Add documentation file.

doc/devices/HI-K0402WS.md

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+# Hisource Hi-KO4022WS
+
+Following is documentation for unmanaged switch marked as `Hi-KO4022WS` . 
+Original software is running UART on 9600 baud rate. 
+Using SPI clamp in-board is the only method for initial installation.
+
+The board has two flash chips `BY25Q16BS` with 16M-bit size. The front switch, switches between the two flash chips.
+These can be programed independently by using said switch - so it is e.g. possible to run the original and new firmware in parallel.
+
+### Label specifications
+
+- **Name**: 2.5G Ethernet Switch  
+- **Model**: Hi-KO4022WS  
+- **Ports**:  
+  - 4 × RJ45: 10/100/1000/2500 Mbps  
+  - 2 × SFP: 1000 / 2500 / 10000 Mbps  
+
+### What works (expected from label + similar devices)
+
+- All four 2.5GBASE-T RJ45 ports at 10/100/1000/2500 Mbps  
+- Both SFP ports supporting 1G, 2.5G and 10G modules 
+- LEDs
+
+### PCB overview
+
+**Board markings**  
+- Top silkscreen: PCB-KO4022W-V3.0 / DIP-KO4022WS-V3.0  
+
+Top side
+
+<img src="photos/K0402W-V3.0-unmanaged\PCB-top.jpg" width="300" />
+
+Bottom
+
+<img src="photos/K0402W-V3.0-unmanaged\PCB-bottom.jpg" width="300" />
+
+### T2, serial console
+
+| `J2` pin | Signal      |
+| -------- | ----------- |
+| 1        | 3V3         |
+| 2        | RX (Input) |
+| 3        | TX (Output)  |
+| 4        | GND         |
+
+
+## Power supply
+
+Input power is delivered via barell plug, `12V 1A` adapter was provided.
+Board has two supply rails. `0.95` and `3.3` volt.
+
+### `0.95` Core Voltage
+
+Voltage is made by a `Techcode TD1720` .
+
+### `3.3` Voltage
+
+Voltage is created by chip marked as `Techcode TD1720`.
+There seems to have been a miscalculation when choosing the inductor and the device is ~25% more efficient with an 5V power supply.