AtTiny85 and MMA8452Q accelerometer cannot connect using i2c

Question

I am trying to connect the MMA8452Q with AtTiny85 using the TinyWireM library. I made the proper hardware setup with pull-ups on SCL and SDA pins - tested it on Arduino Uno and it works, however when i connect the Tiny I cannot even get the handshake. The following code produces output 13 instead of 42 (0x2A)

HARDWARE SETUP

MMA8452 Breakout ------------AtTiny85

3.3V -------voltage regulator------ 5V

SDA -----------^^(330)^^----------- PB0 (on attiny programmer marked as 5)

SCL -----------^^(330)^^----------- PB3 (on attiny programmer marked as 2)

GND --------------------------------- GND

(I also tried different pins with no luck)

CODE

//---bluetooth console communication stuff---//
#include <SoftwareSerial.h>
#define rxPin 1 //not important - needed for initialization
#define txPin A2
SoftwareSerial blueTooth(rxPin, txPin);
//      ---     //


#include "TinyWireM.h" // Used for I2C

// The SparkFun breakout board defaults to 1, set to 0 if SA0 jumper on the bottom of the board is set
#define MMA8452_ADDRESS 0x1D  // 0x1D if SA0 is high, 0x1C if low


#define WHO_AM_I   0x0D



void setup() {

  blueTooth.begin(9600);
  delay(1000);
  TinyWireM.begin(); //Join the bus as a master
  delay(1000);
 }

void loop() {
   byte c =0;
   c = readRegister(WHO_AM_I); 
   blueTooth.println(c); 
   delay(1000);
  }


 // Read a single byte from addressToRead and return it as a byte
byte readRegister(byte addressToRead) {
  TinyWireM.beginTransmission(MMA8452_ADDRESS);
  TinyWireM.send(addressToRead);
  TinyWireM.endTransmission(); //endTransmission but keep the connection active

  TinyWireM.requestFrom(MMA8452_ADDRESS, 1); //Ask for 1 byte, once done, bus is released by default

  while(!TinyWireM.available()) ; //Wait for the data to come back
  return TinyWireM.receive(); //Return this one byte
} 

Would anyone know how to get it right?

Answer 1

On an ATtiny85 pin 5 (PB0) is SDA and pin 7 (PB2) is SCL for the I2C interface. You're using the wrong pin for SCL.

Additionally, your pull-up resistors are too strong. Somewhere between 4.7K Ohm and 10K Ohm are recommended for pull-up's on the SDA and SCL lines.

Answer 2

using the TinyWireM library.

Make sure the i2c actually works. Arduino libraries are generally of poor quality and just because someone wrote it doesn't mean it actually works. Get a logic analyzer or scope on the bus and see what's going on.

Once you can confirm that, get a datasheet for the 8452 and see how it works and code to it.