Introduction¶

The "Grove - 3 axis Accelerometer" is a 3 axis accelerometer that is accessible via a digital I2C interface.
This accelerometer module is based on MMA7660FC chip and it can be used for sensor data changes, product orientation, and gesture detection through an interrupt pin (INT).
It is a very low power, low profile capacitive MEMS sensor.

An accelerometer is a device that will measure the static acceleration and the dynamic acceleration.

• Static acceleration force is generated by a gravitational pull. When measuring static acceleration with a capacitive accelerometer, the measurements would portray the angle of tilt at which the device is being held or suspended in relation to the Earth's gravitational pull.

• Dynamic acceleration force is generated by vibrations from movement. Measuring vibrations allows a capacitive accelerometer to determine in which direction the device is moving and at what speed.

Required Components¶

• Grove - I2C 3-axis Accelerometer
• mbed

Product Specifications¶

• Digital Output (I2C)
• 3 axis motion/orientation sensing
• Acceleration Range: ±1.5g (1g = 9.8m/s^2)
• Sensitivity: 21.33 counts/g
• Working voltage: 3V - 5V
• Resolution: 6bit

Pin Description¶

Connectivity¶

NOTE: The I2C bus is already connected to pull-up resistor.

Registers Summary¶

NOTE: The device must be placed in Standby Mode to change the value of the registers.

Test Code¶

00001 /* Author: Edoardo De Marchi 
00002  * Name: Test Code for MMA7660FC
00003  */
00004 #include "mbed.h"
00005 #include "MMA7660FC.h"
00006 
00007 #define ADDR_MMA7660 0x98                   // I2C SLAVE ADDR MMA7660FC
00008 
00009 MMA7660FC Acc(p28, p27, ADDR_MMA7660);      //sda, scl, Addr
00010 Serial pc(USBTX, USBRX);
00011 
00012 
00013 float G_VALUE[64] = {0, 0.047, 0.094, 0.141, 0.188, 0.234, 0.281, 0.328, 0.375, 0.422, 0.469, 0.516, 0.563, 0.609, 0.656, 0.703, 0.750, 0.797, 0.844, 0.891, 0.938, 0.984, 1.031, 1.078, 1.125, 1.172, 1.219, 1.266, 1.313, 1.359, 1.406, 1.453, -1.500, -1.453, -1.406, -1.359, -1.313, -1.266, -1.219, -1.172, -1.125, -1.078, -1.031, -0.984, -0.938, -0.891, -0.844, -0.797, -0.750, -0.703, -0.656, -0.609, -0.563, -0.516, -0.469, -0.422, -0.375, -0.328, -0.281, -0.234, -0.188, -0.141, -0.094, -0.047};
00014 
00015 
00016 
00017 int main() 
00018 {
00019 
00020     Acc.init();                                                     // Initialization
00021     pc.printf("Value reg 0x06: %#x\n", Acc.read_reg(0x06));         // Test the correct value of the register 0x06
00022     pc.printf("Value reg 0x08: %#x\n", Acc.read_reg(0x08));         // Test the correct value of the register 0x08
00023     pc.printf("Value reg 0x07: %#x\n\r", Acc.read_reg(0x07));       // Test the correct value of the register 0x07
00024            
00025     while(1)
00026     {   
00027         float x=0, y=0, z=0;
00028         
00029         Acc.read_Tilt(&x, &y, &z);                                  // Read the acceleration                    
00030         pc.printf("Tilt x: %2.2f degree \n", x);                    // Print the tilt orientation of the X axis
00031         pc.printf("Tilt y: %2.2f degree \n", y);                    // Print the tilt orientation of the Y axis
00032         pc.printf("Tilt z: %2.2f degree \n", z);                    // Print the tilt orientation of the Z axis
00033 
00034         wait_ms(100);
00035 
00036         pc.printf("x: %1.3f g \n", G_VALUE[Acc.read_x()]);          // Print the X axis acceleration
00037         pc.printf("y: %1.3f g \n", G_VALUE[Acc.read_y()]);          // Print the Y axis acceleration
00038         pc.printf("z: %1.3f g \n", G_VALUE[Acc.read_z()]);          // Print the Z axis acceleration
00039         pc.printf("\n");
00040         wait(5);
00041           
00042     }
00043 }
00044 

The g values or tilt values are converted using the "MM7660FC ACQUISITION CODE TABLE" that you can find it on page 28 of the MMA7660FC datasheet.

Library¶

The initialization is needed because it sets the "Mode" register to "Active Mode" which is required for read the data of the x,y,z axis.

Output¶

Other information you can be found in my notebook page HERE

Update¶

I wrote a C# software that test the principal characteristics of this accelerometer.

The software is here: 3D Accelerometer Tester v1.1
Is required Microsoft .NET Framework 3.5 or above.

This is the new pins assignment (I changed the I2C pins and I connected the interrupt pin):

The mbed requires this firmware:

Some screens:

First you must set the Serial Port and press the button "Connect".
Then you can read the Sensor data (G acc and Orientation) manually pressing the button "Read DATA" or triggered by interrupt if you enabled in the second tab the checkbox "G Interrup Enable".
The red ball changes when the checkbox "SHAKE enable" is enabled.

In this screen you can set and read the registers manually or you can set easy the sample rate, the Shake Interrupt or the G Interrupt.

In this graph are plotted the G data read from the sensor.