Abnormal echo on Serial

[bilboquet]

Hello,
This sketch is causing me some troubles.

#include <Arduino.h>

void setup() {
    Serial.begin(115200);
    Serial.println("Starting !");
}

void loop() {
    int byte, count;
    while (count = Serial.available()) {
        digitalWrite(13, HIGH);
        delay(900);
        digitalWrite(13, LOW);
        delay(100);
        Serial.print("print to Serial : ");
        Serial.println(count);
        byte = Serial.read();
        Serial.print((char) byte);
    }
}

The board is connected to the PC (running Ubuntu 16.04) through the programming port. In a terminal I do a cat /dev/ttyACM0 and nothing more.
For me the sketch should print:

Starting!

Then silently loop forever…
But it prints (blank lines have been removed):

Starting !
print to Serial : 3
Sprint to Serial : 25
tprint to Serial : 48
aprint to Serial : 63
rprint to Serial : 62
tprint to Serial : 63
iprint to Serial : 62
nprint to Serial : 63
gprint to Serial : 63

What is really strange is that the read characters are the printed characters… (see beginning of each line).
Somewhere there is a loop which I don't understand.
Any help will be much appreciated as this "echo" is blocking me.

Regards,

[sandeepmistry]

Hi @bilboquet,

What board are you using?

[bilboquet]

Hello,
Thank you very much for the reply.
I'm on a M0 pro, flashed with the bootloader of a Zero (so I can use eclipse C++ IDE for Arduino to develop…).
Actually, I've found my mistake. I'm full of shame but if it can save someone some time I'm gonna explain it:
I've spent long hours reading the code and running it step by step with gdb… But the problem was in fact on the tty. stty -a -F /dev/ttyACM0 showed that echo was enable on it :/ so stty -echo /dev/ttyACM0 brought everything back to normal.

Thanks again

(PS: I still have some minor annoyances but I don't think they are related to this code)

[sandeepmistry]

@bilboquet no worries, and thanks for the follow up!

I didn't know stty could be using enable echo on a tty device, thanks for sharing.