Arduino Mega 2560 (main headers)

Fig. 1. Arduino Mega 2560 (main headers), Mega shield form factor, 101.5 x 53.3 mm. Hover or tap a signal to light up every pin that carries it.

• Four hardware serial ports is the reason to pick a Mega. Serial0 on D0 and D1 is shared with USB; Serial1, Serial2 and Serial3 on D18/D19, D16/D17 and D14/D15 are free.
• SPI is NOT on these headers. MISO, MOSI, SCK and SS are D50 to D53 on the double-row header, and also on the 6-pin ICSP header in the Uno-standard position.
• I2C is D20 and D21, not A4 and A5. Uno code that assumes the analog pins will not find the bus. D20 and D21 appear twice on the header; both positions are the same net.
• PWM is available on D2 to D13 and on D44, D45 and D46, which are on the double-row header. Fifteen in total.
• The analog pins double as digital D54 to D69, so A0 is also pin 54 in code.
• 20 mA per I/O pin maximum and 50 mA total from the 3V3 pin, which comes from the USB bridge rather than a real regulator.
• VIN takes 6 to 20 V, with 7 to 12 V recommended. Above that the linear regulator cooks under any real load.
• This is the board inside most RAMPS-based 3D printers, which is why it stays in production long after an ESP32 would do the job for less.

Table 1. Pin assignments.