Sending float from arduino to raspberry pi using Wire (arduino) and smbus (python) libraries

Send multiple floats from Arduino to Raspberry through I2C

http://bradsrpi.blogspot.fr/2013/03/sending-data-from-arduino-to-raspberry.html

i2c_float.c

#include <Wire.h>

#define SLAVE_ADDRESS 0x04

#define FLOATS_SENT 2

float temperature = 10.5;
float luminosity = 5.2;
float data[FLOATS_SENT];

void setup() {
    pinMode(13, OUTPUT);
    Serial.begin(9600);
    
    data[0] = temperature;
    data[1] = luminosity;
    
    // initialize i2c as slave
    Wire.begin(SLAVE_ADDRESS);

    // define callbacks for i2c communication
    Wire.onRequest(sendData);
}

void loop() {
    delay(100);
}

void sendData(){
  Wire.write((byte*) &temperature, FLOATS_SENT*sizeof(float));
}

i2c_float.py

import time
import struct

import smbus

# for RPI version 1, use "bus = smbus.SMBus(0)"
bus = smbus.SMBus(1)

# This is the address we setup in the Arduino Program
address = 0x04


def get_data():
    return bus.read_i2c_block_data(address, 0, 8)


def get_float(data, index):
    bytes = data[4*index:(index+1)*4]
    return struct.unpack('f', "".join(map(chr, bytes)))[0]


while True:
    try:
        data = get_data()
        print(get_float(data, 0))
        print(get_float(data, 1))
    except Exception as e:
        print(e)
        continue
    finally:
        time.sleep(1)

Hi, I have the code implemented as given below (pretty much the same as given above, minus error catching to get to the error message).
I get an IOError: [Errno 121] Remote I/O error on the Raspberry Pi (running Python 2.7), once it tries bus.read_i2c_block_data(address, 0, 8).

When, however, I try to do a bus.read_byte(address), no error is encountered.

I'm quite possibly being an idiot, but does anyone have any ideas why this 121 keeps on happening?

i2c_float.py

import struct
import smbus
import time

bus = smbus.SMBus(1)

address = 0x04

def get_data():
    return bus.read_i2c_block_data(address, 0, 8);

def get_float(data, index):
    bytes = data[4*index:(index+1)*4]
    return struct.unpack('f', "".join(map(chr, bytes)))[0]

while True:
    time.sleep(1);
    data = get_data()
    print(get_float(data, 0))
    print(get_float(data, 1))

and i2c_float.c on Arduino Mega

#include <Wire.h>

#define SLAVE_ADDRESS 0x04

#define FLOATS_SENT 2

float temperature = 10.5;
float luminosity = 5.2;
float data[FLOATS_SENT];

void setup() {
    pinMode(13, OUTPUT);
    Serial.begin(9600);
    
    data[0] = temperature;
    data[1] = luminosity;
    
    // initialize i2c as slave
    Wire.begin(SLAVE_ADDRESS);

    // define callbacks for i2c communication
    Wire.onRequest(sendData);
}

void loop() {
    delay(100);
}

void sendData(){
  Wire.write((byte*) &temperature, FLOATS_SENT*sizeof(float));
}

thanks @Tarkahn For Your Help

Thank you for this post it has been very useful for me.
I tried to add a third floating value. On the Arduino side everything goes well. I added the following lines in the code:

#define FLOATS_SENT 3;

then I declare my third floating value

float` humidity = 19.6;

and added the line

data [2] = humidity;

On the side of the Raspberry, I added a print (get_float (data, 2)) , but I have the following error message:

return struct.unpack ('f', "" .join (map (chr, bytes))) [0]
struct.error: unpack requires a string argument of length 4

Any ideas please?

This is the program we setup in the Rpi

import smbus
import time
import struct
bus = smbus.SMBus(1)
address = 0x04

def get_data():
return bus.read_i2c_block_data(address, 0);
def get_float(data_bytes,index):
bytes = data_bytes[4index:4index+4]
aux = bytearray(bytes)
data_float=struct.unpack('<f',aux)[0]
return data_float
while True:
try:
data = get_data()
print(str(get_float(data, 0))+" dB")
print(str(get_float(data, 1))+" index")
print(str(get_float(data, 2))+" ppm")
print(" ")
except:
continue
time.sleep(1);

This is the program we setup in the Arduino Program

#include <Wire.h>

#define SLAVE_ADDRESS 0x04

#define FLOATS_SENT 3

float audio = 10.5;
float uv = 5.2;
float aire = 1500.8;
float data[FLOATS_SENT];

void setup() {
pinMode(13, OUTPUT);
Serial.begin(9600);

data[0] = audio;
data[1] = uv;
data[2] = aire;
// initialize i2c as slave
Wire.begin(SLAVE_ADDRESS);

// define callbacks for i2c communication
Wire.onRequest(sendData);

}

void loop() {
delay(100);
}

void sendData(){
Wire.write((byte*) &data, FLOATS_SENT*sizeof(float));
}

I'm a bit late to the party, I'm sure you guys have figured it out by now. I've only done 1 unit in JAVA data structures so this it might not be the best way but its what worked for me.

Arduino slave

#include "ph_grav.h" //my headerfile
#include <Wire.h>
#define SLAVE_ADDRESS 0x08 // arduino slave address
volatile float pHreading;
Gravity_pH pH = Gravity_pH(A0);
//int FLOATS_TO_SEND = 4;

void setup() {
Wire.begin(SLAVE_ADDRESS);
Wire.onReceive(receiveData);
Wire.onRequest(sendData); //send data when raspberry askes for it

}

void loop() {
noInterrupts(); // inssure the reading and data sending not a the same time
pHreading = (pH.read_ph()); // my sensor function, use what ever float you want
interrupts();
delay(1000);
}

void sendData()
{
Wire.write((uint8_t*) &pHreading, sizeof(float)); // writes the float to wire (4 bytes)
}
//just make a for loop to send an array of floats e.g
//for ( int i = 0; i < FLOATS_TO_SEND; i++){
//Wire.write((uint8_t*) &pHreading[i], sizeof(float));}
//and the same on the raspberry pi side

void receiveData(int bytecount)
{
//if you want to do something when the rasberry sends data
}

Raspberry master

import struct
import smbus
import time

bus = smbus.SMBus(1) # raspberry pi address this might be (0) if on older raspberry pi's

address = 0x08 #address of arduino

def get_data():
temp = bytearray(bus.read_i2c_block_data(address, 1, 1)) # through away the first byte as this is the ack byte
return bytearray(bus.read_i2c_block_data(address, 1, 4));# return 4 bytes in a array ( bytes sent from arduino)

while True:
time.sleep(1);
data = get_data()
data2 = struct.unpack('f', data) # convert the array of bytes to floats
print(data2); #check the it worked

I am also late to the party ;-) hopefully there is still somebody around.

Basically, it does exactly what I want. The modification
read_i2c_block_data(address, 0, 8);
is essential (and python would not complain about the semicolon???).
Otherwise, without the third parameter, the Arduino runs wild and keeps sending 255 and would not stop.

But I have a problem:
The parameter "cmd", which is 0 in this case, is not understood by the Arduino.
When reading from the raspi, the Arduino confuses this 0 with other incoming data.
So this solution works one-way only.

I found another solution here:
https://stackoverflow.com/questions/14420372/how-to-read-data-from-arduino-with-raspberry-pi-with-i2c?rq=2

i2c_msg can be used to request the amount of data needed.

Wow, thanks for the interest in the script !

I cleaned up the script and implemented the read size based on @wicusverhoef's suggestion. However I can't test it right now, I count on you lads :)