ATmega328P: Reading from ADCL make ADC stop conversion

Question

I am using a simple code to read from ADC pin A0. When I read from ADCH and ADCL. The result is correct only once in the startup. When I change the analog value in A0, output from the ADC don't change at all. Unless I have reset the mcu.

In the datasheet it is not state clearly that there is a register called ADC. They highlight ADCH and ADCL.

reading from register ADC work fine.

This is my simple code that don't work.

/*
 * Section: Function Defintion
 */
void ADC_Setup(){
    // Enable the ADC
    ADCSRA |= 1<<ADEN;
    // Select the refence voltage to be AVcc, which is connected to Vcc in the Arduino Nano board
    ADMUX |= 1<<REFS0;
    // Set Prescaler to 128, so that the ADC clock is 16M/128= 125Khz
    ADCSRA |= (1<<ADPS2)|(1<<ADPS1)|(1<<ADPS0);
    ADCSRA |= (1<<ADSC) | (1<<ADATE) | (1<<ADIF) | (1<<ADIF);
    sei();
    
}
void ADC_StartConvert(){
    ADCSRA |= (1<<ADSC);
}
uint16_t ADC_read(){
    uint16_t i = ADCH*256 + ADCL;
    // if changed to i = ADC; it work fine!!
    return i;
}

What is could be the cause to make the ADC hold conversion because of reading from ADCL?

Answer 1

ADCL must be read first before ADCH, as stated in the datasheet.

Use this code

/*
 * Section: Function Defintion
 */
void ADC_Setup(){
    // Enable the ADC
    ADCSRA |= 1<<ADEN;
    // Select the refence voltage to be AVcc, which is connected to Vcc in the Arduino Nano board
    ADMUX |= 1<<REFS0;
    // Set Prescaler to 128, so that the ADC clock is 16M/128= 125Khz
    ADCSRA |= (1<<ADPS2)|(1<<ADPS1)|(1<<ADPS0);
    ADCSRA |= (1<<ADSC) | (1<<ADATE) | (1<<ADIF);
    sei();
    
}
void ADC_StartConvert(){
    ADCSRA |= (1<<ADSC);
}
uint16_t ADC_read(){
    uint16_t i = ADCL + (((uint16_t)(ADCH))<<8);
    return i;
}