docs: Move Mega 2560 information to separate file

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

doc/debugging.md

@@ -13,27 +13,12 @@ external USB keyboard or SSH session.
 
 Requirements:
 - Arduino Mega 2560 compatible board
-- 24 pin flexible printed circuit
+- 24 pin FPC breakout board and cables
     - 0.5mm or 1.0mm pitch, depending on target connector
-- 24 pin FPC breakout board with connectors
 - USB-C cable
 
-### Configuring the Mega 2560
+For details on configuring the Mega 2560 and breakout board, see
-
+[mega2560](./mega2560.md).
-If the breakout board did not come preassembled, assemble it before
-starting. This will require soldering.
-
-1. Connect the breakout board to the Mega 2560
-    - Orientation will affect the mapping of GPIO pins
-2. Connect the Mega 2560 to the host
-3. Configure GPIO pin mapping in `parallel.c` based on how it will
-    connect to the target parallel port
-    - Trace pin 1 on motherboard connector to Mega's GPIO pins
-4. Build and flash the firmware for the Mega 2560
-```
-make BOARD=arduino/mega2560
-make BOARD=arduino/mega2560 flash
-```
 
 ### Setup
 
@@ -44,10 +29,10 @@ make BOARD=arduino/mega2560 flash
 3. Remove bottom panel
 4. Unplug keyboard cable
     - May require removing keyboard depending on port location
-5. Connect Mega 2560 to host
+5. Ground target to host
-    - This will create an ACM device at `/dev/ttyACM*`
-6. Ground target to host
     - Connect USB cable from USB-C port on target to host
+6. Connect Mega 2560 to host
+    - This will create an ACM device at `/dev/ttyACM*`
 7. Connect Mega 2560 to target
 8. Start the console
 ```
@@ -57,7 +42,7 @@ make BOARD=system76/<model> console_external
 EC logs should now print to the console on the host. This can be tested
 by removing or inserting the AC adapter to trigger a power event.
 
-To return the Mega to host mode, reset the device.
+To return the Mega 2560 to host mode, reset the device.
 
 If logs are corrupted, try power cycling the Mega or reseating the cable.
 

doc/mega2560.md

@@ -0,0 +1,72 @@
+# 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