Difference between revisions of "Reading Temperature with AVR ADC"

 
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[[category:AVR Tutorials]]
 
[[category:AVR Tutorials]]
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The Atmega32 comes with a 10-bit, 8 channel inbuilt Ananlog to Digital Convertor (ADC). We will set it up and read temperature from LM35 and light with a simple (light dependent Resistor)LDR. So let's get started!
 +
=Basics=
 +
The ADCSRA register allows us to enable the adc and set the sampling rate.
 +
'''ADC  Control and Status Register A (ADCSRA):'''<br/>
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{| class = "table table-responsive table-striped "
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|-
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|7||6||5||4||3||2||1||0
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|-
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|ADEN||ADSC||ADATE ||ADIF ||ADIE ||ADPS2 ||ADPS1 ||ADPS0
 +
|-
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|1||0||0||0||0||0||0||1
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|}
 +
 +
♦ '''Bit 7 - ADEN: ADC Enable'''
 +
 +
Writing this bit to one enables the ADC. By writing it to zero, the ADC is turned off.
 +
 +
♦ '''Bit 2:0 - ADPS2:0: ADC Prescaler Select Bits'''
 +
 +
These bits determine the division factor between the XTAL frequency and the input clock to the ADC.
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 +
'''ADC Prescaler Selection'''
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{| class="wikitable"
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|-
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! ADPS2 !! ADPS1 !! ADPS0 !! Division Factor
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|-
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| 0 || 0 || 0 || 2
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|-
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| 0 || 0 || 1 || 2
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|-
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| 0 || 1 || 0 || 4
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|-
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| 0 || 1 || 1 || 8
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|-
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| 1 || 0 || 0 || 16
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|-
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| 1 || 0 || 1 || 32
 +
|-
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| 1 || 1 || 0 || 64
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|-
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| 1 || 1 || 1 || 128
 +
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|} 
 +
 +
We will first initialize the ADC with the following code
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# Enable ADC by Setting ADEN bit of ADCSRA and set the sampling frequency to half of oscillator frequency (i.e 8 MHz)
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# Select channel zero by default using the ADMUX register shown below.
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'''ADC Multiplexer Select (ADMUX)''' <br/>
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 +
{| class = "table table-responsive table-striped "
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|-
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|7||6||5||4||3||2||1||0
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|-
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|REFS1||REFS0||ADLAR||MUX4||MUX3||MUX2||MUX1||MUX0
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|-
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|0||0||0||0||0||0||0||0
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|}
 +
 +
♦ '''Bit 7:6 - REFS1:0: Reference Selection Bits
 +
 +
These bits select the voltage reference for the ADC, as shown in below table.
 +
{| class="wikitable"
 +
|-
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! REFS1 !! REFS0 !! Voltage Reference Selection
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|-
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| 0 || 0 || AREF,internal Vref turned off
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|-
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| 0 || 1 || AVCC with external Capacitor at AREF pin
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|-
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| 1 || 0 || Reserved
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|-
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| 1 || 1 || Internal 2.56V Voltage Reference with external Capacitor at AREF pin.
 +
|}
 +
♦ '''Bits 4:0 - MUX 4:0 : Analog Channel and Gain Selection Bits'''
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 +
The value of these bits selects which combination of analog inputs are connected to the ADC.
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 +
 +
<syntaxhighlight>
 +
void ADC_Init()
 +
{
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  ADCSRA=(1<<ADEN) | (1<<ADPS0); /* Enable ADC , sampling freq=osc_freq/2 */
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  ADMUX=0x00;                    /* Result right justified, select channel zero */
 +
}
 +
</syntaxhighlight>
 +
Let us write another function to select the required channel and read the analog data as below :
 +
# Select the required channel passed as input to the function
 +
# Start the ADC conversion by setting ADSC bit in ADCSRA
 +
# Wait till the conversion is over by checking the ADIF bit in ADCSRA
 +
# Read the ADCW register and return the result of conversion
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<syntaxhighlight>
 +
uint16_t ADC_GetAdcValue(uint8_t v_adcChannel_u8)
 +
{
 +
 
 +
  ADMUX = v_adcChannel_u8;              /* Select the required channel */
 +
  DELAY_us(10);                          /* Wait for some time for the channel to get selected */
 +
  util_BitSet(ADCSRA,ADSC);              /* Start the ADC conversion by setting ADSC bit */
 +
 
 +
  while(util_IsBitCleared(ADCSRA,ADIF)); /* Wait till the conversion is over */
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                                          /* ADIF will be set once ADC conversion is complete */
 +
    return(ADCW);                        /* Return the 10-bit result */
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}
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</syntaxhighlight>
 +
 +
=Raw Data=
 +
Let's put all of the above together to read raw data from the ADC. We will connect  a Potentiometer(Pot) to channel 0, vary it and read the raw voltage.
 +
==Hookup Pot==
 +
[[File:Reading Temperature with AVR ADC bb.png|none]]
 +
 +
==Code to Read Voltage==
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<html>
 +
<script src="https://gist.github.com/Xplorer001/d46092fcad54a3b628f0.js"></script>
 +
</html>
 +
 +
=Temperature=
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Let's connect the LM35 temperature sensor to channel zero of the ADC and read the temperature.
 +
Resolution with 5V reference can be calculated as:
 +
$$ResolutionOfADC =  (5v/1023)=4.887mv = 5mv$$
 +
For every degree celcius the Lm35 provides 10mv voltage change, therefore
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$$Temperature =  ADCValue/2 $$
 +
 +
==Hookup LM35==
 +
[[File:0Reading Temperature with AVR ADC TEMP bb.png|none]]
 +
 +
==Code to Read Temperature==
 +
<html>
 +
<script src="https://gist.github.com/Xplorer001/b5324c706391fa1e4a77.js"></script>
 +
</html>
 +
[[File:0Digital Temp.png|none]]
 +
 +
=Video Tutorial=
 +
For those of you, who would like to watch instead of read we have made a video with all the gyan.
 +
<br>
 +
{{#ev:youtubehd|-lNAmSNV2-Q|640}}
 +
 +
 +
= Downloads=
 +
Download the complete project folder from the below link:
 +
https://github.com/ExploreEmbedded/ATmega32_ExploreUltraAvrDevKit/archive/master.zip<br>
 +
 +
 +
 +
Have a opinion, suggestion , question or feedback about the article let it out here!
 +
{{DISQUS}}

Latest revision as of 11:57, 19 April 2016

The Atmega32 comes with a 10-bit, 8 channel inbuilt Ananlog to Digital Convertor (ADC). We will set it up and read temperature from LM35 and light with a simple (light dependent Resistor)LDR. So let's get started!

Basics

The ADCSRA register allows us to enable the adc and set the sampling rate. ADC Control and Status Register A (ADCSRA):

7 6 5 4 3 2 1 0
ADEN ADSC ADATE ADIF ADIE ADPS2 ADPS1 ADPS0
1 0 0 0 0 0 0 1

♦ Bit 7 - ADEN: ADC Enable

Writing this bit to one enables the ADC. By writing it to zero, the ADC is turned off.

♦ Bit 2:0 - ADPS2:0: ADC Prescaler Select Bits

These bits determine the division factor between the XTAL frequency and the input clock to the ADC.

ADC Prescaler Selection

ADPS2 ADPS1 ADPS0 Division Factor
0 0 0 2
0 0 1 2
0 1 0 4
0 1 1 8
1 0 0 16
1 0 1 32
1 1 0 64
1 1 1 128

We will first initialize the ADC with the following code

  1. Enable ADC by Setting ADEN bit of ADCSRA and set the sampling frequency to half of oscillator frequency (i.e 8 MHz)
  2. Select channel zero by default using the ADMUX register shown below.

ADC Multiplexer Select (ADMUX)

7 6 5 4 3 2 1 0
REFS1 REFS0 ADLAR MUX4 MUX3 MUX2 MUX1 MUX0
0 0 0 0 0 0 0 0

♦ Bit 7:6 - REFS1:0: Reference Selection Bits

These bits select the voltage reference for the ADC, as shown in below table.

REFS1 REFS0 Voltage Reference Selection
0 0 AREF,internal Vref turned off
0 1 AVCC with external Capacitor at AREF pin
1 0 Reserved
1 1 Internal 2.56V Voltage Reference with external Capacitor at AREF pin.

♦ Bits 4:0 - MUX 4:0 : Analog Channel and Gain Selection Bits

The value of these bits selects which combination of analog inputs are connected to the ADC. 


  1. void ADC_Init()
  2.  {
  3.    ADCSRA=(1<<ADEN) | (1<<ADPS0); /* Enable ADC , sampling freq=osc_freq/2 */
  4.    ADMUX=0x00;                    /* Result right justified, select channel zero */
  5.  }

Let us write another function to select the required channel and read the analog data as below :

  1. Select the required channel passed as input to the function
  2. Start the ADC conversion by setting ADSC bit in ADCSRA
  3. Wait till the conversion is over by checking the ADIF bit in ADCSRA
  4. Read the ADCW register and return the result of conversion 
  1. uint16_t ADC_GetAdcValue(uint8_t v_adcChannel_u8)
  2.  {
  3.  
  4.    ADMUX = v_adcChannel_u8;               /* Select the required channel */
  5.    DELAY_us(10);                          /* Wait for some time for the channel to get selected */
  6.    util_BitSet(ADCSRA,ADSC);              /* Start the ADC conversion by setting ADSC bit */
  7.  
  8.    while(util_IsBitCleared(ADCSRA,ADIF)); /* Wait till the conversion is over */
  9.                                           /* ADIF will be set once ADC conversion is complete */
  10.      return(ADCW);                        /* Return the 10-bit result */
  11.  }

Raw Data

Let's put all of the above together to read raw data from the ADC. We will connect a Potentiometer(Pot) to channel 0, vary it and read the raw voltage.

Hookup Pot

Reading Temperature with AVR ADC bb.png

Code to Read Voltage

#include "adc.h"
#include "uart.h"
int main()
{
int adcValue;
float volt;
ADC_Init(); /* Initialize the ADC module */
UART_Init(9600); /* Initialize UART at 9600 baud rate */
while(1)
{
adcValue = ADC_GetAdcValue(0); // Read the ADC value of channel zero
volt = (adcValue*5.00)/1023; //10bit resolution, 5vReference
UART_Printf("ADC0 Value:%4d Equivalent Voltage:%f\n\r",adcValue,volt); // Send the value on UART
}
return (0);
}
view raw RawADCVoltageAtmega32.c hosted with ❤ by GitHub

Temperature

Let's connect the LM35 temperature sensor to channel zero of the ADC and read the temperature. Resolution with 5V reference can be calculated as: $$ResolutionOfADC = (5v/1023)=4.887mv = 5mv$$ For every degree celcius the Lm35 provides 10mv voltage change, therefore $$Temperature = ADCValue/2 $$

Hookup LM35

0Reading Temperature with AVR ADC TEMP bb.png

Code to Read Temperature

#include "adc.h"
#include "uart.h"
int main()
{
int adcValue;
int temp;
ADC_Init(); /* Initialize the ADC module */
UART_Init(9600); /* Initialize UART at 9600 baud rate */
while(1)
{
adcValue = ADC_GetAdcValue(0); // Read the ADC value of channel zero where the temperature sensor(LM35) is connected
/* Convert the raw ADC value to equivalent temperature with 5v as ADC reference
Step size of AdC= (5v/1023)=4.887mv = 5mv.
for every degree celcius the Lm35 provides 10mv voltage change.
1 step of ADC=5mv=0.5'c, hence the Raw ADC value can be divided by 2 to get equivalent temp*/
temp = adcValue/2.0; // Divide by 2 to get the temp value.
UART_Printf("ADC0 Value:%4d Equivalent Temperature:%dC\n\r",adcValue,temp); // Send the value on UART
}
return (0);
}
view raw LM35_TempSensor_with_Atmega32.c hosted with ❤ by GitHub

0Digital Temp.png

Video Tutorial

For those of you, who would like to watch instead of read we have made a video with all the gyan.


Downloads

Download the complete project folder from the below link: https://github.com/ExploreEmbedded/ATmega32_ExploreUltraAvrDevKit/archive/master.zip


Have a opinion, suggestion , question or feedback about the article let it out here!

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