// this code scans ADC1 for an analog signal upon request, using 8Mhz processor clock

#include <avr/io.h>
#include <stdint.h>       // needed for uint8_t
    
int ADCsingleREAD(uint8_t adctouse)
{
    int ADCval;

    ADMUX = adctouse;        // use #1 ADC
    ADMUX |= (1 << REFS0);   // use AVcc as the reference
    ADMUX &= ~(1 << ADLAR);  // clear for 10 bit resolution
    
    // 128 prescale for 8Mhz
    ADCSRA |= (1 << ADPS2) | (1 << ADPS1) | (1 << ADPS0);
        
    ADCSRA |= (1 << ADEN);    // Enable the ADC
    ADCSRA |= (1 << ADSC);    // Start the ADC conversion
    
    while(ADCSRA & (1 << ADSC)) // waits for the ADC to finish 
        ;
    
    ADCval = ADCL;
    ADCval = (ADCH << 8) + ADCval; // ADCH is read so ADC can be updated again
    
    return ADCval;
}
   

int main(void)
{
    int ADCvalue;

    while (1)
    {
        ADCvalue = ADCsingleREAD(1);
        // ADCvalue now contains an 10bit ADC read
    }
}

// this code scans ADC1 for an analog signal upon request, using 8Mhz processor clock

#include <avr/io.h>
#include <stdint.h>       // needed for uint8_t
    
int ADCsingleREAD(uint8_t adctouse)
{
    int ADCval;

    ADMUX = adctouse;        // use #1 ADC
    ADMUX |= (1 << REFS0);   // use AVcc as the reference
    ADMUX &= ~(1 << ADLAR);  // clear for 10 bit resolution
    
    // 128 prescale for 8Mhz
    ADCSRA |= (1 << ADPS2) | (1 << ADPS1) | (1 << ADPS0);
        
    ADCSRA |= (1 << ADEN);    // Enable the ADC
    ADCSRA |= (1 << ADSC);    // Start the ADC conversion
    
    while(ADCSRA & (1 << ADSC)); // waits for the ADC to finish
    
    ADCval = ADCL;
    ADCval = (ADCH << 8) + ADCval; // ADCH is read so ADC can be updated again
    
    return ADCval;
}
   

int main(void)
{
    int ADCvalue;

    while (1)
    {
        ADCvalue = ADCsingleREAD(1);
        // ADCvalue now contains an 10bit ADC read
    }
}

For a quick example from Arduino's website

    DDRD is the direction register for Port D (Arduino digital pins 0-7). The bits in this register control whether the pins in PORTD are configured as inputs or outputs so, for example:    

DDRD = B11111110; // sets Arduino pins 1 to 7 as outputs, pin 0 as input DDRD = DDRD | B11111100; // this is safer as it sets pins 2 to 7 as outputs     // without changing the value of pins 0 & 1, which are RX & TX 

//See the bitwise operators reference pages and The Bitmath Tutorial in the Playground

PORTD is the register for the state of the outputs. For example;

PORTD = B10101000; // sets digital pins 7,5,3 HIGH

You will only see 5 volts on these pins however if the pins have been set as outputs using the DDRD register or with pinMode().

For analog read

// this code scans ADC1 for an analog signal upon request, using 8Mhz processor clock
 

#include <avr/io.h>
 
#include <stdint.h>       // needed for uint8_t

 

int ADCsingleREAD(uint8_t adctouse)
 
{
 
    int ADCval;
 

    ADMUX = adctouse;          // use #1 ADC
 
    ADMUX |= (1 << REFS0);     // use AVcc as the reference
 
    ADMUX &= ~(1 << ADLAR);    // clear for 10 bit resolution
 
      

      ADCSRA |= (1 << ADPS2) | (1 << ADPS1) | (1 << ADPS0);     // 128 prescale for 8Mhz

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

 
    ADCSRA |= (1 << ADSC);    // Start the ADC conversion
 

    while(ADCSRA & (1 << ADSC));      // waits for the ADC to finish 

    ADCval = ADCL;
 
    ADCval = (ADCH << 8) + ADCval;     // ADCH is read so ADC can be updated again
 

    return ADCval;
 
}
   

   int main(void)
 
{
 
    int ADCvalue;
 

    while (1)
 
    {
 
            ADCvalue = ADCsingleREAD(1);
 
             // ADCvalue now contains an 10bit ADC read
 
    }
 
}

The following is an example from the Arduino website.

DDRD is the direction register for Port D (Arduino digital pins 0-7). The bits in this register control whether the pins in PORTD are configured as inputs or outputs so, for example:

DDRD = B11111110; // sets Arduino pins 1 to 7 as outputs, pin 0 as input 
DDRD = DDRD | B11111100; // this is safer as it sets pins 2 to 7 as outputs 
                         // without changing the value of pins 0 & 1, which are RX & TX 

See the bitwise operators reference pages and The Bitmath Tutorial in the Playground

PORTD is the register for the state of the outputs. For example;

PORTD = B10101000; // sets digital pins 7,5,3 HIGH

You will only see 5 volts on these pins however if the pins have been set as outputs either by using the DDRD register directly or by using the pinMode() function.

For analog read

// this code scans ADC1 for an analog signal upon request, using 8Mhz processor clock

#include <avr/io.h>
#include <stdint.h>       // needed for uint8_t
    
int ADCsingleREAD(uint8_t adctouse)
{
    int ADCval;

    ADMUX = adctouse;        // use #1 ADC
    ADMUX |= (1 << REFS0);   // use AVcc as the reference
    ADMUX &= ~(1 << ADLAR);  // clear for 10 bit resolution
    
    // 128 prescale for 8Mhz
    ADCSRA |= (1 << ADPS2) | (1 << ADPS1) | (1 << ADPS0); 
        
    ADCSRA |= (1 << ADEN);    // Enable the ADC
    ADCSRA |= (1 << ADSC);    // Start the ADC conversion
    
    while(ADCSRA & (1 << ADSC)) // waits for the ADC to finish 
        ;
    
    ADCval = ADCL;
    ADCval = (ADCH << 8) + ADCval; // ADCH is read so ADC can be updated again
    
    return ADCval;
}
   

int main(void)
{
    int ADCvalue;

    while (1)
    {
        ADCvalue = ADCsingleREAD(1);
        // ADCvalue now contains an 10bit ADC read
    }
}