Teensy 4.0

Fig. 1. Teensy 4.0, 2x14 2.54 mm, 35 x 18 mm. Hover or tap a signal to light up every pin that carries it.

• Not 5 V tolerant. The pins take 0 to 3.3 V and nothing more, which catches people moving over from an Uno or a Mega.
• 600 MHz with 1 MB of RAM, which is roughly thirty times an Arduino Due and in a smaller package. The Audio Library is the usual reason to pick one.
• Seven hardware serial ports, three SPI and three I2C. Serial1 is on pins 0 and 1, Serial2 on 7 and 8, Serial3 on 15 and 14, Serial4 on 16 and 17, Serial5 on 21 and 20.
• Wire is on pins 18 and 19, Wire1 on 17 and 16. The second I2C bus shares pins with Serial4, so you get one or the other.
• Pin 13 is the on-board LED and the SPI clock at once. PJRC warn against INPUT_PULLUP on it.
• Vin accepts 3.6 to 5.5 V, and the 3.3 V output is good for 250 mA. To run from an external supply with USB also plugged in, cut the VUSB pad on the underside first.
• Another sixteen pads sit on the underside for pins 24 to 39, plus five on the back edge for program, on/off, ground and VBat. Those are not on this diagram.
• Every digital pin can raise an interrupt and 31 of them do PWM, so the usual hunt for a free interrupt or timer pin does not apply here.
• The Teensy 4.1 is the longer version of the same chip, adding Ethernet, a microSD slot, a USB host port and 55 I/O.

Table 1. Pin assignments.