diff --git a/libraries/AP_HAL_ChibiOS/hwdef/Pixhawk4/README.md b/libraries/AP_HAL_ChibiOS/hwdef/Pixhawk4/README.md
index 34390661bd..0ff62dc54d 100644
--- a/libraries/AP_HAL_ChibiOS/hwdef/Pixhawk4/README.md
+++ b/libraries/AP_HAL_ChibiOS/hwdef/Pixhawk4/README.md
@@ -37,6 +37,111 @@ have RTS/CTS.
The UART7 connector is labelled debug, but is available as a general
purpose UART with ArduPilot.
+### TELEM1, TELEM2 ports
+
+
+
+
+ | Pin |
+ Signal |
+ Volt |
+
+
+ | 1 (red) |
+ VCC |
+ +5V |
+
+
+ | 2 (blk) |
+ TX (OUT) |
+ +3.3V |
+
+
+ | 3 (blk) |
+ RX (IN) |
+ +3.3V |
+
+
+ | 4 (blk) |
+ CTS |
+ +3.3V |
+
+
+ | 5 (blk) |
+ RTS |
+ +3.3V |
+
+
+ | 6 (blk) |
+ GND |
+ GND |
+
+
+
+
+
+### GPS port
+
+
+
+
+ | Pin |
+ Signal |
+ Volt |
+
+
+ | 1 (red) |
+ VCC |
+ +5V |
+
+
+ | 2 (blk) |
+ SERIAL3 TX (OUT) |
+ +3.3V |
+
+
+ | 3 (blk) |
+ SERIAL3 RX (IN) |
+ +3.3V |
+
+
+ | 4 (blk) |
+ SCL |
+ +3.3 (pullups) |
+
+
+ | 5 (blk) |
+ SDA |
+ +3.3 (pullups) |
+
+
+ | 6 (blk) |
+ SafetyButton |
+ +3.3V |
+
+
+ | 7 (blk) |
+ SafetyLED |
+ +3.3V |
+
+
+ | 8 (blk) |
+ VDD 3.3 (OUT) |
+ +3.3V |
+
+
+ | 9 (blk) |
+ Buzzer |
+ +3.3V |
+
+
+ | 10 (blk) |
+ GND |
+ GND |
+
+
+
+
## RC Input
RC input is configured on the port marked DSM/SBUS RC. This connector
@@ -113,6 +218,168 @@ The Pixhawk4 has 7 analog inputs
- ADC Pin11 -> ADC 3.3V Sense
- ADC Pin103 -> RSSI voltage monitoring
+## I2C Buses
+
+ - the internal I2C port is bus 0 in ArduPilot (I2C3 in hardware)
+ - the port labelled I2CA is bus 3 in ArduPilot (I2C1 in hardware)
+ - the port labelled I2CB is bus 2 in ArduPilot (I2c2 in hardware)
+ - there is no external connector for I2C4
+
+### Pinout for I2CA
+
+
+
+
+ | Pin |
+ Signal |
+ Volt |
+
+
+ | 1 (red) |
+ VCC |
+ +5V |
+
+
+ | 2 (blk) |
+ SCL |
+ +3.3 (pullups) |
+
+
+ | 3 (blk) |
+ SDA |
+ +3.3 (pullups) |
+
+
+ | 4 (blk) |
+ GND |
+ GND |
+
+
+
+
+### Pinout for I2CB+UART
+
+
+
+
+ | Pin |
+ Signal |
+ Volt |
+
+
+ | 1 (red) |
+ VCC |
+ +5V |
+
+
+ | 2 (blk) |
+ SERIAL4 TX (OUT) |
+ +3.3V |
+
+
+ | 3 (blk) |
+ SERIAL4 RX (IN) |
+ +3.3V |
+
+
+ | 4 (blk) |
+ SCL |
+ +3.3 (pullups) |
+
+
+ | 5 (blk) |
+ SDA |
+ +3.3 (pullups) |
+
+
+ | 6 (blk) |
+ GND |
+ GND |
+
+
+
+
+## CAN
+
+The Pixhawk4 has two independent CAN buses, with the following pinouts.
+
+### CAN1&2
+
+
+
+
+ | Pin |
+ Signal |
+ Volt |
+
+
+ | 1 (red) |
+ VCC |
+ +5V |
+
+
+ | 2 (blk) |
+ CAN_H |
+ +12V |
+
+
+ | 3 (blk) |
+ CAN_L |
+ +12V |
+
+
+ | 4 (blk) |
+ GND |
+ GND |
+
+
+
+
+## Debug
+
+The Pixhawk4 supports SWD debugging on the debug port
+
+
+
+
+ | Pin |
+ Signal |
+ Volt |
+
+
+ | 1 (red) |
+ FMU VDD 3.3 |
+ +3.3V |
+
+
+ | 2 (blk) |
+ UART TX Debug (OUT) |
+ +3.3V |
+
+
+ | 3 (blk) |
+ UART RX Debug (IN) |
+ +3.3V |
+
+
+ | 4 (blk) |
+ SWDIO |
+ +3.3V |
+
+
+ | 5 (blk) |
+ SWCLK |
+ +3.3V |
+
+
+ | 6 (blk) |
+ GND |
+ GND |
+
+
+
+
+
## Loading Firmware
The board comes pre-installed with an ArduPilot compatible bootloader,