Arduino to Arduino I2C, getting 0xFF

Forum 2005-2010 (read only) Hardware Interfacing
#1

Hello, I’m trying to simulate an I2C part using an arduino, so I set up this little test case to try to pass information from an Arduino as a slave on the I2C bus.

Here’s the issue:
I should be receiving 0x68, 0x65.
I am receiving: 0x65, 0xFF.

Any ideas?

I have a common ground between the boards.

Master Sketch:

#include <Wire.h>

void setup()
{
  Wire.begin();        // join i2c bus (address optional for master)
  Serial.begin(9600);  // start serial for output
}

void loop()
{
  Wire.requestFrom(2,2 );    // request 2 bytes from slave device #2

  while(Wire.available())    // slave may send less than requested
  { 
    byte b = Wire.receive();
    Serial.print(b, HEX);         // print the character
  }
  delay(1000);
}

Slave Sketch

#include <Wire.h>

void setup()
{
  Wire.begin(2);                // join i2c bus with address #2
  Wire.onRequest(requestEvent); // register event
}

void loop()
{
  delay(100);
}

// function that executes whenever data is requested by master
// this function is registered as an event, see setup()
void requestEvent()
{ 
  byte b;
  b = 0x68;
  Wire.send(b);
  b = 0x65;
  Wire.send(b);

}
#2

and the default sketch works just fine....

#3

There is also Wire.send(data, quantity)
Its not really documented
http://arduino.cc/en/Reference/WireSend
I never made a slave but give it a try:

char [] answer={0x68,0x65};
Wire.send(answer,2);

Eberhard

#4

Are you using version 0017?

You say you have a common ground between the two boards. What about pullup resistors, length of wires between the boards? Are both boards 5V?

Have you tried requesting/returning three bytes to see if the issue is consistent (first byte lost)?

#5

Wayoda,

I tried it....changed declaration to: 

char answer[2]={0x68,0x65};

Arduino says invalid conversion from char* to unit8_t;

BenF, The bus isn't the issue, the default sketch works just fine, I just can't seem to send and receive bytes correctly.

I rewrote the slave code to send 3 bytes:

  b = 0x68;
  Wire.send(b);
  b = 0x65;
  Wire.send(b);
  b = 0x62;
  Wire.send(b);

What I get back is: 62FFFF

So it looks like I'm receiving the last byte first, and then everything else is 1's.

#6

Oh, and I am using 017, but I had the same issues in 016

#7

Wayoda, THANK YOU.

I changed the code to uint8_t array, and it works!

Here’s the new sending code that works.

#include <Wire.h>

void setup()
{
  Wire.begin(2);                // join i2c bus with address #2
  Wire.onRequest(requestEvent); // register event
}

void loop()
{
  delay(100);
}

// function that executes whenever data is requested by master
// this function is registered as an event, see setup()
void requestEvent()
{ 
  uint8_t b[3] = {0x68, 0x65, 0x62};
  Wire.send(b,3);
  /* This code does not work, but should (in my opinion!)
  b = 0x68;
  Wire.send(b);
  b = 0x65;
  Wire.send(b);
  b = 0x62;
  */

}
#8

Hi, sorry about the char issue , but I'm glad it works now. I looked at the sources for the Wire lib and found out , you have to use the send(data, length) method if you want to return more than 1 byte in a request to a I²C slave. The way it is implemented is correct, but as I wrote earlier it is very badly documented. I will file a bug report for this...

Thanks for keeping us updated!

Eberhard

#9

This code does not work, but should (in my opinion!)

Looking at the source I can see why, but as you say - it is far from obvious.

#10

BenF,

I'm not trying to slam anyone, just adding a running commentary in my code. I should have removed it before posting.

I'd love to write up my experiences and document them...if anyone knows the right person to update the Arduino wire library documentation.

#11

I'd love to write up my experiences and document them...if anyone knows the right person to update the Arduino wire library documentation.

There are only a few people who have permissions to update the docs on the reference-section of the site. I filed a request for this, which is about all one can do.

http://code.google.com/p/arduino/issues/detail?id=180 Eberhard

#12