I have a Teensy 4.1 and a RaspberryPi connected via I2C with a working connection. I reproduced the results from this tutorial: (https://dronebotworkshop.com/i2c-arduino-raspberry-pi/) (Code also below), where you get the teensy onboard LED to flash with a python program using smbus2 on the raspberry.

I can flash the LED as intended, but I noticed that the I2C bus is full of messages when I print them to the serial monitor. When I use Seriell.println() in the callback function to see arriving bytes, my terminal is flooded with the one byte I tried to send, even though there is only a single call from the master to write to the bus. I get around 3500 calls/second to the ReceiveEvent() callback function which indicates that a I2C message was received.

When I print out the number of bytes in a I2C message using Wire.available(), it is always capped at the maximum of 40 bytes. This should be only 1 byte, because the master only calls bus.write_byte() once.

This continues even if I shutdown Ubuntu on the raspberry, but ends when I pull its power plug.

Question: Why are there so many bytes sent over the I2C bus? As I currently understand there should only be a single byte sent, or am I missunderstanding something?

EDIT: I used a differerent Teensy (4.1) and the same code worked as expected, not flooding the I2C connection... So problem successfully defered!

Teensy/Arduino Code (I2C Slave):

// Include the Wire library for I2C
#include <Wire.h>
volatile int i = 0;
// Function that executes whenever data is received from master
void receiveEvent(int howMany) {
  while (Wire.available()) { // loop through all but the last
    char c = Wire.read(); // receive byte as a character
    digitalWrite(LED_BUILTIN, c);

void setup() {
  // Join I2C bus as slave with address 8
  // Call receiveEvent when data received                
  digitalWrite(LED_BUILTIN, HIGH);
  digitalWrite(LED_BUILTIN, LOW);

void loop() {
  int j = i;
  Serial.print(" +");

Raspberry Python Code (I2C Master):

from smbus import SMBus
addr = 0x8 # bus address
bus = SMBus(1) # indicates /dev/ic2-1
numb = 1
print ("Enter 1 for ON or 0 for OFF")
while numb == 1:
    ledstate = input(">>>>   ")
    if ledstate == "1":
        bus.write_byte(addr, 0x1) # switch it on
    elif ledstate == "0":
        bus.write_byte(addr, 0x0) # switch it on
        numb = 0
  • 2
    you are asking a question about a problem with an unknown connection ... please provide detailed information about the connection between the two devices
    – jsotola
    Sep 18 at 16:40
  • Do you see >>>> once or more than once? Do you have pull-up resistors on SDA and SCL?
    – Nick Gammon
    Sep 18 at 22:07
  • I'm using the I2C pins 18 & 19 on the Teensy LC with the I2C pins on the arduino, which have integrated pullup resistors. This exact setup (with cables and everything) was made and used by someone else before me. Apperantly it was working correctly then. Do you mean >>>> in python? No, just once.
    – Phi
    Sep 19 at 8:31
  • Please add such information to your question by editing it, this is not a forum. -- Do you have access to an oscilloscope or logic analyzer? Then please watch the lines SDA and SCL, and add your findings, too. Sep 19 at 8:50


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