Hello,

Giving the information that i am new to the C language, i want to use my mbed to drive 8 led's on the I²C bus by using a PCF8574AP. This becouse i am using the I²C bus in my dissertation for my high school studies and i want to get used with it. Previously i tried it with the library for the PCF8574, but it did not work (the led's where glowing very faintly. So i wrote my own program, very simple : turn all the led's on, and turn them back off, so here it is:

#include "mbed.h"

I2C i2c(p28, p27);


int main() {
while (1) {

void start(void);    //start condition
i2c.write(0x40);    //write adress
i2c.write(0xf);       //write data
void stop(void) ;  //stop condition

wait(1);

void start(void);    //start condition
i2c.write(0x40);    //write adress
i2c.write(0x0);      //write data
void stop(void);   //stop condition

wait(1);
}
}

The compiler is not giving any errors/warnings, but the led's are still just glowing faintly and do not seem to be affected by my code.

All help is welcome, thanks in advance :)

Hi Stef,

In your example, you need to call start() and stop() as methods of the i2c object. So something like:

#include "mbed.h"

I2C i2c(p28, p27);

int main() {
  while (1) {
    i2c.start();    //start condition
    i2c.write(0x40);    //write adress
    i2c.write(0xf);       //write data
    i2c.stop();  //stop condition

    wait(1);

    i2c.start();    //start condition
    i2c.write(0x40);    //write adress
    i2c.write(0x0);      //write data
    i2c.stop();   //stop condition

    wait(1);
  }
}

Nothing to say this will make it work, but you'll be closer at least :)

Simon

Try this, it works with a pcf8574 and a pcf8574A

#include "mbed.h"

I2C i2c(p28, p27);

int main() {
int addr;
char cmd[3];

while (1) {

    addr = 0x40;                // adres van pcf8574
    cmd[0] = 0xFF;              //
    i2c.write(addr, cmd, 1);    // Send command string
    i2c.stop();

    wait(0.5);

    addr = 0x40;                // adres van pcf8574
    cmd[0] = 0x00;              //
    i2c.write(addr, cmd, 1);    // Send command string
    i2c.stop();

    wait(0.5);

    addr = 0x70;                // adres van pcf8574A version
    cmd[0] = 0xFF;              //
    i2c.write(addr, cmd, 1);    // Send command string
    i2c.stop();

    wait(0.5);

    addr = 0x70;                // adres van pcf8574A version
    cmd[0] = 0x00;              //
    i2c.write(addr, cmd, 1);    // Send command string
    i2c.stop();

    wait(0.5);

wait(1);
}
}

The program works indeed. From this base i will be able to continue with my project.

Thanks alot for getting me on track :)

One question tough, in the previous post there is the i2c command written like i2c.write(adress, data, 1); but, what does this "1" refer to, and is it neccesary?

The "1" means the number of bytes send in the complete command. See the next sample code when using a ds1621. Its initialize the registers at starrt.

  addr = 0x90;                // adres van ds1621
    cmd[0] = 0xac;              // access config prot
    cmd[1] = 0x02;              // output pol high , continu conv
    i2c.write(addr, cmd, 2);    // Send command string
    i2c.stop();

    wait(0.07);

    addr = 0x90;                // adres van ds1621
    cmd[0] = 0xa1;              // access TH
    cmd[1] = 0x18;              // set TH = 24 graden
    cmd[2] = 0x00;              // set TH = 00
    i2c.write(addr, cmd, 3);    // Send command string
    i2c.stop();

    wait(0.07);

    addr = 0x90;                // adres van ds1621
    cmd[0] = 0xa2;              // access TL
    cmd[1] = 0x16;              // set TL = 22 graden
    cmd[2] = 0x00;              // set TL = 00
    i2c.write(addr, cmd, 3);    // Send command string
    i2c.stop();

    wait(0.07);

    addr = 0x90;                // adres van ds1621
    cmd[0] = 0xee;              // start conv t
    i2c.write(addr, cmd, 1);    // Send command string
    i2c.stop();