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Ever wanted to use any pair of pins for I2C on Arduino, not just the dedicated pins on Analog 4 & 5? Me too, so I made a quick little Arduino library called “SoftI2CMaster”, available in the “blinkm-projects” Googlecode repository.

Get it here: SoftI2CMaster.h, SoftI2CMaster.cpp.

It’s still a work in progress, but it can write data pretty successfully and do it over longer cables than normal.

For the VIMBY/Scion Hackerspace Challenge, I created an array of BlinkM MaxM-powered accent lights for the device we made. Because the I2C cable was longer than a few feet, the normal Wire library that BlinkM_funcs.h uses to communicate with BlinkMs couldn’t be used. This is because the Wire library assumes a perfect bus. If there is any noise or other bus problems, the Wire library will currently lock up. For the SoftI2CMaster library, I wanted it to be very tolerant, even lazy, about bus problems and also have more tunable timing to let you slow the bus down. Of course, you still need pull-up resistors on the two lines. I’ve found using 2.2k resistors to be good.

The SoftI2CMaster API follows Wire’s API pretty closely:

• SoftI2CMaster(sdaPin,sclPin) — create an new SoftI2CMaster for the two pins specified
• beginTransmission(address) — begin sending data
• send(data) — send some data (byte or byte arrays)
• endTransmission() — stop sending data

In use it looks something like this:

#include "SoftI2CMaster.h"
const byte sdaPin = 7;
const byte sclPin = 6;
SoftI2CMaster i2c = SoftI2CMaster( sdaPin,sclPin );

i2c.beginTransmission( 9 );  // send to address 9
i2c.send('c');
i2c.send(255);
i2c.send(0);
i2c.send(200);
i2c.endTransmission();

There is a simple demo for BlinkMs that this library currently lives in. It’s called “BlinkMSoftI2CDemo” and shows off a simplified BlinkM_funcs called “BlinkM_funcs_soft.h“. The entirely of BlinkMSoftI2CDemo is shown below.

const byte sdaPin = 7;  // digital pin 7 wired to 'd' on BlinkM
const byte sclPin = 6;  // digital pin 6 wired to 'c' on BlinkM

#include "SoftI2CMaster.h"
SoftI2CMaster i2c = SoftI2CMaster( sdaPin,sclPin );

// must define "i2c" before including BlinkM_funcs_soft.h
#include "BlinkM_funcs_soft.h"

byte blinkm_addr = 9;

//
void setup()
{
  Serial.begin( 19200 );
  Serial.println("BlinkMSoftI2CDemo");

  BlinkM_off(0);

  for( int i=0; i< 100; i++ ) {  // flash the blinkms
    BlinkM_setRGB( blinkm_addr, 255,255,255 );
    delay(10);
    BlinkM_setRGB( blinkm_addr, 0,0,0 );
    delay(10);
  }
}

void loop()
{
  byte r = random(255);
  byte g = random(255);
  byte b = random(255);

  BlinkM_setRGB( blinkm_addr, r,g,b );
  delay(10);
  BlinkM_fadeToRGB( blinkm_addr, 0,0,0 );
  delay(1000);
}

void BlinkM_off(byte addr)
{
  BlinkM_stopScript( addr );
  BlinkM_setFadeSpeed(addr,20);
  BlinkM_setRGB(addr, 0,0,0 );
}