The ADC of the AVR


Analog to Digital Conversion

AVR Series

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Most real world data is analog. Whether it be temperature, pressure, voltage, etc, their variation is always analog in nature. For example, the temperature inside a boiler is around 800°C. During its light-up, the temperature never approaches directly to 800°C. If the ambient temperature is 400°C, it will start increasing gradually to 450°C, 500°C and thus reaches 800°C over a period of time. This is an analog data.

Signal Acquisition Process

Signal Acquisition Process

Now, we must process the data that we have received. But analog signal processing is quite inefficient in terms of accuracy, speed and desired output. Hence, we convert them to digital form using an Analog to Digital Converter (ADC).

Signal Acquisition Process

In general, the signal (or data) acquisition process has 3 steps.

  • In the Real World, a sensor senses any physical parameter and converts into an equivalent analog electrical signal.
  • For efficient and ease of signal processing, this analog signal is converted into a digital signal using an Analog to Digital Converter (ADC).
  • This digital signal is then fed to the Microcontroller (MCU) and is processed accordingly.
ADC Pins - ATMEGA16/32

ADC Pins – ATMEGA16/32

Interfacing Sensors

In general, sensors provide with analog output, but a MCU is a digital one. Hence we need to use ADC. For simple circuits, comparator op-amps can be used. But even this won’t be required if we use a MCU. We can straightaway use the inbuilt ADC of the MCU. In ATMEGA16/32, PORTA contains the ADC pins.

The ADC of the AVR

The AVR features inbuilt ADC in almost all its MCU. In ATMEGA16/32, PORTA contains the ADC pins. Some other features of the ADC are as follows:

ADC Features - ATMEGA16/32

ADC Features – ATMEGA16/32

Right now, we are concerned about the 8 channel 10 bit resolution feature.

  • 8 channel implies that there are 8 ADC pins are multiplexed together. You can easily see that these pins are located across PORTA (PA0…PA7).
  • 10 bit resolution implies that there are 2^10 = 1024 steps (as described below).
8 channel 10 bit ADC

8 channel 10 bit ADC

Suppose we use a 5V reference. In this case, any analog value in between 0 and 5V is converted into its equivalent ADC value as shown above. The 0-5V range is divided into 2^10 = 1024 steps. Thus, a 0V input will give an ADC output of 0, 5V input will give an ADC output of 1023, whereas a 2.5V input will give an ADC output of around 512. This is the basic concept of ADC.

To those whom it might concern, the type of ADC implemented inside the AVR MCU is of Successive Approximation type.

Apart from this, the other things that we need to know about the AVR ADC are:

  • ADC Prescaler
  • ADC Registers – ADMUX, ADCSRA, ADCH, ADCL and SFIOR

ADC Prescaler

The ADC of the AVR converts analog signal into digital signal at some regular interval. This interval is determined by the clock frequency. In general, the ADC operates within a frequency range of 50kHz to 200kHz. But the CPU clock frequency is much higher (in the order of MHz). So to achieve it, frequency division must take place. The prescaler acts as this division factor. It produces desired frequency from the external higher frequency. There are some predefined division factors – 2, 4, 8, 16, 32, 64, and 128. For example, a prescaler of 64 implies F_ADC = F_CPU/64. For F_CPU = 16MHz, F_ADC = 16M/64 = 250kHz.

Now, the major question is… which frequency to select? Out of the 50kHz-200kHz range of frequencies, which one do we need? Well, the answer lies in your need. There is a trade-off between frequency and accuracy. Greater the frequency, lesser the accuracy and vice-versa. So, if your application is not sophisticated and doesn’t require much accuracy, you could go for higher frequencies.

ADC Registers

We will discuss the registers one by one.

ADMUX – ADC Multiplexer Selection Register

The ADMUX register is as follows.

ADMUX

ADMUX Register

The bits that are highlighted are of interest to us. In any case, we will discuss all the bits one by one.

  • Bits 7:6 – REFS1:0 – Reference Selection Bits – These bits are used to choose the reference voltage. The following combinations are used.
Reference Voltage Selection

Reference Voltage Selection

ADC Voltage Reference Pins

ADC Voltage Reference Pins

The ADC needs a reference voltage to work upon. For this we have a three pins AREF, AVCC and GND. We can supply our own reference voltage across AREF and GND. For this, choose the first option. Apart from this case, you can either connect a capacitor across AREF pin and ground it to prevent from noise, or you may choose to leave it unconnected. If you want to use the VCC (+5V), choose the second option. Or else, choose the last option for internal Vref.

Let’s choose the second option for Vcc = 5V.

  • Bit 5 – ADLAR – ADC Left Adjust Result – Make it ‘1’ to Left Adjust the ADC Result. We will discuss about this a bit later.
  • Bits 4:0 – MUX4:0 – Analog Channel and Gain Selection Bits – There are 8 ADC channels (PA0…PA7). Which one do we choose? Choose any one! It doesn’t matter. How to choose? You can choose it by setting these bits. Since there are 5 bits, it consists of 2^5 = 32 different conditions as follows. However, we are concerned only with the first 8 conditions. Initially, all the bits are set to zero.
Input Channel and Gain Selections

Input Channel and Gain Selections

Thus, to initialize ADMUX, we write

ADMUX = (1<<REFS0);

ADCSRA – ADC Control and Status Register A

The ADCSRA register is as follows.

ADCSRA Register

ADCSRA Register

The bits that are highlighted are of interest to us. In any case, we will discuss all the bits one by one.

  • Bit 7 – ADEN – ADC Enable – As the name says, it enables the ADC feature. Unless this is enabled, ADC operations cannot take place across PORTA i.e. PORTA will behave as GPIO pins.
  • Bit 6 – ADSC – ADC Start Conversion – Write this to ‘1’ before starting any conversion. This 1 is written as long as the conversion is in progress, after which it returns to zero. Normally it takes 13 ADC clock pulses for this operation. But when you call it for the first time, it takes 25 as it performs the initialization together with it.
  • Bit 5 – ADATE – ADC Auto Trigger Enable – Setting it to ‘1’ enables auto-triggering of ADC. ADC is triggered automatically at every rising edge of clock pulse. View the SFIOR register for more details.
  • Bit 4 – ADIF – ADC Interrupt Flag – Whenever a conversion is finished and the registers are updated, this bit is set to ‘1’ automatically. Thus, this is used to check whether the conversion is complete or not.
  • Bit 3 – ADIE – ADC Interrupt Enable – When this bit is set to ‘1’, the ADC interrupt is enabled. This is used in the case of interrupt-driven ADC.
  • Bits 2:0 – ADPS2:0 – ADC Prescaler Select Bits – The prescaler (division factor between XTAL frequency and the ADC clock frequency) is determined by selecting the proper combination from the following.
ADC Prescaler Selections

ADC Prescaler Selections

Assuming XTAL frequency of 16MHz and the frequency range of 50kHz-200kHz, we choose a prescaler of 128.

Thus, F_ADC = 16M/128 = 125kHz.

Thus, we initialize ADCSRA as follows.

ADCSRA = (1<<ADEN)|(1<<ADPS2)|(1<<ADPS1)|(1<<ADPS0);
// prescaler = 128

ADCL and ADCH – ADC Data Registers

The result of the ADC conversion is stored here. Since the ADC has a resolution of 10 bits, it requires 10 bits to store the result. Hence one single 8 bit register is not sufficient. We need two registers – ADCL and ADCH (ADC Low byte and ADC High byte) as follows. The two can be called together as ADC.

ADC Data Registers (ADLAR = 0)

ADC Data Registers (ADLAR = 0)

ADC Data Registers (ADLAR = 1)

ADC Data Registers (ADLAR = 1)

You can very well see the the effect of ADLAR bit (in ADMUX register). Upon setting ADLAR = 1, the conversion result is left adjusted.

SFIOR – Special Function I/O Register

In normal operation, we do not use this register. This register comes into play whenever ADATE (in ADCSRA) is set to ‘1’. The register goes like this.

SFIOR Register

SFIOR Register

The bits highlighted in yellow will be discussed as they are related to ADATE. Other bits are reserved bits.

  • Bits 7:5 – ADC Auto Trigger Source – Whenever ADATE is set to ‘1’, these bits determine the trigger source for ADC conversion. There are 8 possible trigger sources.
ADC Auto Trigerring Source Selections

ADC Auto Triggering Source Selections

These options are will be discussed in the posts related to timers. Those who have prior knowledge of timers can use it. The rest can leave it for now, we won’t be using this anyway.

ADC Initialization

The following code segment initializes the ADC.

void adc_init()
{
    // AREF = AVcc
    ADMUX = (1<<REFS0);

    // ADC Enable and prescaler of 128
    // 16000000/128 = 125000
    ADCSRA = (1<<ADEN)|(1<<ADPS2)|(1<<ADPS1)|(1<<ADPS0);
}

Reading ADC Value

The following code segment reads the value of the ADC. Always refer to the register description above for every line of code.

uint16_t adc_read(uint8_t ch)
{
  // select the corresponding channel 0~7
  // ANDing with ’7′ will always keep the value
  // of ‘ch’ between 0 and 7
  ch &= 0b00000111;  // AND operation with 7
  ADMUX = (ADMUX & 0xF8)|ch; // clears the bottom 3 bits before ORing

  // start single convertion
  // write ’1′ to ADSC
  ADCSRA |= (1<<ADSC);

  // wait for conversion to complete
  // ADSC becomes ’0′ again
  // till then, run loop continuously
  while(ADCSRA & (1<<ADSC));

  return (ADC);
}

Physical Connections

Let’s connect two LDRs (Light Dependent Resistors) to pins PA0 and PA1 respectively. The connection is as follows. The function of potentiometers is explained in a later section, Sensor Calibration. You can scroll down to it. 😉

LDR Connections

LDR Connections

Now suppose we want to display the corresponding ADC values in an LCD. So, we also need to connect an LCD to our MCU. Read this post to know about LCD interfacing.

Since it is an LDR, it senses the intensity of light and accordingly change its resistance. The resistance decreases exponentially as the light intensity increases. Suppose we also want to light up an LED whenever the light level decreases. So, we can connect the LED to any one of the GPIO pins, say PC0.

Note that since the ADC returns values in between 0 and 1023, for dark conditions, the value should be low (below 100 or 150) whereas for bright conditions, the value should be quite high (above 900).

Now let’s write the complete code.

Example Code

To learn about LCD interfacing, view this post. You can type, compile and build it in AVR Studio 5. View this page to know how. To know about the I/O port operations in AVR, view this page.

#include <avr/io.h>
#include <util/delay.h>

#include "lcd.h"

#define LTHRES 500
#define RTHRES 500

// initialize adc
void adc_init()
{
    // AREF = AVcc
    ADMUX = (1<<REFS0);

    // ADC Enable and prescaler of 128
    // 16000000/128 = 125000
    ADCSRA = (1<<ADEN)|(1<<ADPS2)|(1<<ADPS1)|(1<<ADPS0);
}

// read adc value
uint16_t adc_read(uint8_t ch)
{
    // select the corresponding channel 0~7
    // ANDing with '7' will always keep the value
    // of 'ch' between 0 and 7
    ch &= 0b00000111;  // AND operation with 7
    ADMUX = (ADMUX & 0xF8)|ch;     // clears the bottom 3 bits before ORing

    // start single conversion
    // write '1' to ADSC
    ADCSRA |= (1<<ADSC);

    // wait for conversion to complete
    // ADSC becomes '0' again
    // till then, run loop continuously
    while(ADCSRA & (1<<ADSC));

    return (ADC);
}

int main()
{
    uint16_t adc_result0, adc_result1;
    char int_buffer[10];
    DDRC = 0x01;           // to connect led to PC0

    // initialize adc and lcd
    adc_init();
    lcd_init(LCD_DISP_ON_CURSOR);

    // display the labels on LCD
    lcd_puts("left  ADC = ");
    lcd_gotoxy(0,1);
    lcd_puts("right ADC = ");

    _delay_ms(50);

    while(1)
    {
        adc_result0 = adc_read(0);      // read adc value at PA0
        adc_result1 = adc_read(1);      // read adc value at PA1

        // condition for led to glow
        if (adc_result0 < LTHRES && adc_result1 < RTHRES)
            PORTC = 0x01;
        else
            PORTC = 0x00;

        // now display on lcd
        itoa(adc_result0, int_buffer, 10);
        lcd_gotoxy(12,0);
        lcd_puts(int_buffer);

        itoa(adc_result0, int_buffer, 10);
        lcd_gotoxy(12,1);
        lcd_puts(int_buffer);
        _delay_ms(50);
    }
}

Sensor Calibration

Calibration means linking your real world data with the virtual data. In the problem statement given earlier, I have mentioned that the LED should glow if the light intensity reduces. But when should it start to glow? The MCU/code doesn’t know by itself. You get the readings from the sensor continuously in between 0 and 1023. So, the question is how do we know that below ‘such and such’ level the LED should glow?

This is achieved by calibration. You need to physically set this value. What you do is that you run the sensor for all the lighting conditions. You have the ADC values for all these levels. Now, you need to physically see and check the conditions yourself and then apply a threshold. Below this threshold, the light intensity goes sufficiently down enough for the LED to glow.

The potentiometer connected in the circuit is also for the same reason. Now, by the basic knowledge of electronics, you could easily say that upon changing the pot value the ADC value changes. Thus, for various reasons (like poor lighting conditions, you are unable to distinguish between bright and dark conditions, etc), you can vary the pot to get desired results.

This is why I have given the two thresholds (RTHRES anf LTHRES) in the beginning of the code.

So, this is all with the ADC. I hope you enjoyed reading this. Please post the comments below for any suggestion, doubt, clarification, etc.

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200 responses to “The ADC of the AVR

  1. it seem’s that you miss stdlib.h
    i use Atmel Studio 6.1 and when i copy the code it show error abt using itoa() func that it’s been declared implicit and isn’t valid in C99

  2. Sir:
    Cant find the reply button so i create a new one here.
    I will try to combine it into a RS232 data format, with 2 bytes each. The reason to combine the data is cause i just want to use 1 MCU in the receiving instate of 3 MCU for the receiving. As later on i will only need one cable to send to either PC or phone app.

  3. What if I want to simultaneously sample the input at multiple pins? What do I set the MUX bits to then? And will all pins have separate places to store the converted values?

      • yeah i got ur point.but if i want to generate PWM of 1hz with 8 prescaller so what should be the TCCRO statement and what should be stored in OCR1A.i thought that
        TCCR0 |= (1<<WGM00)|(1<<COM01)|(1<<WGM01)|(1<<CS01);
        is it true??? but what to do 1Khz freq.. facing trouble….

        • Rida, did you try to implement it what result did it produce? What value of OCR0 did you set?
          PS. Since you are using Timer0, you’ll have to use OCR0 register and NOT OCR1A (as you mentioned in the question above).

  4. hellow sir ,
    ur posts are best.. i have 1 confusion that if i want to read 10-bit ADC and save its value in some variable ‘x’ then how can i reflect it on 8-bit PWM..

  5. sir, I am beginner in embedded learning. I didn’t understand the following line code in above tutorial. please guide.

    ch &= 0b00000111;
    ADMUX = (ADMUX & 0xF8)|ch;

    while(ADCSRA & (1<<ADSC));

      • Thank you sir, for your the link. I have read the article and understood the bitwise manipulation. But still did not understood that how 0b00000111 no came from. please guide

        • Shrikant, 0b00000111 is the binary representation of decimal 7. In Atmega16/32, there are only 8 ADC channels (0-7). We don’t want to read inappropriate value if the user inputs any value greater than 7. This is why we have masked the other bits. Makes sense?

  6. hi sir.
    i have a query in “Reading ADC Value”
    u have written
    return (ADC); in code line 18 whereas u used ch as variable.
    as far as i know we write the variable name in return statement. why did you write adc in return statement and what does it signifies?

    thanku!!

  7. hey can you provide me with code for interfacing lcd in 4-bit mode with portc as rs is PC1, en as PC2, and 4 data pins as PC4-PC7 i stuck with it and i won’t get any solution and R/W pin is connected to groung by default and can you also help me out with interfacing LM35 Temperature Sensor

    Please help me out:::

    my code that i had tested is: http://pastebin.com/iJXEpKzd

  8. This code doesn’t work in studio 4. sir, could you please give a code for studio 4…need for a college project

    • Atmel has released version 6.1 for the software. Are you from a country where people like to use outdated and obsolete software?
      Btw, the code should remain the same, only the settings should vary, which I’m not aware of. I have never used the version you’re talking about.

  9. hello sir, i hav a bunch of problems,plz clarify me regarding the adc test code which i hav written from different sources,
    http://pastebin.com/teCuXB8J
    the code was good n no errors were found,but when i dumped into atmega8(proteus simulation),
    1)it says “simulation not running in real time due to excessive CPU load”
    2)i tried decreasing the clock frequency to 12Mhz,it was better (CPU load=85%),dont know y!!,no error ,but no output.
    3)set the CKSEL fuses setting to “(1111)Ext.crystal high frequency”, is there anything wrong in it ..(or have to use Int RC 1MHz)
    plz help in sorting out the problem…
    ….hoping for a sooner reply

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