docs: Add documentation for debugging

Add the steps required to wire a supported laptop for EC debugging,
along with setting up an environment to receive EC logs.

doc/debugging.md

@@ -0,0 +1,55 @@
+# Debugging the EC firmware
+
+**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
+
+These steps apply to the following models:
+- darp5
+- darp6
+- galp3-c
+- galp4
+
+1. Power off system
+2. Unplug AC adapter
+3. Remove bottom panel
+4. Disconnect battery
+5. Attach one wire to battery pin 4 (`CLK`)
+6. Attach one wire to battery pin 5 (`DATA`)
+7. Route wires through the Kensington lock slot
+    - Secure wires to board with tape
+8. Attach female connector and housing to wires
+9. Reconnect battery
+10. Replace bottom panel
+
+## Setup
+
+Requirements:
+- Target wired for EC debugging
+- Adafruit Trinket M0
+- USB-C cable
+
+1. Enable I2C debugging in the EC firmware for the target
+    - Uncomment `I2C_DEBUGGER` in `board.mk`
+    - Build and flash firmware
+2. Connect Trinket M0 to host
+    - This will create an ACM device at `/dev/ttyACM*`
+3. Connect to ACM device from host
+```
+sudo tio -b 115200 -m INLCRNL /dev/ttyACM0
+```
+4. Ground target to host
+    - Connect USB cable from USB-C port on target to host
+5. Connect target to host
+    - Connect `CLK` wire to `SCL` on Trinket M0
+    - Connect `DATA` wire to `SDA` on Trinket M0
+
+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.