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@brihijoshi
brihijoshi committed Sep 26, 2017
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30 changes: 24 additions & 6 deletions final.js
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Original file line number Diff line number Diff line change
Expand Up @@ -2,7 +2,10 @@ var tessel = require('tessel');

// Connect to device
var port = tessel.port.A; // Use the SCL/SDA pins of Port A

//This address of the Slave has been taken from https://github.com/tessel/accel-mma84/blob/master/index.js#L15
var slaveAddress = 0x1D; // Specific to device

var i2c = new port.I2C(slaveAddress); // Initialize I2C communication

// Details of I2C transfer
Expand All @@ -12,24 +15,29 @@ var numBytesToRead = 1; // Read back this number of bytes
i2c.read(numBytesToRead, function (error, dataReceived) {
// Print data received (buffer of hex values)
console.log('Buffer returned by I2C slave device ('+slaveAddress.toString(16)+'):', dataReceived);

});

// Read/Receive data over I2C using i2c.transfer
// 0x0D is the WHO_AM_I Register which sends back an acknowledgement to the master for starting the communication

i2c.transfer(new Buffer([0x0D]), numBytesToRead, function (error, dataReceived) {
// Print data received (buffer of hex values)
// The returned buffer from the I2C slave device should be [0x2A]
console.log('Buffer returned by I2C slave device ('+slaveAddress.toString(16)+'):', dataReceived);

});
// dataReceived contains all of the the bits for x, y, z - OUT_X_MSB as well as OUT_X_LSB - 6 bytes which is 48 bits

/*
In the i2c.transfer function the dataReceived consists of all the 6 bytes of data 2 bytes each for the x, y and z values.
Each of the x, y and z coordinates have two registers associated with them for storing the 12 bit long sample.
The first 8 bits are stored in their respective OUT_MSB registers. These are the Most Significant first 8 bits.
The next 4 bits are stored in their respective OUT_LSB registers. The remaining 4 bits are occupied
by 0s. These lower 4 bits are redundant bits which are not required.
by 0s. These lower 4 bits are redundant bits which are not required. The OUT_LSB and OUT_MSB store the 2's complement form of the coordinates.
The organisation of the registers can be seen in the datasheet inside section 6.1 (Data Registers), page number - 21
The register address for OUT_X_MSB is 0x01. This can be found at Page 19 of https://www.nxp.com/docs/en/data-sheet/MMA8452Q.pdf
*/
i2c.transfer(new Buffer([0x01]), 6, function (error, dataReceived) {
Expand All @@ -40,6 +48,7 @@ i2c.transfer(new Buffer([0x0D]), numBytesToRead, function (error, dataReceived)
//This array is going to store the final x,y,z values
var out=[];


/*
The for loop is iterated 3 times, in order to extract the x,y,z values.
Expand All @@ -48,25 +57,34 @@ i2c.transfer(new Buffer([0x0D]), numBytesToRead, function (error, dataReceived)
of the OUT_LSB.
2. The OUT_LSB of the respective coordinate.
The OUT_LSB of the respective coordinate right shifted by 4 to get rid of the redundant lower 0 bits which are 4 in number.
The OUT_LSB of the respective coordinate is right shifted by 4 to get rid of the redundant lower 0 bits which are 4 in number.
Check whether the most significant bit of the OUT_MSB is 1 or 0 i.e whether the coordinate value if negative or
positive. If it is negative (checked using 0x7F, which is the maximum possible number that can be made from 7 bits), the if condition changes
it to a 2's complement form, thus making it positive and adding a "-" sign in front of it.
Finally, check whether the number is negtative. If it is negative
Lastly, normalise the coordinate to get a value between 0 and 1, dividing the gCount by 2^10.
*/

for (var i=0;i<3;i++){

var gCount=(dataReceived[i*2] << 8) | dataReceived[(i*2)+1];

console.log(gCount);

gCount=gCount >> 4;

// 127 is checking whether we have a 0 or a 1 at the first position - basically its sign.
if (dataReceived[i*2] > 0x7F) {

gCount = -(1 + 0xFFF - gCount); // Transform into negative 2's complement

}
console.log(gCount);
// / - normal division
// we are scaling it down to 0 - 1 (normalising the values)

out[i] = gCount / ((1<<12)/(2*2));

}

console.log('The x, y, z values are :',out);
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88 changes: 61 additions & 27 deletions tut-i2c.md
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Expand Up @@ -9,56 +9,90 @@ Let us get the accelerometer values using the I2C protocol. We would be using th
![Fritzing Diagram](http://i.imgur.com/zK4U4S3.png)

```js
var tessel = require('tessel'); //Import tessel
var tessel = require('tessel');

// Connect to device
var port = tessel.port.A; // Select Port A of Tessel
var port = tessel.port.A; // Use the SCL/SDA pins of Port A

//This address of the Slave has been taken from https://github.com/tessel/accel-mma84/blob/master/index.js#L15
//More about registers can be found at Page 19 of https://www.nxp.com/docs/en/data-sheet/MMA8452Q.pdf
var slaveAddress = 0x1D; // Specefic for accelerometer module
var slaveAddress = 0x1D; // Specific to device

var i2c = new port.I2C(slaveAddress); // Initialize I2C communication

// Details of I2C transfer
var numBytesToRead = 1; // Read back this number of bytes

// Read data over I2C using i2c.transfer
i2c.read(numBytesToRead, function (error, dataReceived) {

// Print data received (buffer of hex values)
console.log('Buffer returned by I2C slave device ('+slaveAddress.toString(16)+'):', dataReceived);

});

// Read/Receive data over I2C using i2c.transfer
// 0x0D is the WHO_AM_I Register which sends back an acknoledgement to the master for starting the communication
i2c.transfer(new Buffer([0x0D]), numBytesToRead, function (error, dataReceived) {
// 0x0D is the WHO_AM_I Register which sends back an acknowledgement to the master for starting the communication

i2c.transfer(new Buffer([0x0D]), numBytesToRead, function (error, dataReceived) {
// Print data received (buffer of hex values)
// The returned buffer from the I2C slave device should be [0x2A]
console.log('Buffer returned by I2C slave device ('+slaveAddress.toString(16)+'):', dataReceived);

});

/*
In the i2c.transfer function the dataReceived consists of all the 6 bytes of data 2 bytes each for the x, y and z values.
Each of the x, y and z coordinates have two registers associated with them for storing the 12 bit long sample.
The first 8 bits are stored in their respective OUT_MSB registers. These are the Most Significant first 8 bits.
The next 4 bits are stored in their respective OUT_LSB registers. The remaining 4 bits are occupied
by 0s. These lower 4 bits are redundant bits which are not required. The OUT_LSB and OUT_MSB store the 2's complement form of the coordinates.
The organisation of the registers can be seen in the datasheet inside section 6.1 (Data Registers), page number - 21
The register address for OUT_X_MSB is 0x01. This can be found at Page 19 of https://www.nxp.com/docs/en/data-sheet/MMA8452Q.pdf
*/
i2c.transfer(new Buffer([0x01]), 6, function (error, dataReceived) {

// This is an exception. If an error is generated, it will throw the error
if (error) throw error;

//This array is going to store the final x,y,z values
var out=[];

//Now, try to print the accelerometer data using i2c.transfer
// The register address for OUT_X_MSB is 0x01. This can be found at Page 19 of https://www.nxp.com/docs/en/data-sheet/MMA8452Q.pdf
// 6 Bytes are used for pairwise MSB and LSB of the x,y and z axis
i2c.transfer(new Buffer([0x01]), 6, function (error, dataReceived) {
if (error) throw error;
//Create a blank array for the output
var out=[];
//iterating three times the x, y, z values
for (var i=0;i<3;i++) {
var gCount=(dataReceived[i*2] << 8) | dataReceived[(i*2)+1]; //Converting the 8 bit data into a 12 bit
gCount=gCount >> 4;
if (dataReceived[i*2] > 0x7F) {
gCount = -(1 + 0xFFF - gCount); // Transform into negative 2's complement

/*
The for loop is iterated 3 times, in order to extract the x,y,z values.
The gCount variable is a bitwise OR operation between two binary numbers:
1. The OUT_MSB of the respective coordinate, that is left shifted by 8 bits in order to make space for the remaining 8 bits
of the OUT_LSB.
2. The OUT_LSB of the respective coordinate.
The OUT_LSB of the respective coordinate is right shifted by 4 to get rid of the redundant lower 0 bits which are 4 in number.
Check whether the most significant bit of the OUT_MSB is 1 or 0 i.e whether the coordinate value if negative or
positive. If it is negative (checked using 0x7F, which is the maximum possible number that can be made from 7 bits), the if condition changes
it to a 2's complement form, thus making it positive and adding a "-" sign in front of it.
Lastly, normalise the coordinate to get a value between 0 and 1, dividing the gCount by 2^10.
*/

for (var i=0;i<3;i++){
var gCount=(dataReceived[i*2] << 8) | dataReceived[(i*2)+1];
console.log(gCount);
gCount=gCount >> 4;

// 127 is checking whether we have a 0 or a 1 at the first position - basically its sign.
if (dataReceived[i*2] > 0x7F) {
gCount = -(1 + 0xFFF - gCount); // Transform into negative 2's complement
}
console.log(gCount);
out[i] = gCount / ((1<<12)/(2*2));
}
out[i] = gCount / ((1<<12)/(2*2));
}
console.log('The x, y, z values are :',out); //Log the Array containing the x,y,z values

});
console.log('The x, y, z values are :',out);

});
```

[Datasheet for accelerometer module](http://www.nxp.com/docs/en/data-sheet/MMA8452Q.pdf)
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