Most programs written for Arduino work on Teensy. All of the standard Arduino functions (digitalWrite, pinMode, analogRead, etc) all work on Teensy. Teensyduino is also compatible with many Arduino libraries.

Teensy is not limited to only serial device type. The Tools > USB Type menu can select the type of device Teensy will become when it runs your sketch.

All communication is performed at full native 12 Mbit/sec USB speed. Serial includes built-in flow control, so the effective speed will be as fast as your code can read, but unlike Arduino, you will not lose incoming data if your sketch does not use Serial.read() quickly. When implementing Serial mode, maximum speeds of approximately 1 Mbit/sec are commonly achievable.

Teensy has the same built-in peripherals as Arduio: analog inputs, SPI, I²C, PWM, and a real serial port. For applications that need the serial port (MIDI, GPS modules, etc), Teensy is very easy to use, because uploading takes place on the USB port, which is not shared with serial.

How Does Teensy Compare With Arduino?

Specification	Teensy	Teensy++	Arduino Uno
Processor	ATMEGA32U4	AT90USB1286	ATMEGA328
Flash Memory	32256	130048	32256
RAM Memory	2560	8192	2048
EEPROM Memory	1024	4096	1024
Total I/O	25	46	20
Analog Inputs	12	8	6
PWM (Analog Out)	7	9	6
USB Port	1	1	(serial only)

Teensy uses a similar Atmel AVR processor as the Arduino board, but there are differences. The most important is direct, built-in USB on the Teensy compared to serial converted to USB by a separate chip on the Arduino. Teensy communications MUCH faster than Arduino, and can implement many types of devices.

Teensy is designed for easy use on breadboards in the smallest possible size, which is incompatible with the form-factor used by Arduino shields.

While many types of sketches can run, Teensy is NOT an "Arduino clone". The Teensy hardware and Teensyduino software add-on are NOT endorsed or supported by the Arduino developer team.