Add T-Watch Ultra developer notes documenting hard-won lessons

Covers: I2C pin default mismatch (XPowersLib), flash workflow
(esptool vs JTAG), deep sleep PMU caution, display power gate,
GPS/RTC differences from T-Beam Supreme, and I2C bus architecture.

Documentation/T-Watch-Ultra-Notes.md

@@ -0,0 +1,73 @@
+# T-Watch Ultra Development Notes
+
+Hard-won lessons from porting RNode firmware to the LilyGo T-Watch Ultra.
+
+## I2C Pin Default Mismatch (Critical)
+
+The generic Arduino ESP32-S3 FQBN (`esp32:esp32:esp32s3`) defines default I2C pins as **SDA=8, SCL=9** (in `variants/esp32s3/pins_arduino.h`). The T-Watch Ultra uses **SDA=3, SCL=2**.
+
+The XPowersLib `XPowersAXP2101` constructor stores the variant's default SDA/SCL internally:
+
+```cpp
+// BROKEN — uses default SDA=8, SCL=9 from variant
+PMU = new XPowersAXP2101(Wire);
+PMU->init();  // Calls Wire.begin(8, 9) — wrong pins!
+
+// CORRECT — pass explicit pins
+PMU = new XPowersAXP2101(Wire, I2C_SDA, I2C_SCL);
+PMU->init();  // Calls Wire.begin(3, 2) — correct pins
+```
+
+**Symptom**: `Wire.begin(3, 2)` returns true, but no I2C device responds. SDA/SCL lines read HIGH (pullups working). Looks like a hardware failure but is entirely a firmware bug.
+
+**Root cause**: `XPowersCommon::begin()` calls `__wire->begin(__sda, __scl)` where `__sda` and `__scl` were set to 8/9 by the constructor defaults. This silently reinitialises the I2C bus on the wrong pins.
+
+This applies to **any ESP32-S3 board using non-default I2C pins with XPowersLib**. Always pass explicit SDA/SCL to the constructor.
+
+## Flash Workflow
+
+The T-Watch Ultra's built-in USB JTAG/serial interface does NOT support esptool auto-reset. Two flash methods:
+
+### Method 1: esptool (faster, for large images)
+1. Hold **BOOT** button, press **RST**, release BOOT → download mode
+2. Flash: `esptool --chip esp32s3 --port /dev/ttyACM4 --baud 921600 write_flash 0x0 bootloader.bin 0x8000 partitions.bin 0xe000 boot_app0.bin 0x10000 app.bin`
+3. Press **RST** to exit download mode (battery keeps ESP32 running after USB cycle)
+
+### Method 2: JTAG via OpenOCD (for small images <2MB)
+```bash
+openocd -f board/esp32s3-builtin.cfg -c "program_esp app.bin 0x10000 verify reset exit"
+```
+Full flash images (16MB factory binaries) exceed JTAG flash stub limits.
+
+## Physical Buttons
+
+- **BOOT**: Left edge of PCB (GPIO 0) — download mode entry
+- **RST**: Right edge of PCB — hardware reset
+- **<< and >|**: Side buttons — media/navigation, NOT power key
+- **Power key**: Connected to AXP2101 PWR_KEY input — controls PMU power on/off
+
+## Deep Sleep Caution
+
+Do NOT call `PMU->enableSleep()` before `esp_deep_sleep_start()`. The AXP2101 retains its sleep mode state across battery-backed resets. If the PMU enters sleep mode, its I2C slave interface may become unresponsive, and the only recovery is a full battery disconnect (including the RTC button cell on the PCB).
+
+Setting GPIO pins to `OPEN_DRAIN` before deep sleep is correct for power savings, but do NOT set the I2C pins (GPIO 2/3) to OPEN_DRAIN — this can leave the bus in a state that's difficult to recover from.
+
+## Display Power Gate
+
+The CO5300 AMOLED display's VCI power is gated by the XL9555 GPIO expander (VC_EN signal, confirmed from schematic sheet 3). The display will retain its last frame buffer content as long as ALDO2 (display power rail) stays on from the battery.
+
+The XL9555 pin mapping for VC_EN needs verification — the BHI260AP GPIO enum values (6, 14, 15) used by LilyGoLib may not directly correspond to XL9555 port pin numbers. The schematic (sheet 3) shows the exact wiring.
+
+## GPS Module
+
+The MIA-M10Q (u-blox) outputs standard NMEA at 38400 baud by default. No vendor-specific init commands needed (unlike the L76K which requires PCAS commands). TinyGPSPlus parses the output without modification.
+
+## RTC
+
+The PCF85063A has time registers at offset 0x04-0x0A (vs PCF8563's 0x02-0x08). Same I2C address (0x51), similar BCD encoding, but different control register layout. The oscillator stop bit is at seconds register bit 7 (same as PCF8563).
+
+## I2C Bus Architecture
+
+All I2C devices share a single bus on GPIO 3 (SDA) and GPIO 2 (SCL) with 2.2K pull-up resistors to VDD3V3. The bus routes through the LCD connector (pins 1-2), meaning the display flex cable must be properly seated for I2C to function.
+
+The I2C bus is used by: AXP2101 (PMU), PCF85063A (RTC), CST9217 (touch), BHI260AP (IMU), DRV2605 (haptic), XL9555 (GPIO expander).