usb serial available() returning true even if buffer is empty()

[tbowmo]

I discovered a fault in USB driver in SAMD 1.6.7, where a call to SerialUSB.available() always will return true, after a single character has been sent to the USB endpoint.

following test sketch will show the error:

void setup() {
  // put your setup code here, to run once:
  SerialUSB.begin(115200);
  while (!Serial) {}
  SerialUSB.println("Test sketch");
}

void loop() {
  // put your main code here, to run repeatedly:
  SerialUSB.print("test ");
  SerialUSB.println(millis());
  delay(1000);
  if (SerialUSB.available()) {
    SerialUSB.print(" Received ");
    SerialUSB.println(SerialUSB.read());
  }
}

The output with SAMD 1.6.7 package is:

Test sketch
test 1120
test 2120
test 3120
test 4120
test 5120
test 6120
 Received 32
test 7120
 Received -1
test 8120
 Received -1
test 9120
 Received -1
test 10120
 Received -1
test 11120
 Received -1
test 12120
 Received -1
test 13120

while with SAMD 1.6.6 package installed:

Test sketch
test 810
test 1810
test 2810
test 3810
test 4810
test 5810
 Received 32
test 6810
test 7810
test 8810
 Received 32
test 9810
test 10810
test 11810

The error wil result in an endless loop, if you have a while(SerialUSB.available()) {} to read characters from the serial device.

[cmaglie]

@tbowmo
thanks for the bug report, I confirm that there is a bug.

May you test if the following patch works for you?
#173
https://github.com/arduino/ArduinoCore-samd/pull/173/files

[tbowmo]

@cmaglie

I have verified the patch, and it is working..

When can we expect to have this out in a release?

[cmaglie]

Thanks for testing.

Expect a release very soon (maybe tomorrow).