How to read data from Arduino with Raspberry Pi with I2C

Question

I am trying to read data from an Arduino UNO to Raspberry Pi with the python smbus module. The only documentation I could find on the smbus module was here. I am not sure what the cmd means in the module. I can use the write to send data to the Arduino. I have written two simple programs one for read and one for write

The one for write

import smbus
b = smbus.SMBus(0)
while (0==0):
    var = input("Value to Write:")
    b.write_byte_data(0x10,0x00,int(var))

The one for read

import smbus
bus = smbus.SMBus(0)
var = bus.read_byte_data(0x10,0x00)
print(var)

The Arduino code is

#include <SoftwareSerial.h>
#include <LiquidCrystal.h>
#include <Wire.h>
LiquidCrystal lcd(8,9,4,5,6,7);

int a = 7;

void setup() 
{ 
  Serial.begin(9600);
  lcd.begin(16,2);
  // define slave address (0x2A = 42)
  #define SLAVE_ADDRESS 0x10

  // initialize i2c as slave
  Wire.begin(SLAVE_ADDRESS);

  // define callbacks for i2c communication
  Wire.onReceive(receiveData);
  Wire.onRequest(sendData); 
}
void loop(){
}

// callback for received data
void receiveData(int byteCount) 
{
 Serial.println(byteCount);
  for (int i=0;i <= byteCount;i++){
  char c = Wire.read();
  Serial.println(c);
 }
}

// callback for sending data
void sendData()
{ 
  Wire.write(67);
  lcd.println("Send Data");
}

When I run the read program it returns "33" every time. The Arduino returns that the the sendData function is called.

I am using a Data Level Shifter and the description says it might be a little sluggish.

Has anyone gotten this to work?

Answer 1

I know this is an old post but I have had some similar success and this seemed like a descent place to append my success.

I've been attempting to offload the calculation of high RPM to an ATTiny85 for a long time. I wanted to send the RPM on i2c request to the Pi rather than utilizing a GPIO pin interrupt and processor time on the pi. I finally accomplished this today. Here's what I came up with. Hope it can help someone. If anyone wants to critique my code, have at it!

AtTiny 85 ino code: 1 Mhz clock speed

#include <TinyWire.h>

//            _____
//     RST  -|o AT |- VDD (1.8-5.5v)
//   A3/D3  -| Tiny|- D2
//   A2/D4  -|  85 |- D1     (PWM)  Can use analogWrite(pin_0_or_1, 255);
//     GND  -|_____|- D0 SDA (PWM)  on pwm pins. (8 bit resolution=0-255)

#define own_address 0x26    // I2C address - change this in arduino code if changed
#define LED_pin 4 
#define hallPin 3 //Melexis US5881 Hall Effect Sensor  (Put a 50K Ohm pull up resistor
                  //                                    between VDD and OUT pins
bool pulseState = false;
const int sample_rate_ms = 1000;

volatile uint32_t timeCheck = 0;  //uint32_t is same as "unsigned long"

volatile uint32_t pulse1 = 0;  //uint32_t is same as "unsigned long"
volatile uint32_t pulse2 = 0;  //uint32_t is same as "unsigned long"    

volatile uint8_t i2c_regs[2] = { 0, 0 };
volatile uint16_t RPM = 0; 
volatile int counter = 0;   //int is same as short

void setup() {
  pinMode(LED_pin, OUTPUT);      // If you want to enable an LED for status.
  pinMode(hallPin, INPUT);       
  digitalWrite(LED_pin, HIGH);   // Turn on the LED to indicate alive status
  delay(2000);                   // Leave it on for 2 seconds
  digitalWrite(LED_pin, LOW);    // Turn it off
  
  TinyWire.begin( own_address );
  TinyWire.onRequest( onI2CRequest );
  
  timeCheck = millis() + sample_rate_ms;   // Set a time in the future to calculate
  pulse1 = micros();                       // Take an initial time hack
  sei();     // Enable interrupts
}

void loop() {
  if (digitalRead(hallPin)!=pulseState){       // Look at the hall state
    pulseState = !pulseState;                  // If the pin isn't what it was before
    if (!pulseState){                          // Check if the pin was pulled low
      pulse1 = pulse2;                         // Start time was previous pulse
      pulse2 = micros();                       // End time is this pulse  
      ++counter;                               // Count pulses for determining 0 RPM
      delay(0.0016);                           // Wait for 1/600th of a second (to 
                                               //  prevent bounces) Should be good up 
                                               //  to about 20,000rpm. This may not be
    }                                          //  necessary but it works.
  }

  if(millis() > timeCheck){   // Every so often (once per second in my code)
                              // calculate RPM or lack thereof. 
        timeCheck = millis() + sample_rate_ms;
        if (counter < 2){     //  Less than 2 pulses, it's not rotating
          RPM = 0;
        }else{
          RPM = 60000000 / (pulse2 - pulse1);  // Revs per minute at 1000000 ticks per 
                                               // second using micros()
        }
        i2c_regs[0] = (RPM & 0xFF00) >>8;    // Prepare the two bytes to send
        i2c_regs[1] = (RPM & 0x00FF);        // and place them into an array
        counter = 0;                         // Reset the counter for the next loop
     }
  }
void onI2CRequest() {
  TinyWire.send(i2c_regs, sizeof(i2c_regs));   // Send the two bytes over i2c
}

Here's the Raspberry Pi Python 3 code:

from smbus2 import SMBus, i2c_msg
# https://pypi.org/project/smbus2/
# To install on raspberry pi execute the following:
# pip install smbus2
# or 
# sudo python3 -m pip install smbus2
# or
# sudo python3 -m pip3 install smbus2

import time
from datetime import datetime, timedelta


def bytes_to_int(bytes):
    result = 0

    for b in bytes:
        result = result * 256 + int(b)

    return result

def int_to_bytes(value, length):
    # This routine is unused for now
    result = []

    for i in range(0, length):
        result.append(value >> (i * 8) & 0xff)

    result.reverse()

    return result


last_good_data_at = datetime.utcnow()
with SMBus(1) as bus:
    address=0x26
    while True:
     
        time.sleep(0.25)

        try:
            read = i2c_msg.read(address, 2)
            bus.i2c_rdwr(read)
            bytes = list(read)
        except:
            bytes = []

        if bytes:
            last_good_data_at = datetime.utcnow()
            rpm = bytes_to_int(bytes)

            print(f"                     " + "\r", end="")
            print(f"RPM: {rpm}   Bytes: {bytes}" + "\r", end="")

        elif datetime.utcnow() > last_good_data_at + timedelta(seconds=10):
            break
    print("No data received for 10 seconds.")
    print("Terminating program")

Answer 2

I managed to initiate a communication between an Arduino and a Raspberry Pi. The two are connected using two 5k pullup resistors (see this page). The arduino write a byte on the i2c bus for each request. On the Raspberry Pi, hello is printed every second.

Arduino code:

#include <Wire.h>
#define SLAVE_ADDRESS 0x2A

void setup() {
    // initialize i2c as slave
    Wire.begin(SLAVE_ADDRESS);
    Wire.onRequest(sendData); 
}

void loop() {
}

char data[] = "hello";
int index = 0;

// callback for sending data
void sendData() { 
    Wire.write(data[index]);
    ++index;
    if (index >= 5) {
         index = 0;
    }
 }

Python code on the Raspberry Pi:

#!/usr/bin/python

import smbus
import time
bus = smbus.SMBus(1)
address = 0x2a

while True:
    data = ""
    for i in range(0, 5):
            data += chr(bus.read_byte(address));
    print data
    time.sleep(1);

On my Raspberry Pi, the i2c bus is 1. Use the command i2c-detect -y 0 or i2c-detect -y 1 to verify if your Raspberry Pi detect your Arduino.