How to clear output buffer of Arduino Uno?

Question

I have coded arduino for DHT11 which used to print humidity in first and temperature in second line. But some times when I open SerialPort I get absurd data.enter image description here

• v7d8dpo4: Possible duplicate of stackoverflow.com/questions/10105666/…
• Shubham Rajput: I am sending this data from arduino to pi serially and from pi to cloud. So some times I used to receive humidity as 6767.0 on the cloud. I am printing on console just for showing an example.

Answer 1

You can't.

The data you are seeing has already been sent by the previous execution of the sketch before the main MCU was reset. That data resides in a small buffer within the USB interface chip (or even within the USB CDC/ACM / USB UART driver itself, depending on your OS) to which you have no access.

You just have to ignore that data. The simplest way is to put a marker at the start of your program to indicate that whatever is receiving the data should start its receiving from this point on. Something like:

Serial.println();
Serial.println("START");

• Gerben: You could disable reset, by adding a capacitor between GND and RESET. Though this make uploading new code less convenient.
• Majenko: @Gerben It also means that you can start reading partway through a line of digits giving you similar garbage at the start of your reception. Doesn't really cure the problem. Better is to use a proper protocol rather than plain ASCII text.
• Gerben: Good point. Didn't think of that.
• Shubham Rajput: I am sending this data from arduino to pi serially and from pi to cloud. So some times I used to receive humidity as 6767.0 on cloud. I am printing on console just for showing an example. So is there no way to clean output buffer of arduino when ever it starts.?

Answer 2

I've circumvented this issue by sending a byte to the Arduino whenever I'm ready to receive data using this quick-and-dirty method:

if(Serial.available() > 0) { // At least one byte is available
 Serial.println(millis());
 Serial.read(); // Read the (one) character to empty the buffer
}