this is the proper type to encapsulate a byte when data is available
or -1 when there is no data.

Fixed PWM frequency glitch

Port of https://github.com/arduino/Arduino/pull/4718 to SAMD core.

Use TX buffer for Wire slave writes

Enable digitalRead on output pins, only enable pull-up in digitalWrite if pin is not output

Add Serial_::availableForWrite()

Fix #149

Add Uart::availableForWrite()

Correct SerialUSB return value

Port of https://github.com/arduino/Arduino/pull/4996

Previous 0 was always returned.