Update README with features, flash space, and simulator fixes

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

README.md

@@ -1,6 +1,90 @@
 # ku1255cfw
 Custom open firmware for the Lenovo KU-1255 compact USB keyboard
 
+## Overview
+
+The Lenovo ThinkPad Compact USB Keyboard with TrackPoint (KU-1255) uses a
+Sonix SN8F2288FG 8-bit MCU with 12K words (24KB) flash and 512 bytes RAM.
+The TrackPoint is connected via bit-banged I2C (P2.4 SCL, P2.5 SDA, address
+0x2A, Synaptics proprietary protocol).
+
+This firmware is a from-scratch rewrite based on the original by
+[ranma](https://github.com/ranma/ku1255cfw), adding standalone middle-button
+scroll and other missing features so the keyboard works fully without an
+external converter.
+
+## Features added over base firmware
+
+### Middle-button scroll
+3-state state machine (IDLE / UNDECIDED / SCROLLING) with 150ms timeout:
+- **Short press** (<150ms): sends a normal middle click (deferred on release)
+- **Hold + TrackPoint movement**: converts XY deltas to scroll wheel events
+- **FN + middle button**: passes middle button through directly (no scroll logic)
+
+### CapsLock LED feedback
+Host LED output reports (SET_REPORT) are parsed and CapsLock state (bit 1) drives
+the power LED on P5.3/PWM0 (active-low). The flasher magic byte sequence detection
+is preserved.
+
+### FN+F7 through FN+F12
+| Key | Normal | With FN held |
+|-----|--------|-------------|
+| F7  | F7     | LGUI+P (display settings) |
+| F8  | F8     | F8 (passthrough) |
+| F9  | F9     | LGUI+I (settings) |
+| F10 | F10    | LGUI (search) |
+| F11 | F11    | LCTRL+LALT+TAB (task switch) |
+| F12 | F12    | F12 (passthrough) |
+
+### USB compliance fixes
+- **HID GET_REPORT**: returns proper per-interface empty reports (8B keyboard / 5B mouse)
+  instead of stale buffer contents. Some OSes query this on resume from suspend.
+- **SET/CLEAR FEATURE**: tracks DEVICE_REMOTE_WAKEUP state instead of just ACKing.
+
+## Flash space
+
+| Firmware | Words used | Free | Utilization |
+|----------|-----------|------|-------------|
+| OEM      | 10,226 / 10,239 | 13 | 99.9% |
+| This     | 10,238 / 10,239 | 1  | 100.0% |
+
+Key debouncing is not implemented (and not needed) — the 8ms scan cycle naturally
+debounces scissor switches (<1ms bounce time). The OEM firmware also does not debounce.
+
+## Flashing
+
+Requires [vpelletier/dissn8](https://github.com/vpelletier/dissn8) tools.
+
+1. Build: `asn8 main.s -o ku1255cfw.bin`
+2. Enter bootloader: hold **Return** while plugging in the keyboard
+3. Flash: `flashsn8 ku1255cfw.bin`
+
+## Simulator testing
+
+`test_scroll.py` runs 23 automated tests against the
+[dissn8 simulator](https://github.com/vpelletier/dissn8) via the `ku1255_sim.py`
+harness. Tests cover all scroll state transitions, deferred click timing, FN modifier
+interaction, and edge cases (rapid clicks, timeout behavior).
+
+**Note:** The simulator requires a fix for PnUR register read handlers (see below).
+
+## Simulator fixes (for vpelletier/dissn8)
+
+Running this firmware in the dissn8 simulator exposed three bugs:
+
+1. **PnUR read handlers crash** — `_volatile_dict` maps P0UR-P5UR read handlers to
+   `None`. B0BSET/B0BCLR (read-modify-write) on these registers causes
+   `TypeError: 'NoneType' object is not callable`. Fixed by adding `readPullUp()`
+   returning the latch value. Branch: `pnur_readpullup` (PR pending upstream).
+
+2. **Hardcoded HID descriptor sizes** — `ku1255_sim.py` assumed 0x51/0xD3 byte
+   HID report descriptors (OEM sizes). This firmware uses 91/61 bytes. Fixed by
+   parsing sizes dynamically from the config descriptor.
+
+3. **Wrong HID descriptor recipient** — HID report descriptor requests must use
+   interface recipient (0x81), not device (0x80), per USB spec. The OEM firmware
+   happened to accept both.
+
 ## Dev setup
 - [OpenViszla](https://github.com/openvizsla/ov_ftdi) USB protocol analyzer
 - 5V-tolerant PL2303 UART interface (e.g. https://www.adafruit.com/product/954)