Update README with current runtime state and RTC GPIO pin mapping

README.md

@@ -8,6 +8,65 @@ Headless Raspberry Pi Zero W project:
 - RTC boot restore + NTP sync + RTC write-back
 - Autostart via systemd
 
+## Current State
+
+Current implementation status:
+- Boot flow restores system time from RTC (`hwclock -s`) before starting logging.
+- If WLAN + internet is unavailable, AP fallback starts on `wlan0`:
+  - SSID: `serial`
+  - Password: `serialserial`
+  - AP IP: `192.168.4.1/24`
+  - DHCP range: `192.168.4.10` to `192.168.4.200`
+- Web portal is available on port `80`:
+  - `/` WiFi scan + connect UI
+  - `/serial` live serial console (SSE via `/events/serial`)
+- ESP32 serial bridge:
+  - Auto-detects `/dev/ttyUSB*`, `/dev/ttyACM*`, `/dev/serial/by-id/*`
+  - Reconnects automatically on unplug/replug
+  - Daily log rollover at midnight with datetime filename
+- Once internet is available, NTP sync runs and writes corrected time back to RTC (`hwclock -w`).
+
+Runtime check commands:
+```bash
+systemctl status serial-bridge
+journalctl -u serial-bridge -f
+ip a show wlan0
+ls -l /home/pi/xxx.log /home/pi/xxx_*.log
+sudo hwclock -r
+```
+
+## RTC GPIO Wiring (Raspberry Pi Zero W)
+
+Use I2C1 pins on the 40-pin header:
+
+| RTC module pin | Raspberry Pi pin | BCM GPIO | Notes |
+|---|---|---|---|
+| `VCC` | `Pin 1` | 3V3 | Use 3.3V |
+| `GND` | `Pin 6` | GND | Common ground |
+| `SDA` | `Pin 3` | GPIO2 (`SDA1`) | I2C data |
+| `SCL` | `Pin 5` | GPIO3 (`SCL1`) | I2C clock |
+
+Minimal pin marker (header top):
+```text
+Pin 1 (3V3)   Pin 2 (5V)
+Pin 3 (SDA1)  Pin 4 (5V)
+Pin 5 (SCL1)  Pin 6 (GND)
+```
+
+Enable I2C + RTC overlay (example DS3231):
+```bash
+sudo raspi-config nonint do_i2c 0
+echo 'dtoverlay=i2c-rtc,ds3231' | sudo tee -a /boot/firmware/config.txt
+sudo reboot
+```
+
+Verify:
+```bash
+ls -l /dev/rtc0
+sudo i2cdetect -y 1
+sudo hwclock -r
+```
+
 ## Quick install (on Raspberry Pi)
 
 ```bash