For some reason when I loop my code through a separate function the serial port hangs up. The serial connection appears fine when I call cases within the loop but not when I call outside.

I tried to figure out where the problem is and for some reason using the Serial.print(ComControlValue); and the output is


But the error still persists.

Previous it was this before I changed the baudrate

-b⸮⸮⸮⸮ Waiting for Command

It's not printing out the -1 I'm using as the main case. Could this be the problem?

#include <Wire.h>
/* RPi Army variables */
int RPi[3] = {3,6,9};                                        // SDA/SCL code names for the different Raspberry pi's

/* Define parameters for computer control */
int ComControlValue = -1;

void setup() {
  Serial.begin(115200);                                     // Establish Serial connection to computer
  Wire.begin();                                             // join i2c bus as master

void loop() {
  /* -1 : nothing happened yet */
  case -1:
     Serial.println("Waiting for Command");
     while (Serial.available()==0){}
     ComControlValue=Serial.parseInt();                             // Pull value from serial port

  case 111:
     Serial.println("Eat more Toast");
     ComControlValue = -1;

  case 210:
     ComControlValue= -1;

void Sendsignals(int Rpi, int code){
   char str[4];
   sprintf(str, "%3d\n", code);
   Wire.beginTransmission(Rpi);                                 // transmit to device of choice
   Wire.write(str);                                             // sends 16 bytes
   Wire.endTransmission();                                      // stop transmitting
  • Consider adding more debug lines during your Sendsignals function. Perhaps it hangs up during the Wire transmission. You should also consider changing sprintf to snprintf - apparently it's more robust against buffer issues, and requires the buffer size to be set manually.
    – towe
    Nov 16, 2018 at 7:16
  • I tried that and it seems everything runs fine up until the break; when going through my separate function. Nov 16, 2018 at 7:43
  • Arduino Uno with an Adafruit motor shield Nov 16, 2018 at 8:17
  • reconnecting Serial Monitor resets the board. while (Serial.available()==0){} blocks the loop
    – Juraj
    Nov 16, 2018 at 8:18
  • Everything seems to work fine though when not calling Wire.h . Once I call the Wire.h functions the serial connection messes up. Nov 16, 2018 at 8:21

1 Answer 1


Your str variable in function Sendsignals is short. sprintf writes 5 bytes to it, 3 digits, the \n and terminating zero. The terminating zero is written outside of the allocated array and causes the crash.

  • This causes the serial terminal to disconnect? How to fix? Should I increase the str to 8 bits? Nov 16, 2018 at 8:47
  • Thanks. I changed the size of the str to 16 and it fixed the issue. Nov 16, 2018 at 8:58
  • str[5] would be enough, if your codes have always 3 digits
    – Juraj
    Nov 16, 2018 at 9:07

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