docs: Add info for logging through parport

doc/debugging.md

@@ -1,15 +1,76 @@
 # Debugging the EC firmware
 
+Terms used:
+- *target*: The laptop system that has the EC to be tested
+- *host*: The system that will have all devices connected to it and
+    will receive the EC logs
+
+## Parallel port
+
+This method replaces the keyboard with a device used for debug logging.
+An alternate method of interacting with the target is needed; e.g., an
+external USB keyboard or SSH session.
+
+Requirements:
+- Arduino Mega 2560 compatible board
+- 24 pin flexible printed circuit
+    - 0.5mm or 1.0mm pitch, depending on target connector
+- 24 pin FPC breakout board with connectors
+- USB-C cable
+
+### Configuring the Mega 2560
+
+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
+
+1. Enable parallel port debugging in the EC firmware
+    - Uncomment `PARPORT_DEBUG` in `board.mk`
+    - Build and flash the firmware for the target
+2. Power off target
+3. Remove bottom panel
+4. Unplug keyboard cable
+    - May require removing keyboard depending on port location
+5. Connect Mega 2560 to host
+    - This will create an ACM device at `/dev/ttyACM*`
+6. Connect to ACM device from host
+```
+sudo tio -b 1000000 -m INLCRNL /dev/ttyACM0
+```
+7. Set Mega to peripheral mode
+```
+echo 'C' > /dev/ttyACM0
+```
+8. Ground target to host
+    - Connect USB cable from USB-C port on target to host
+9. Connect Mega 2560 to target
+
+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.
+
+If logs are corrupted, try power cycling the Mega.
+
+## I2C connection
+
 **Failure to follow steps in order, or performing steps on an
 unsupported board, may result in damaged board components.**
 
-Terms used:
-
-- *target*: The laptop system that has the EC to be tested.
-- *host*: The system that will have all devices connected to it and
-    will receive the EC logs.
-
-## Wiring the target
+### Wiring the target
 
 These steps apply to the following models:
 - darp5
@@ -29,7 +90,7 @@ These steps apply to the following models:
 9. Reconnect battery
 10. Replace bottom panel
 
-## Setup
+### Setup
 
 Requirements:
 - Target wired for EC debugging