add docs

README.md

@@ -76,10 +76,39 @@ Hardware: FT232 USB-UART adapter connected to UPDI with a 4.7 kΩ resistor.
    ```
 ## Hardware Setup (FT232 → ATtiny202 UPDI)
    ```bash
-    FT232 TX ----[4.7kΩ]---- UPDI (pin 6)
+    FT232 TX ----[4.7kΩ]---- UPDI (Pin 6)
     FT232 GND --------------- GND
     VCC -------------------- VCC (3.3V or 5V, must match FT232 logic)
    ```
+*Hint:* The GPIO PA0 acts as UPDI of ATtiny202 and needs to be disconnected from the PCB during programming. 
+
+## ATtiny202-SSN Pin Mapping (PCB Usage)
+![Image of the PCB in doc/ folder.](doc/bottom_side.png "Back of PCB")
+![Image of the PCB in doc/ folder.](doc/front_side.png "Front of PCB")
+
+| Pin # | Pin Name | Port | PCB Usage                | Direction                                |
+|------:|----------|------|--------------------------|------------------------------------------|
+|     1 | VDD      | —    | +VDD (Power)             | Power Input                              |
+|     2 | PA6      | IO   | Green LED                | Output                                   |
+|     3 | PA7      | IO   | Red LED                  | Output                                   |
+|     4 | PA1      | IO   | LED 1–2                  | Output                                   |
+|     5 | PA2      | IO   | LED 3–6                  | Output                                   |
+|     6 | PA3      | IO   | LED 7–8                  | Output                                   |
+|     7 | PA0      | IO   | UPDI / Activation Button | Input (button, pull-up) / UPDI (program) |
+|     8 | GND      | —    | Ground                   | Power Ground                             |
+
+---
+
+### Logic Levels (LEDs & Button)
+
+| Signal | PCB Usage         | LOW                      | HIGH                      |
+|--------|-------------------|--------------------------|---------------------------|
+| PA6    | Green LED         | LED **ON**               | LED OFF                   |
+| PA7    | Red LED           | LED **ON**               | LED OFF                   |
+| PA1    | LED 1–2           | LED(s) **ON**            | LED(s) OFF                |
+| PA2    | LED 3–6           | LED(s) **ON**            | LED(s) OFF                |
+| PA3    | LED 7–8           | LED(s) **ON**            | LED(s) OFF                |
+| PA0    | Activation Button | Button **pressed** (GND) | Button released (pull-up) |
 
 ## License