docs: Move Mega 2560 information to separate file

Make a more comprehensive document describing the Mega 2560 and FPC
breakout board.

doc/mega2560.md

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+# Mega 2560
+
+The Mega 2560 is the recommended tool for EC flashing and debugging.
+
+The Mega 2560 is an open-source hardware design by [Arduino] based on the 8-bit
+Atmel ATmega2560. Arduino's official model sells for around 40 USD. Multiple
+clones exist, and can be bought for around 15 USD.
+
+## Clone compatibility
+
+Multiple clones of the Arduino Mega 2560 exist. Clones can be used, but must be
+compatible with the Arduino model. Some have replaced the ATmega16U2 chip used
+for USB communication with a CH340G chip. If it does not explicitly state that
+it uses the ATmega16U2, find a different model.
+
+## FPC breakout board
+
+A flexible printed circuit (FPC) breakout board is required. It should have:
+
+- 24 pins with 0.1" (2.54mm) pitch
+- One side with 0.5mm pitch FPC connector (for lemp9)
+- One side with 1.0mm pitch FPC connector (for all other models)
+
+Depending on the vendor, the connectors may not come soldered to the board (or
+at all). A header block will likely not be be provided, so male breakaway
+headers will need to be purchased separately.
+
+### Connecting to the Mega 2560
+
+The FPC board should be assembled so the 1.0mm pitch connector faces up. When
+connected to the Mega 2560, the FPC connector should face away from the Mega
+2560. In this orientation, pin 1 of the FPC connector connects to pin 22 (PA0)
+of the Mega 2560.
+
+### Connecting to the laptop
+
+A 24 pin flexible flat cable (FFC) is used to connect the Mega 2560 to the
+laptop. It may be worth buying both a set of standard cables (traces exposed on
+same side at each end) and reversed cables (traces exposed on opposite sides at
+each end). With both, it will always be possible to have pin 1 of the breakout
+board go to pin 1 of the keyboard port.
+
+The orientation of the FFC traces can be determined by looking at how the
+keyboard connects to its port.
+
+The laptop keyboard may use a 26-pin connection. Ensure that the FFC is aligned
+so the traces line up with pins 1-24 on the keyboard port.
+
+A second cable (typically, USB-C) must be used for grounding the target laptop
+to the host system (what the Mega 2560 is connected to).
+
+## Firmware
+
+`mega2560/parallel.c` must be modified for the mapping of the keyboard pins to
+the GPIO pins.
+
+- [Arduino Mega 2560 pin mapping][PinMapping2560]
+
+Using the orientation described above, pin 1 of the keyboard maps to
+
+- pin 22 (PA0) of the Mega 2560 when using the 1.0mm pitch connector
+- pin 45 (PL4) of the Mega 2560 when using the 0.5mm pitch connector
+
+Once the GPIO mapping is updated, the firmware can be compiled and flashed.
+
+```
+make BOARD=arduino/mega2560
+make BOARD=arduino/mega2560 flash
+```
+
+[Arduino]: https://www.arduino.cc/
+[PinMapping2560]: https://www.arduino.cc/en/Hacking/PinMapping2560