Accelerometers are recently developed solid state electronics devices that makes it very easy to measure acceleration. They are completely modular and very tiny devices which gives voltage proportional to acceleration. These type are called analog accelerometers as their output is voltage. Some other gives a PWM output or direct binary digital data, they are called digital accelerometers.




Accelerometers are used widely in modern devices.

  • Apple iPhone,iPad and Nokia series 60v5 devices for automatic screen orientation changing.
  • Also for motion gaming and other showoff stuff like Xpress Beer in above devices.
  • Portable Hard disk and Notebooks for fall detection.
  • Anti-theft devices.
  • Motion Gaming Consoles like Nintendo Wii.
  • Balancing Robots and UAVs.
  • Experiments which needs to find force, like car crash experiments.
  • And Possibly many other.

Accelerometers can measure acceleration in 2 dimensional or 3 dimensional space. They are called 2D and 3D accelerometers respectively. Accelerometers have certain range, i.e. the maximum acceleration they can measure. It is specified in terms of g. 'g' is the acceleration due to gravity of earth and it is equal to 9.80665m/s2. Common accelerometers can have a range of 1.5g to 6g. It is obvious that 1.5g accelerometer is more precise than 6g. So use 1.5g where more accuracy is needed while use 6g for much more harsh experiments. Renault R26 can pick up 100km/hr is about 1.7 sec! Even this beast don't give more acceleration that 1.6g, so a 6g sensor can be used for much harsh experiments. A high g roller coaster can give a g-force of about 3.5 to 6.3g (source Wikipedia)

One interesting fact about accelerometer is that they always measure acceleration relative to the earths gravity. That means if it is NOT at all accelerating, like being placed on table or held in hand, it will show an acceleration along the direction of earths gravitational field. And when it free falls (that means actually accelerating due to g) it will show a 0 acceleration.

This fact is used to sense tilt of device by using simple trigonometry.

The MMA7260QT Based Accelerometer Module.

In this tutorial we will learn how to interface a 3axis accelerometer with AVR Microcontroller. The accelerometer which we will use is MMA7260QT from Freescale Semiconductors. Most accelerometer chip (including MMA7260) are available only in very small SMT packages, thus bit hard to work with (unless you can fabricate a PCB for it). To solve this issue we have introduced a PCB Module based on MMA726 to make it easy for most of us to work with. It also has a 3.3v regulator chip built in, so you can easily hook it up with 5v development boards and MCUs. You can buy it from the following links.

NOTE: Sparkfun Version is Very basic and does NOT has inbuilt 3.3v regulator, so to interface it with 5v logic you need a LM1117-3.3 chip.

3 axis accelerometer module

3-Axis Accelerometer Module.

 

3 axis accelerometer module

3-Axis Accelerometer Module.

PIN OUT

GND Ground Supply
VDD 3.3 V OUT
Sel2 G Select Pin2 (See datasheet)
Sel1 G Select Pin1 (See datasheet)
X X axis output (g bias point @ 1.65v)
Y Y axis output (g bias point @ 1.65v)
Z Z axis output (g bias point @ 1.65v)
Sleep (Active Low) Low= Sleep High* = Wake
GND Ground Supply
+5v 5 Volts IN
DO NOT USED
DI NOT USED
CLK NOT USED
EN NOT USED
RXD NOT USED
TXD NOT USED

 

The MMA7260 Accelerometer Interface.

The block diagram of the MMA7260 Accelerometer Module is given below.

mma7260 accelerometer block diagram

MMA7260 Accelerometer Module Block Diagram

The signals on left hand side are the input and on the right side are output. The MMA7260 accelerometer module supports 4 selectable g options.

g-Select2 g-Select1 Range Sensitivity
0 0 1.5g 800mV/g
0 1 2.0g 600mV/g
1 0 4.0g 300mV/g
1 1 6.0g 200mV/g

To select any g Range, simply apply proper logic at the g-Select Pins. You can use the 3.3V out pin to get logic high. Note: Never apply 5v as logic high as it will damage the chip. In this example we will use the first mode, i.e. 1.5g so we leave the g-Select Pins unconnected. The internal PULL DOWN resistors will make both pins logic 0.

For Normal operation the SLEEP pin must be at HIGH logic, so tie it up to the 3.3v Out pin.

Output of MMA7260 accelerometer module.

The output of any axis is an analog voltage proportional to the acceleration in that axis. As the acceleration can be positive, negative or zero. So the output has a zero bias point. That means the output is held at this point for zero acceleration. A negative acceleration will result in voltage less than the zero g point. Normally this zero bias point is half of the supply voltage for mma7260. In our case it is 3.3V so zero bias point is 1.65V.

This is illustrated with the help of following example.

Y axis output.

In CASE#1

The acceleration due to gravity is in the direction of Y axis so the output is positive hence voltage greater than 1.65V

In CASE#2

The device is inverted, now the acceleration due to gravity is opposite to the Y axis hence a negative acceleration or a Value which is less than 1.65v

If the device is made to lie flat so that Y axis is perpendicular to earths gravitational field then the output will be close to 1.65v

Interfacing Accelerometer with AVR Microcontrollers

Interfacing the module is fairly easy, you just need to connect the X,Y,Z output to the ADC input channels. The inbuilt ADC will convert the analog voltage into a digital number. It is recommended you try out basic AVR ADC programming as described in this tutorial.

After reading the above tutorial you know that AVR ADC will give us a number between 0-1023 for voltage between 0-5V. The relation between adc_value and input voltage is.

Vin=(adc_value/1023)x5

Using the above relation we get the adc_value for zero g point, i.e. 1.65v

Using simple math we get adc_value=337.59 for Vin=1.65

Rounding Up we get adc_value=338 for zero g point.

So we now know that if ADC readings are less than 338 it means negative value. So we subtract 338 from the value provided by the ADC to get the actual acceleration in that direction.

int x; 		//Acceleration in x direction
x=ReadADC(0);	//Read Analog Channel 0 (connected to X output)
x=x-338; 		//Actual Acceleration with sign!

We will write a small demo app that will run on ATmega32 AVR Microcontroller that will read accelerometer data and present it in a 16x2 LCD Module. User is recommended to see the LCD Interfacing tutorial for AVR using the following link.

Those who are using xBoard v2.0 will like to read the following page.

The AVR Must be clocked at 16MHz to run the demo supplied in this page. If you are using a brand new chip don't forget to change the Fuse Bits to following value.

  • HIGH Fuse Byte = 0xC9
  • LOW Fuse Byte = 0xFF

The above setting will disable JTAG and switch to external crystal. Before doing so please solder a crystal of 16MHz to the XTAL pins.

Hardware Setup for Testing 3-Axis Accelerometers with AVR

You can make a small circuit as shown in the schematic below. Otherwise you may use one of our ready made development boards. If you cannot get the expected result with home made circuit then it is suggested that you purchase a development board to ease your life.

avr and accelerometer circuit

Schematic for AVR ATmega32 and LCD Connection.

Use a 5v supply system using LM7805 to power both the accelerometer and the above circuit. Connect Accelerometers Xout to ADC0, Yout to ADC1, Zout to ADC2 respectively. Connect Accelerometers SLEEP pin to 3.3v out from the accelerometer module. Leave both the g-Select pins unconnected. Now you are ready to run the demo program.

If you are using the xBoard v2.0 then you don't need to built any circuit. Just hookup the LCD Module as shown below.

A 16x2 Character Alphanumeric LCD Module Supplied with xBoard

 

A 12 PIN Connector is already soldered to the LCD Module

 

12 PIN Header For LCD Module Connection on xBoard.

 

LCD Module Connected.

 

The Whole Setup.

Use the extra 5v out and GND pins to supply the accelerometer module. Then connect X,Y and Z out pins from accelerometer to the xBoard's Analog in pins(ADC0,ADC1,ADC2 etc). The location all these pins is show below.

PORTA/ADC Port

avr-gcc program to test Accelerometer Module.

The following program reads the acceleration in 3 axis of the accelerometer module and displays the result on 16x2 lcd module. The program is written in C language and compiled using avr-gcc compiler using the AVR Studio Development Environment. These tools must be installed correctly and user must have basic knowledge of usage. If you are new to them please read the following tutorials.

User is assumed to have basic knowledge of C language and programming concepts. Teaching basic programming techniques (like loops or control statements) is NOT in the scope of our tutorial series. We focus only on programming embedded system and closely related techniques for people who already know what is programming. If you are new to programming (or C) itself then jumping into system programming is NOT for you. Go learn programming is an environment that is more suited for beginner, like a PC or MAC. You may first get started with simple languages like LOGO (for Kids) or BASIC, then move to professional languages like C/C++ or Java.


/*********************************************************************

                 xBoard(TM) v2.0 Sample Programs

               ------------------------------------


Description :  Demonstrate the use of Accelerometer to sense tilt angle.
            Shows the raw X,Y,Z readings on LCD Disp.

Accelerometer: MMA7260 - 3 axis accelerometer breakout board.

Author      : Avinash Gupta

Web         : www.eXtremeElectronics.co.in

               
            

            
            Copyright 2008-2010 eXtreme Electronics, India
                   
**********************************************************************/

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


#include "lcd.h"

/*

Function To initialize the on-chip ADC

*/
void InitADC()
{
   ADMUX=(1<<REFS0);                                     // For Aref=AVcc;
   ADCSRA=(1<<ADEN)|(1<<ADPS2)|(1<<ADPS1)|(1<<ADPS0);    //Rrescalar div factor =128

}

/*

Function to read required analog channel and returns the value

Argument:
   Channel number from 0-7

Return Vale:
   Result of conversion on selected channel

   ranges from 0-1023

*/
uint16_t ReadADC(uint8_t ch)
{
   //Select ADC Channel ch must be 0-7
   ch=ch&0b00000111;
   ADMUX&=0b11100000;
   ADMUX|=ch;

   //Start Single conversion

   ADCSRA|=(1<<ADSC);

   //Wait for conversion to complete
   while(!(ADCSRA & (1<<ADIF)));

   //Clear ADIF by writing one to it

   //Note you may be wondering why we have write one to clear it

   //This is standard way of clearing bits in io as said in datasheets.
   //The code writes '1' but it result in setting bit to '0' !!!

   ADCSRA|=(1<<ADIF);

   return(ADC);
}

/*

Simple Delay function

*/
void Wait(uint8_t t)
{
   uint8_t i;
   for(i=0;i<t;i++)
      _delay_loop_2(0);
}


void main()
{
   int16_t x,y,z; //X,Y,Z axis values from accelerometer.

   //Wait for LCD to Startup
   _delay_loop_2(0);

   //Initialize LCD, cusror style = NONE(No Cursor)
   LCDInit(LS_NONE);
   LCDClear();

   //Initialize ADC
   InitADC();

   //Put some intro text into LCD

      LCDWriteString("  Accelerometer ");
      LCDWriteStringXY(0,1,"      Test      ");
      Wait(150);

   LCDClear();
      LCDWriteString("     eXtreme ");
      LCDWriteStringXY(0,1,"   Electronics  ");
   Wait(150);

   LCDClear();

   while(1)
   {
         x=ReadADC(0);           // Read Analog value from channel-0

      y=ReadADC(1);           // Read Analog value from channel-1
      z=ReadADC(2);           // Read Analog value from channel-2

       //Make it signed value (zero point is at 338)
      x=x-338;
      y=y-338;
      z=z-338;

      //Print it!

      LCDWriteStringXY(0,0,"X=");
      LCDWriteInt(x,4);

      LCDWriteString(" Y=");
      LCDWriteInt(y,4);

      LCDWriteStringXY(0,1,"Z=");
      LCDWriteInt(z,4);

      Wait(20);


   }


}

Downloads

  • AVR Studio Project for MMA7260 based accelerometer module test (For ATmega32)
  • HEX File for MMA7260 based accelerometer module test (For ATmega32)

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Avinash Gupta
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