9 years, 2 months ago.

In general I would advice you to use the full functions for I2C, so for example i2c.write(address, data, 2); These are easier to use and better supported.

Now what you are doing might be rejected by the NXP I2C hardware: They have an internal state machine and might refuse to send data when no start condition has been sent. So first try adding a start condition and then the write.

Finally your LEDs glowing lightly is intended behavior :). By default pins on the MCU have a weak pull up enabled, this causes a very small amount of current to go through the LEDs and they will light up very faintly. How much they light up can differ because this pull up current is not well defined.

9 years, 2 months ago.

Here is a snip along the lines of what Erik is saying. I use this method on all platforms that works for me (up to now).

You may also change the pull up's to a lower value, I have used 10k but generally I stick to 4k7.

    reset[0]=0x0C;   // device register I'm writing to
    reset[1]=0x00;   // byte value I'm sending

    i2c.write(0xD0,reset,2);   // 0xD0 is the device i2c address, sending one byte to register 0x0C here to reset the device.

Here is an i2c test that will enable you to test any standard i2c device.

// I2C - Search for devices on an I2C bus,
//  - Continuously cycles through each device address, checking for a response
//  - Prints 8-bit address for each device found
//  - if the program hangs, the bus isn't working (check pull-ups etc)
//  - if it doesn't find your device, check power/bus wiring etc

#include "mbed.h"
 
Serial pc(USBTX, USBRX);

I2C i2c(p9, p10); // I2C device sda, scl


int main() {
    i2c.frequency(10000); // set required i2c frequency
    
    pc.printf("I2C Searching!\n\n");
    pc.printf("Starting....\n\n");    
    
    while (1) {
        int count = 0;
        for (int address=0; address<256; address+=2) {
            if (!i2c.write(address, NULL, 0)) { // 0 returned is ok                
                pc.printf("I2C address 0x%02X\n", address);
                count++;
            }
        }        
        pc.printf("%d devices found\n\n\n", count);
        wait(2);       
    }
}

As it says on the tin, if you do not get any results there will be a hardware problem with either your Mbed, i2c device or wiring.

I have used this on all the Mbed platforms.

9 years, 2 months ago.

I2C issues seem to crop up every few weeks. Reference this checklist to help in debugging your I2C problems.

https://developer.mbed.org/questions/5910/I2C-problem/#answer6122