Arduino Mega 2560 pinout: every pin, its port and its timer

The top header reads, left to right, SCL, SDA, AREF, GND, 13 down to 0, then 14 to 21. The bottom header reads IOREF, RESET, 3V3, 5V, GND, GND, VIN, then A0 to A15. The double row on the short edge is 22 to 53 in pairs, 22 and 23 first, with a 5V and a GND hole at the end. SDA and SCL beside AREF are the same two traces as pins 20 and 21, not A4 and A5 as on an Uno.

The chip pins are not in order, and that is the real board’s doing: 22 to 29 count up through port A, 30 to 37 count down through port C, and 42 to 49 count down through port L. A sketch never sees it, because digitalWrite carries the table, but one that writes PORTA itself does. A0 to A15 are also digital pins 54 to 69.

Fifteen pins do PWM with analogWrite: 2 to 13 and 44 to 46. Pins 4 and 13 are on timer 0 at 976 Hz, which also runs millis(); the rest are at 490 Hz on timers 1 to 5, three pins to a timer except timers 1 and 2, which have two each here. Mokxi runs every one on the same timer at the same rate as the board.

Arduino’s Servo reference says a Mega drives up to twelve servos without touching PWM, and that more than twelve stop PWM on 11 and 12. That is one more thing a Mega does differently from an Uno, where the first servo costs 9 and 10.

attachInterrupt works on 2, 3, 18, 19, 20 and 21. The interrupt numbers are not the chip’s INT numbers: interrupt 0 is pin 2, which is INT4, and interrupts 2 to 5 are pins 21 down to 18, which are INT0 to INT3. Write digitalPinToInterrupt(pin) and let the table do it. Pin-change interrupts, which SoftwareSerial needs to receive, are on 10 to 15, 50 to 53 and A8 to A15 only.

I2C is 20 for SDA and 21 for SCL, which the SDA and SCL holes beside AREF also reach. SPI is 50, 51, 52 and 53 for MISO, MOSI, SCK and SS, and the ICSP header carries the same three data lines. In Mokxi both buses run in software on those pins, because the chip’s I2C and SPI blocks are not modeled. A0 to A15 read ten bits against the 5 V rail, and the Mega has an internal 2.56 V reference an Uno does not.