RasPi 3 - Arduino Uno I2C communication - Block transfer error

Question

I am trying to connect an Arduino Uno with my Raspberry using the i2c bus, based on the tutorial given here, sending only one byte from RPi to Arduino and back. I was able to implement this and it works fine.

I needed to transfer more than a byte of data, so I referred to this page and tried to implement the given solution with python 3.4.2. However when I implement the solution, it gives me following error:

>>>
[45, 0, 25, 2, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255]
Traceback (most recent call last):
  File "/home/pi/codeSampleMaster2.py", line 15, in <module>
    number = readLong()
  File "/home/pi/codeSampleMaster2.py", line 10, in readLong
    n = struct.unpack('<l',(''.join([chr(i) for i in block[:4]])))[0]
TypeError: 'str' does not support the buffer interface

The data in the square bracket is what I received as part of block read. What I require is to be able to extract 45002502 as part of my program.

Please guide me as to where I might have made a mistake. Im fairly new to working with python and Rasp Pi.

Arduino Code:

#include <Wire.h>
#define SLAVE_ADDRESS 0x04
int senseLeft = A0;     // Left side distance sensor input pin.
long senseLeftVal = 0;   // Variable to store the Left side sensor value.

void setup() {
  Serial.begin (9600); // start serial for output 
  Wire.begin(SLAVE_ADDRESS);
  Wire.onRequest(requestEvent); // register event
  Serial.println("Ready!");
}

void loop() {
//  Serial.println(senseLeftVal);
  delay(1);
}

void requestEvent()
{
  senseLeftVal = analogRead(senseLeft);
  Wire.write((const uint8_t*)&senseLeftVal, sizeof(long));
  Serial.println(senseLeftVal);
}

Rasp Pi Code:

import smbus
import time
import struct

bus = smbus.SMBus(1)
address = 0x04

def readLong():
    block = bus.read_i2c_block_data(address,0)
    print(block)
    n = struct.unpack('<l',(''.join([chr(i) for i in block[:4]])))[0]
    return n

while True:
    time.sleep(1)
    number = readLong()
    print("Arduino: ", number)

Answer 1

I have a feeling the following line is the problem

n = struct.unpack('<l',(''.join([chr(i) for i in block[:4]])))[0]

try replacing it with this

n = block[0] << 24 | block[1] << 16 | block[2] << 8 | block[3]
print(n)

if the numbers dont make sense, you might need to swap the order like so (for endianess)

n = block[3] << 24 | block[2] << 16 | block[1] << 8 | block[0]
print(n)

and check again

---

Also as joan suggests in this post, you might want move the analogRead after the Wire.write to reduce the chances of missed communication as timing is critical in dealing with i2c devices. Ideally you'd want to move the analogRead into the loop and simply send out the last read value when requested in the event handler

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Looking at the documentation and example for smbus over here it makes sense that you are getting 32 bytes of data

Example 2: Read a block of data Note: You can read up to 32 bytes at once.

from smbus2 import SMBusWrapper    
with SMBusWrapper(1) as bus:
# Read a block of 16 bytes from address 80, offset 0    
block = bus.read_i2c_block_data(80, 0, 16) # Returned value is a list of 16 bytes    
print(block)

Since you are only sending 4 bytes = (sizeof(long)) from your arduino, there are 28 bytes filled with junk (255 in your case). the simplest fix is just use the first 4 bytes like so

import smbus
import time
import struct

bus = smbus.SMBus(1)
address = 0x04

def readLong():
    block = bus.read_i2c_block_data(address,0)
    print(block) # should print the array with 32 entries

    reading = block[0:4] # select only first 4 bytes
    print(reading)

    n = reading[0] << 24 | reading[1] << 16 | reading[2] << 8 | reading[3]
    print(n)

    return n

while True:
    time.sleep(1)
    number = readLong()
    print("Arduino: ", number)

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