8-Bit AVR Analog to Digital Converter (ADC) Internal Temp Sensor Example

 Objective

This hands-on project steps through a simple example of reading the on-chip temperature sensor. Reading the temperature sensor can be a rewarding project in itself. It confirms that you have completed the software build and the hardware setup of the ADC, and were able to program the microcontroller successfully. Finally, the debugger is used to view the temperature using Studio 7 output window.

The on-chip temperature sensor is coupled to a single ended ADC8 channel. Selecting the ADC8 channel by writing ADMUX.MUX[3:0] to '1000' enables the temperature sensor. The internal 1.1 V voltage reference must also be selected for the ADC voltage reference source in the temperature sensor measurement. When the temperature sensor is enabled, the ADC converter can be used in single conversion mode to measure the voltage over the temperature sensor.
The voltage sensitivity is approximately 1 mV/°C, the accuracy of the temperature measurement is ±10°C.

 Materials

Hardware Tools (Optional)

Tool About Purchase
ATmega328PB-XplainedMini-50px.png
ATmega328PB Xplained Mini
Evaluation Kit

Software Tools

Tool About Installers
Installation
Instructions
Windows Linux Mac OSX
swtool-28px.png
Atmel® Studio
Integrated Development Environment

Exercise Files

File Download
Installation
Instructions
Windows Linux Mac OSX
Main.c Source File

Additional Files

Files

 Procedure

1

Task 1 - Project Creation

  • Open Atmel Studio 7
  • Select File > New > Project
  • Select GCC C Executable Project and give it the name Project1
  • Choose a location to save the project on your computer
step1.png
  • The Device Selection window will appear. In the search bar enter 328P, then select the device Atmega328PB and click OK.
step2.png

2

Task 2 - Main.c

This project reads the internal temperature sensor, converts the result to degrees centigrade, then stores the result in ADC Temperature Result.

1) The main.c file is where the application code is added. The project has a main.c file already created but it only contains a while(1) statement. Modify main.c by entering the lines in the gray code block below.

#include <avr/io.h> is automatically added to the main.c file when it is generated. This should always be placed before the main(void) loop. The header file io.h calls the iom328pb.h file that defines the ADC register definitions.

unsigned int Ctemp;
unsigned int Ftemp;

int main(void)
{

    /* Setup ADC to use int 1.1V reference 
    and select temp sensor channel */
    ADMUX = (1<<REFS1) | (1<<REFS0) | (0<<ADLAR) | (1<<MUX3) | (0<<MUX2) | (0<<MUX1) | (0<<MUX0);

    /* Set conversion time to 
    112usec = [(1/(8Mhz / 64)) * (14 ADC clocks  per conversion)]
     and enable the ADC*/
    ADCSRA = (1<<ADPS2) | (1<<ADPS1) | (1<<ADEN);

    /* Perform Dummy Conversion to complete ADC init */
    ADCSRA |= (1<<ADSC);

    /* wait for conversion to complete */
    while ((ADCSRA & (1<<ADSC)) != 0);

    /* Scan for changes on A/D input pin in an infinite loop */
    while(1)
    {
        /* start a new conversion on channel 8 */
        ADCSRA |= (1<<ADSC);

        /* wait for conversion to complete */
        while ((ADCSRA & (1<<ADSC)) != 0)
        ;

        /* Calculate the temperature in C */
        Ctemp = (ADC - 247)/1.22;
        Ftemp = (Ctemp * 1.8) + 32;
    }

    return -1;

}

3

Task 3 - Build Project

  • Select Build > Build Solution from the Studio 7 menu to compile the code. You will see a Build Succeeded message in the output window. If there are any errors, check main.c for any mistakes in entering the program code.
output.png

4

Task 4 - Programming the Xplained Board

  • Connect the Xplained board to the USB port of the computer using the included cable.
  • In the Solution Explorer area, right-click on the project name and select Properties.
solution.png
  • Under the Tool menu selection, choose the mEDBG and debugWire as the interface.
step42.png
  • Select Debug > Start Without Debugging from the Studio 7 menu. The project will build and then program the xplained board with the project code along with debug control.
step52.png
  • Studio 7 will show a Ready message when the programming is complete.

If the Xplained board will not connect, there may be a fuse setting causing this to occur.

5

Task 5 - Debugging

Debugging a device is essential for determining how a program may be running.

  • Select Debug > Start Debugging and Break. The project will build and the program will be loaded into the Xplained board
  • The I/O View window will open up showing the various peripherals
  • Click on the ADC selection to open the I/O view for the ADC
adcdebug.png
  • Select Debug > Step Over (F10) to single-step through the program on the Xplained board. Monitor the ADC registers in the I/O View while single-stepping
adcdebug2.png

6

Task 6 - Breakpoint and Output

  • Click on the Debug > Break All from the top menu of Studio 7
  • Click on the margin to enable a breakpoint on the command line at the ADCSRA statement within the While loop
breakpoint.png
  • Move the mouse over the breakpoint red circle, to be able to see the setting pop-up option (gear symbol) and click on it
breakpoint2.png
  • Inside the Breakpoint Setting Window, check the Actions box. Inside the “Log a message to Output Window” insert the following: Temperature in C = {Ctemp} and check the Continue execution box. Click on Close
  • Enter Debug mode by clicking on Start Debugging and Break in the top Debug menu
  • Open the Output Window by selecting Debug > Windows > Output to see the value of the temperature variable
  • Click on Debug > Continue and view the temperature in the Output window

 Results

The temperature of the chip is displayed in the output window. Pressing on it with a finger will heat up the reading by a couple of degrees.

breakpoint3.png

7

Task 7 - Disable debugWIRE and Close

The debugWire fuse needs to be reset in order to program the Xplained board in the future. While still running in debug mode select Debug > Disable debugWire and Close. This will release the debugWire fuse.

 Analysis

Using the ADC is quite easy and the debug feature makes it easy to monitor the results.

 Conclusions

This project can become the basis for future ADC related projects.

© 2017 Microchip Technology, Inc.
Information contained on this site regarding device applications and the like is provided only for your convenience and may be superseded by updates. It is your responsibility to ensure that your application meets with your specifications. MICROCHIP MAKES NO REPRESENTATIONS OR WARRANTIES OF ANY KIND WHETHER EXPRESS OR IMPLIED, WRITTEN OR ORAL, STATUTORY OR OTHERWISE, RELATED TO THE INFORMATION, INCLUDING BUT NOT LIMITED TO ITS CONDITION, QUALITY, PERFORMANCE, MERCHANTABILITY OR FITNESS FOR PURPOSE. Microchip disclaims all liability arising from this information and its use. Use of Microchip devices in life support and/or safety applications is entirely at the buyer's risk, and the buyer agrees to defend, indemnify and hold harmless Microchip from any and all damages, claims, suits, or expenses resulting from such use. No licenses are conveyed, implicitly or otherwise, under any Microchip intellectual property rights.