I'm using atmega32 µc with the internal 10-bit ADC. I connected my potentiometer to the ADC 1 and I'm using 5 V Voltage Reference.

using the following code, I did the ADC-config

void ADC_init(void)

    //Turn on the ADC
    ADCSRA |= 1<<ADEN;   // Turn On ADC (Enable ADC)

    //ADMUX  |= 1<<ADLAR ;  // Config shifting to the left (10-bit), otherwise it is right adjusted

    ADMUX  |= 1<<REFS0 ;  // Config VRef (external Vref in this case)
    ADMUX  |= 1<<MUX1 ;  // ADC-Channel 1

//  ADCSRA |= 1<<ADIE;   // Enable ADC Interrupt
    ADCSRA |= 3<<ADPS0;  // Assign ADC-prescaler 128

    ADCSRA |= 1<<ADSC;   //start the the measure conversion

Then in Main function I am calling the following function each 1000ms and send the data to the USART interface

void ADC_measure(void)

    // if the ADC right adjusted
    theLow = ADCL;  // ADC Value Low

    ADC_TenBit = ADCH<<8 | theLow  ;  // because of lift adjustment of ADC


    sprintf( ADCBuffer, "The ADCValue is : %d \n", ADC_TenBit );



    //start the the measure conversion
    ADCSRA |= 1<<ADSC;

The problem is : when I open the terminal , I see only the ADC Value = 255 or 248 increasing or decreasing the potentiometer value doesn't change the ADC value.

what can I do now?

  • \$\begingroup\$ You're using the pot as a voltage divider, right? \$\endgroup\$ – Ignacio Vazquez-Abrams Sep 7 '14 at 15:59
  • \$\begingroup\$ Why not use the ADC interrupt? I have no idea if your conversion is finished when you're reading the result register, because you're not even checking if it's done -- you're just delaying some arbitrary amount of time, and blindly reading the register. There are two ways I typically see ADC code implemented: using interrupts, or burning time in a while loop waiting for the "conversion finished" flag to be set. \$\endgroup\$ – Jay Carlson Sep 7 '14 at 16:18

You have incorrect multiplexer setting. To select channel 1, it should be

ADMUX  |= 1<<MUX0 ;  // ADC-Channel 1

Edit: Another thing is incorrect clock prescaler. To get \$f_{clk}/128\$:

ADCSRA |= 7<<ADPS0;  // Assign ADC-prescaler 128
  • \$\begingroup\$ thanks for your reply. but it still have the same problem \$\endgroup\$ – user3213767 Sep 7 '14 at 17:05


ADC_TenBit = (uint16_t)ADCH<<8 | theLow;

And you did declare ADC_TenBit uint16_t, right?

  • \$\begingroup\$ thanks for your reply. it works for the first measurement then again fails. the values are 255 or 290 or 245 ...just in this range. independent of the input Signal \$\endgroup\$ – user3213767 Sep 7 '14 at 17:03

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