CODEWITHSUNDEEP
pythonarduinoraspberry-pii2carduino-uno
Jonathan König
Jonathan König

Reputation: 70

Wrong data transmission I2C RasPi->Arduino

I'm tinkering with my RaspberryPi and my Arduino to send some text over I2C. I got it working so far but there is a number appearing which shouldn't exist.

I'm sending "Hello" converting it to an int array and sending it over I2C.

['H','e','l','l','o'] => [72,97,108,108,111] // This should be the result
['H','e','l','l','o'] => [0, 5, 72, 101, 108, 108, 111] // This is what i get

The leading 0 is intentional but the 5 shouldn't exist at all!

The (shortened) code I'm running on the Arduino:

void setup() {
  Serial.begin(9600); // start serial for output
  // initialize i2c as slave
  Wire.begin(SLAVE_ADDRESS);

  // define callbacks for i2c communication
  Wire.onReceive(receiveData);  
  Serial.println("Ready!");
}

receiveData(int byteCount) {
    int i;

    // Iterate through the byte packets
    for(i = 0; Wire.available(); i++) { 
        number = Wire.read();

        if(i != 0) {  // Ignore the first byte
            text[i-1] = (char)number;
        }

        // Output number, byteCount, and i over the serial bus
    }

    Serial.print("Result: ");
    Serial.println(text);
}

The exact output I'm getting:

[CMD]data received: 0, 2char: , byteCount: 7, Iteration: 0 //The cmd byte
data received: 5, 2char: , byteCount: 7, Iteration: 1
data received: 72, 2char: H, byteCount: 7, Iteration: 2
data received: 101, 2char: e, byteCount: 7, Iteration: 3
data received: 108, 2char: l, byteCount: 7, Iteration: 4
data received: 108, 2char: l, byteCount: 7, Iteration: 5
data received: 111, 2char: o, byteCount: 7, Iteration: 6
Result: "Hello"

The code running on the RaspberryPi (Python):

import smbus
import time

# Initiate the SMBus on device 1
bus = smbus.SMBus(1)

# The address of the Arduino
address = 0x04

chars = [] # The character/int array
test = "Hello" # The test text

# Split up the string in individual chars, 
# convert them to int and add them to the array
for c in test:
    chars.append(ord(c))

# send the data... the 0 is the cmd byte! The function accepts an int array
bus.write_block_data(address, 0, chars)

Upvotes: 2

Views: 973

Answers (1)

patthoyts
patthoyts

Reputation: 33223

Just from the values shown it is likely to be the number of chars or bytes being sent in the string part of the transmission. "Hello" has 5 bytes in ASCII representation.

If you look at the description of the smbus protocol in the section for the write_block_data command it documents that an 8 bit Count is sent after the command byte which gives the length of the Data block.

SMBus Block Write: i2c_smbus_write_block_data()

The opposite of the Block Read command, this writes up to 32 bytes to a device, to a designated register that is specified through the Comm byte. The amount of data is specified in the Count byte.

S Addr Wr [A] Comm [A] Count [A] Data [A] Data [A] ... [A] Data [A] P

Upvotes: 1

Related Questions