Sending int over I2C - UNO master, ATtiny85 slabe

Projects Networking, Protocols, and Devices
1

I'm trying to use an ATtiny85 to read a MPX5010DP Freescale Pressure Sensor, then send the reading back to an UNO via I2C. It's just an analog read of the sensor.
I've looked at a number of examples, and this is a far as I've gotten with it. I can read the pin, map the 0-1023 reading down to 0-255 and send it as a byte using the below pair of sketches.

On the UNO:

#include "Wire.h";
int selection=0;
int ATtinyAddress=0x26;
unsigned long marktime; 

void setup(){
  Wire.begin();
  Serial.begin(9600);
}
void loop(){
  if (millis()>marktime)
  {
  Wire.requestFrom(ATtinyAddress,1);
  while(Wire.available())
  {
    byte v = Wire.read();
    Serial.println(v);
  }
  marktime+=1000;
  }
}

on the ATtiny85

#include "TinyWireS.h"                  
#define I2C_SLAVE_ADDR  0x26            
#define Sensor_PIN  3

void setup(){
  pinMode(Sensor_PIN,INPUT);
  TinyWireS.begin(I2C_SLAVE_ADDR);      
  TinyWireS.onRequest(requestEvent);
}

void loop(){
}

void requestEvent()
{
  int pressure=analogRead(Sensor_PIN);
  byte pbyte=map(pressure,0,1023,0,255);
  TinyWireS.send(pbyte);
}

I'd like the full resolution of the Tiny, though. I know I can use lowByte & highByte to get a pair of bytes, and I know I can word() the two back together once I have them on the UNO.
I'm lost at the part of requesting and receiving two bytes, instead of one, and assigning them usable values on the UNO side.

2

its here - Arduino Playground - WireLibraryDetailedReference -

check requestFrom(address, count);

3

robtillaart:
its here - Arduino Playground - HomePage -

check requestFrom(address, count);

Thanks for pointing me in that direction. Between that, and plagerizing other code here from the forum (http://forum.arduino.cc/index.php/topic,160680.0.html), I now have a pair of sketches that work, at least. Despite it's function, I'm not fond of this code on the slave.

First, here's the master code so you can see it.

#include "Wire.h";
int ATtinyAddress=0x26;
unsigned long marktime; 

void setup()
{
  Wire.begin();
  Serial.begin(9600);
}


void loop(){
  if (millis()>marktime)
  {
    byte hb;
    byte lb;
    Wire.requestFrom(ATtinyAddress,2); 
    if (Wire.available()) 
    {
      lb=Wire.read();
      hb=Wire.read();
    }
    int pressure=word(hb,lb);
    Serial.println (pressure); 
    marktime+=1000;
  }
}

Here's the slave code that works.

#include "TinyWireS.h"                  
#define I2C_SLAVE_ADDR  0x26            
#define Sensor_PIN  3
boolean firstbyte = true;

void setup(){
  pinMode(Sensor_PIN,INPUT);
  TinyWireS.begin(I2C_SLAVE_ADDR);      
  TinyWireS.onRequest(requestEvent);
}

void loop(){
}


void requestEvent()
{
  int pressure=analogRead(Sensor_PIN);
  byte plow;
  byte phi;

  if(firstbyte == true)
  {     // on the first byte we do the math
    plow=lowByte(pressure);
    firstbyte = false;      //so next time though we send the next byte    
    TinyWireS.send(plow);
  } 
  else 
  {
    phi=highByte(pressure);
    TinyWireS.send(phi);
    firstbyte = true;
  }
}

I don't understand why this code doesn't work. What purpose does the boolean firstbyte serve in the above sketch?

void requestEvent()
{
  int pressure=analogRead(Sensor_PIN);
  byte plow;
  byte phi;
  plow=lowByte(pressure);
  phi=highByte(pressure);
  TinyWireS.send(plow);
  TinyWireS.send(phi);
}
4

And, in contrast - this works perfectly if I use a Nano as a slave instead of a tiny.

#include "Wire.h"                  
#define I2C_SLAVE_ADDR  0x26            
#define Sensor_PIN  3
int pressure;

void setup(){
  pinMode(Sensor_PIN,INPUT);
  Wire.begin(I2C_SLAVE_ADDR);      
  Wire.onRequest(requestEvent);
}

void loop()
{
}


void requestEvent()
{
  pressure=analogRead(Sensor_PIN);
  byte plow=lowByte(pressure);
  byte phi=highByte(pressure);
  byte data[]={plow, phi};
  Wire.write(data, 2);
  }