I read 7 bytes of info from an IC into an 8 byte array. The first byte returned is the number of bytes to be read to the mbed. That seems right if I output the results to terminal and do the maths. I then copy all info from the "read" array into a "write" array (with the the offset address of the register I want to write to in position [0]). This to ensure that all info I am not manipulating is "as read" from the IC. I then manipulate the bits I want to change. I then write the register to the IC and read all the contents back a second time and display to terminal. What I see after reading it a second time after writing is that the contents have shifted down by 1 from the first time I read. Example: First read array[6] = 8 array[5] = 255 array[4] = 255 array[3] = 246 array[2] = 223 array[1] = 224 Second read array[6] = 8 array[5] = 8 array[4] = 255 array[3] = 255 array[2] = 246 array[1] = 223 I am using a I2C address of 0x20 to address the IC. I am not changing it for read and write. I am not too familiar with I2C. Comments I am also using the same register offset address for read and write. Is this correct? Any suggestions?

#include "mbed.h"

I2C i2c(p9, p10);        // sda, scl
Serial pc(USBTX, USBRX); // tx, rx

const int addr = 0x00 ... 0x07; // define the correct I2C Address    

int main() 
{
    char regaddr[1];
    char readdata[8]; // room for length and 7 databytes
    char writedata[9]; // room for reg address, length and 7 databytes
    while (1) 
    {
        // read the data
        regaddr[0] = 0xE0;
        i2c.write(addr, regaddr, 1, true);  // select the register, no I2C Stop
        i2c.read(addr, readdata, 8);        // read the length byte and the 7 databytes
        
        wait (1);

        // print the data to the screen
        pc.printf("Register 0x%x = 0x%x 0x%x 0x%x 0x%x 0x%x 0x%x 0x%x\r\n",
                  regaddr[0],
                  readdata[1], readdata[2], readdata[3], readdata[4],
                  readdata[5], readdata[6], readdata[7] );
        wait(1);

        // copy the data, starting with register address
        writedata[0] = regaddr[0];  // register address
        writedata[1] = readdata[0]; // length, should be 7
        writedata[2] = readdata[1]; // byte 1
        writedata[3] = readdata[2];
        writedata[4] = readdata[3];
        writedata[5] = readdata[4];
        writedata[6] = readdata[5];
        writedata[7] = readdata[6];
        writedata[8] = readdata[7]; // byte 7

        // write the data
        i2c.write(addr, writedata, 9); // select the register, 
                                       // write the length, write 7 databytes      
        wait(1);

    }
}

I guess you refer to the forum thread that you started [[http://mbed.org/forum/helloworld/topic/3827/?page=1#comment-19090|here]]. The device you used there is the SM72445. I mentioned in my previous comment that this device sends a dummy length byte (value 7) when reading data. It also expects a dummy length byte value (7) when writing data (See [[http://www.ti.com/lit/ds/symlink/sm72445.pdf | datasheet]] Fig 12). You dont seem to send that length byte, this probably explains the shift in your results. So in very step-by-step mode your code should be something like this: <<code>> #include "mbed.h" I2C i2c(p9, p10); // sda, scl Serial pc(USBTX, USBRX); // tx, rx const int addr = 0x00 ... 0x07; // define the correct I2C Address int main() { char regaddr[1]; char readdata[8]; // room for length and 7 databytes char writedata[9]; // room for reg address, length and 7 databytes while (1) { // read the data regaddr[0] = 0xE0; i2c.write(addr, regaddr, 1, true); // select the register, no I2C Stop i2c.read(addr, readdata, 8); // read the length byte and the 7 databytes wait (1); // print the data to the screen pc.printf("Register 0x%x = 0x%x 0x%x 0x%x 0x%x 0x%x 0x%x 0x%x\r\n", regaddr[0], readdata[1], readdata[2], readdata[3], readdata[4], readdata[5], readdata[6], readdata[7] ); wait(1); // copy the data, starting with register address writedata[0] = regaddr[0]; // register address writedata[1] = readdata[0]; // length, should be 7 writedata[2] = readdata[1]; // byte 1 writedata[3] = readdata[2]; writedata[4] = readdata[3]; writedata[5] = readdata[4]; writedata[6] = readdata[5]; writedata[7] = readdata[6]; writedata[8] = readdata[7]; // byte 7 // write the data i2c.write(addr, writedata, 9); // select the register, // write the length, write 7 databytes wait(1); } } <</code>>

Hello really interesting, thanks I'm Just starting with C code for micros. Is there any guide or step by step commented guide of how to read/write parameters or registers in C code? I mean, really for dummies and explaining it from the C point of view. Ir real examples much better. If that examples does come with real AD converters, DACs orTemp sensors from Maxim, even better. Thanks in advance !