Deepak Dandapat

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10 years, 6 months ago.

doubt regarding i2c

sir, i saw your code , its working also.but i didn't get it..

#include "mbed.h"

Serial pc(USBTX,USBRX);     // Tx, Rx Pin, default boud = 9600
I2C i2c(p28, p27);          // SDA, SCL

int main() 
{
    char cmd[2];            // Command
    char echo[2];           // Echo result data (8bit)
    int  addr  = 0xA0;   
     int  addr1  = 0xA1;   // Defalut I2C address of SRF08
    int  range = 0;         // Rage data (16bit, cm)
    
    cmd[0] = 0x02;          // Register(RangeValue)
    cmd[1] = 0x80;          // (RangeValue*43)+43 = (69*43)+43 = 3010mm, maximum range
    i2c.write(addr,cmd,2);  // Send I2C data (8bit*2)

  
     cmd[0] = 0x00;          // Register(Command)
        cmd[1] = 0x51;          // Ranging mode - Result in centimeters
        i2c.write(addr,cmd,2);  // Send I2C data (8bit*2)
        
        wait(0.07);             // Wait 70ms
        
        cmd[0] = 0x02;          // Register(1st Echo)
        i2c.write(addr,cmd,1);  // Send I2C data (8bit*1)
        i2c.read(addr1,echo,3);  // Read I2C data (8bit*2)
        
       range = (echo[0]<<8)+echo[1];   // 8bit+8bit=16bit
    
        pc.printf("Range : %d cm\n",echo[0]);
          pc.printf("Range : %d cm\n",echo[1]);
    
}

I dont know basics.. could u guide me please? i am using AT24C08A EEPROM with LPC11U24 using I2C , but why for this many cmd[]? pls explain me.

Question relating to:

Last updated: 03 Mar 2013

1 Answer

Tristan Hughes
10 years, 6 months ago.

For i2c to work you have to supply several bits of data. The read/write address of the device, the address of the register you wish read or write and if required the data to write to that register.

From the code you posted you can see that the 8-bit device write address is defined in the line:

int  addr  = 0xA0;

and the read address is defined underneath it

int  addr1  = 0xA1;

next the code loads the cmd[] array with the commands to carry out, in this case writing a 0x80 to register 0x02.

cmd[0] = 0x02;        
cmd[1] = 0x80;

The code then calls the i2c.write() function to write the data to the bus.

i2c.write(addr,cmd,2);

the addr is the write address for the device. The cmd is the list of commands we have previously prepared, and the 2 indicates the number of commands we are sending.

To read a value from a device on the bus is slightly different. The first thing to do is send the device the address of the register we want to read. This is done is a similar way to writing data.

cmd[0] = 0x02;          
i2c.write(addr,cmd,1);

The 1 at the end of the i2c.write call indicates that this time we are only sending one command to the device. Now we can put the mbed into i2c read mode. This is done with the following command.

i2c.read(addr1,echo,3);

This function uses addr1, the read address of the device. The function is going to place 3 bytes of data into the echo array.

I hope that helps to explain the basics to you! You can find more information in the datasheet for the AT24C08A.

Accepted Answer

Thank u very much ....it will help me a lot...ok then let me clear one thing that.." this commands will be according to slave specification right?"

posted by Deepak Dandapat 16 Oct 2013