docs: add SPI-based LED Strip Driver analysis documentation

NOTE_SPI.md

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+# SPI-based LED Strip Driver Analysis
+
+## Overview
+
+This driver emulates RMT (Remote Control) signals for WS2812B/SK6812 LED strips using SPI peripheral. It works by encoding each data bit into multiple SPI bits to achieve the required timing characteristics.
+
+## Clock Configuration
+
+- **SPI Clock Frequency**: 2.5 MHz (`LED_STRIP_SPI_DEFAULT_RESOLUTION`)
+- **SPI Bit Period**: 1/2.5MHz = **400 nanoseconds**
+- **Tolerance**: ±300 KHz (2.2 MHz to 2.8 MHz accepted)
+
+## Bit Encoding Scheme
+
+Each **1 RMT data bit** is encoded as **3 SPI bits**, giving a total symbol period of 1.2 μs.
+
+### Encoding Patterns
+
+| Data Bit | SPI Pattern (MSB first) | High Time | Low Time | Total Time |
+| -------- | ----------------------- | --------- | -------- | ---------- |
+| **0**    | `100` (binary)          | 400 ns    | 800 ns   | 1200 ns    |
+| **1**    | `110` (binary)          | 800 ns    | 400 ns   | 1200 ns    |
+
+### WS2812B Timing Compatibility
+
+The encoding matches WS2812B specifications:
+- **T0H** (0 high time): ~400ns (spec: 350±150ns)
+- **T0L** (0 low time): ~800ns (spec: 800±150ns)
+- **T1H** (1 high time): ~800ns (spec: 700±150ns)
+- **T1L** (1 low time): ~400ns (spec: 600±150ns)
+
+## Why 3 Bits per Symbol?
+
+The choice of 3 SPI bits per data bit is driven by timing requirements:
+
+1. **Duty Cycle Ratio**: WS2812B needs approximately 1:2 or 2:1 high/low ratios
+2. **Granularity**: 
+   - 2 bits would only give 50/50 split (insufficient)
+   - 3 bits provides 33/66% or 66/33% ratios (optimal)
+   - 4+ bits would make the period too long (>1.6μs)
+3. **Clock Frequency**: At 2.5 MHz, 3 bits = 1.2μs matches the ~1.25μs WS2812B bit period
+
+## Memory Usage
+
+Each LED color component requires:
+- **1 byte** of actual data
+- **3 bytes** of SPI transmission data (3:1 expansion)
+
+For RGB LEDs:
+- 3 bytes per pixel → 9 bytes SPI data per pixel
+
+For RGBW LEDs:
+- 4 bytes per pixel → 12 bytes SPI data per pixel
+
+Defined as: `SPI_BYTES_PER_COLOR_BYTE = 3`
+
+## Bit Mapping Function
+
+The `__led_strip_spi_bit()` function expands each 8-bit color value into 24 SPI bits (3 bytes):
+
+```c
+static void __led_strip_spi_bit(uint8_t data, uint8_t *buf)
+```
+
+**Input**: 1 byte of color data (8 bits)  
+**Output**: 3 bytes written to buffer (24 SPI bits = 8 RMT symbols)
+
+### Bit Order
+
+The function processes bits from LSB to MSB and maps them across 3 bytes:
+- Bit 0 (LSB) → buf[2] bits 2:1
+- Bit 1 → buf[2] bits 5:4, plus bit 7
+- Bit 2 → buf[2] bit 7, buf[1] bit 0
+- Bit 3 → buf[1] bits 3:2
+- Bit 4 → buf[1] bits 6:5
+- Bit 5 → buf[0] bits 1:0
+- Bit 6 → buf[0] bits 4:3
+- Bit 7 (MSB) → buf[0] bits 7:6
+
+Each data bit becomes a 3-bit pattern:
+- `0` → `100` (binary)
+- `1` → `110` (binary)
+
+## Data Transmission
+
+The `led_strip_spi_refresh()` function handles transmission with chunking:
+
+1. **Total data size**: `strip_len × bytes_per_pixel × 3`
+2. **Max transfer size**: Limited by SPI bus configuration (`max_transfer_sz`)
+3. **Chunked transfers**: Data sent in chunks if it exceeds max transfer size
+4. **Transmission mode**: `spi_device_polling_transmit()` used for blocking transfer
+
+## Advantages of SPI Emulation
+
+1. **No RMT peripheral needed**: Frees up RMT channels for other uses
+2. **DMA support**: Can use DMA for efficient transfers
+3. **Precise timing**: Hardware SPI ensures consistent bit timing
+4. **Scalability**: Can drive long LED strips with proper DMA buffer sizing
+
+## Limitations
+
+1. **3x memory overhead**: Requires 3× the buffer space vs. RMT
+2. **Fixed clock**: Requires SPI clock source with ~2.5 MHz capability
+3. **No flexibility**: Cannot easily adjust timing for non-WS2812B LEDs
+4. **SPI bus exclusive**: Ties up an entire SPI peripheral
+
+## Date
+
+Analysis performed: 2025年11月10日