Sending number as byte through serial

Question

I'm trying to send data from an Arduino to an Android device.

I saw an online guide for that which provided this code:

void setup() 
{ 
  Serial.begin(115200);
} 

void loop() 
{ 
  byte msg[64]; 
  int len = Serial.available();
  if (len > 0)
  {
    len = Serial.readBytes((char*)msg, sizeof(msg)); // readBytes seems to need a char* not a byte*
    Serial.write(msg, len);
  }
}  

I tried that example and it is working perfectly!

But when I put Serial.write("3"); in the setup loop after Serial.begin(115200); it appears as question mark on my Android device, I guess that I'm not sending it as a byte, I searched online but without any luck.

Answer 1

Start by changing it to Serial.write("3", 1);

Does that help, you're missing the second parameter to the Serial.write call, which is the number of characters (bytes) to write. 

Answer 2

I think this is a timing issue. It is fairly normal for a garbage byte or two to be received shortly after doing a Serial.begin() because the transition of "not defined" (or high impedance) output to Serial (which is normally all 1-bits) can cause the receiver to think it got a start bit, and then misinterpret what follows.

Try adding a short delay after Serial.begin. You could also try a pull-up resistor on the serial Tx line to ensure it is high prior to Serial.begin being called.

Your use of Serial.available effectively builds in a delay.