Having some trouble displaying a chr string using Wire.read() and u8g2.drawStr()

Projects Programming
1

Hi all! I'm still getting myself acquainted with C, so I expect this is a very basic issue I'm facing. I've got an OLED screen connected to a Teensy LC and can display text on it just fine. The Teensy receives bytes via I2C, and I want to reassemble those bytes into strings and display them on the OLED. Let's see if this is an X/Y problem!

My relevant code:

#define I2C_ADDRESS 4
char buf[16];

void setup(void) {
  Serial.begin(9600);
  Wire.begin(I2C_ADDRESS);
  Wire.onReceive(receiveData);
  u8g2.begin();
}
void loop(void) {
  // U8G2 setup code snipped
  // This line doesn't output anything on the screen:
  u8g2.drawStr(0, 0, buf);
  // This line does:
  u8g2.drawStr(0, 20, "Foobar!");
  u8g2.sendBuffer();
  delay(1000);
}

void receiveData(int byteCount){
  Serial.print("Data received: ");
  int i;
  for (i=0; i<byteCount; i++)
  {
    buf[i] = Wire.read();
    Serial.println(buf[i]);
  }
  buf[i] = '\0';
  Serial.println(buf);
}

In the receiveData() function, the Serial.println() within the for loop prints out each letter of the payload as it's received, one after the other - but the Serial.println() below the for loop, which in my estimation should print out the whole message, prints out a blank line. I presume that's related to the fact that the received string doesn't show up on the OLED display.

2 
void receiveData(int byteCount){
  Serial.print("Data received: ");

Never call a Serial output method inside interrupt context! Sooner or later this will fail!

Additionally, every variable used in an interrupt handler and normal code must be declared volatile, otherwise the compiler might optimise it away.

3

This topic was automatically closed 120 days after the last reply. New replies are no longer allowed.