion rate.

Commonly we use the ADC to read temperature value, flex sensor and strain gauge etc.

ATMega644P ADC Programming Example

The ADC of the ATMega644P has a 10-bit analog to digital converter with the maximum conversion rate up to 15kSPS. It has up to 8 analog input channel (ADC7:ADC0) via PortA. The conversion triggers by user software (set ADC to start) or by any interrupt sources.

Analog-to-digital Converter Block Schematic

 

The software configuration of the ADC module of AVR micro-controller is simpler than other micro-controllers. It has a few registers to configure. 

I don't write its detail here since the ADC process of the ATMega644P is very similar to the ATMega32. 

ADC and LED Example 1

In this introductory example the ATMega644P read the analog input from ADC0(PA0). The 10-bit ADC result will converts to 8-bit by shifting right that will display on PORTB.

C Source Code:

1. /*
2. * 9-ADC_LED.c
3. *
4. * Created: 1/30/2026 3:28:06 PM
5. * Author : Admin
6. */
7. 
   
8. #include <avr/io.h>
9. 
   
10. int main(void)
11. {
13. DDRB=0xFF;
14. //PA0 or ADC0 as an analog input
15. DDRA=0;
16. //Turn on the ADC module
17. ADCSRA=(1<<ADEN);
18. uint16_t temp;
19. while (1)
20. {
21. //Start the conversion
22. ADCSRA|=(1<<ADSC);
23. //Wait for the completion
24. while((ADCSRA&(1<<ADSC))==1);
25. //Read the result
26. temp=ADCL+(ADCH<<8);
27. PORTB=temp>>2;
28. }
29. }
30.  

Proteus Simulation:

Proteus Simulation #1

AVR Prototype Board Test:

 

ADC and LED Example 2

I use this prototype board's LED and POT to make a LED shifting using the ADC voltage level. It's very similar to a ring counter. However I need to convert between ADC value counter value.

Source Code:

1. /*
2. * 9-ADC_BarGraph.c
3. *
4. * Created: 1/30/2026 4:21:13 PM
5. * Author : Admin
6. */
7. 
   
8. #include <avr/io.h>
9. 
   
10. int main(void)
11. {
13. DDRB=0xFF;
14. //PA0 or ADC0 as an analog input
15. DDRA=0;
16. //Turn on the ADC module
17. ADCSRA=(1<<ADEN);
18. long temp,div;
19. while (1)
20. {
21. //Start the conversion
22. ADCSRA|=(1<<ADSC);
23. //Wait for the completion
24. while((ADCSRA&(1<<ADSC))==1);
25. //Read the result
26. temp=ADCL+(ADCH<<8);
27. div=(temp*8)/1024;
28. PORTB=(1<<div);
29. }
30. }
31. 
    

Proteus Schematic and Simulation:

Proteus Schematic and Simulation

AVR Prototype Board Test: 

 

ADC and LED Example 2

There are two on-board ADC inputs, POT and LM35 temperature sensor. The LCD will display the analog input from POT(ADC0) and LM35(ADC1). The LM35 temperature (in degree Celsius) is equivalent to 10mV that's 2 in digital value (+5.0VDC voltage reference).

LM35DZ in TO-92 Package

 

Source Code:

1. /*
2. * 9-ADC_LCD.c
3. *
4. * Created: 1/30/2026 4:47:25 PM
5. * Author : Admin
6. */
7. 
   
8. #include <avr/io.h>
9. #include <stdio.h>
10. #include <util/delay.h>
11. #define F_CPU 16000000UL
12. 
    
13. void adc_init(void){
14. /*Set PA0 and PA1 to input*/
15. DDRA&=~(1<<0);
16. DDRA&=~(1<<1);
17. ADCSRA=(1<<ADEN);
18. /*Select AD0*/
19. ADMUX=0x03;
20. }
21. 
    
22. unsigned int read_adc(char channel){
23. //Select Channel
24. ADMUX=channel;
25. //Start Convsersion
26. ADCSRA|=(1<<ADSC);
27. //Wait for Completion
28. while(ADCSRA&(1<<ADSC));
29. //Get the 10-bit values
30. return (ADCL+(ADCH<<8));
31. }
32. int main(void)
33. {
34. /* Replace with your application code */
35. lcd_init();
36. lcd_text("ATMega644P ADC");
37. lcd_xy(1,2);
38. lcd_text("POT AND LM35");
39. adc_init();
40. _delay_ms(1000);
41. lcd_clear();
42. unsigned int temp;
43. float voltage;
44. char msg[16];
45. lcd_command(0x0C);
47. while (1)
48. {
49. //Read POT
50. temp=read_adc(0);
51. //Convert To Voltage
52. voltage=temp*5.0/1023;
53. sprintf(msg,"ADC0: %4d %.2fV",temp,voltage);
54. lcd_xy(1,1); lcd_text(msg);
55. //Read LM35
56. temp=read_adc(1);
57. voltage=temp*5.0/1023;
58. voltage*=100;
59. sprintf(msg,"Temp: %0.1f%cC",voltage,223);
60. lcd_xy(1,2); lcd_text(msg);
61. _delay_ms(1000);
62. }
63. }
64. 
    

Proteus Schematic:

Proteus Schematic

AVR Prototype Board: 

Program Tested on my AVR Prototype Board

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