Add stack connector + addressing detail

Stack-Connector-and-Addressing.md

@@ -0,0 +1,42 @@
+# BikeAudio — Stack Connector, Bus & I2C Addressing
+
+Physical/electrical interconnect for the stackable unit (1 SINK + N BROADCASTERS of identical ESP32-WROOM, + ESP32-S3 HUD). Audio 44.1k/16/stereo -> BCK = 1.4112 MHz.
+
+## 1. Stacking connector
+**26-pin (2x13) 2.54 mm THT pass-through stacking header**, polarized/keyed, ~3 A/pin (vibration-rated), **11-12 mm board pitch** (clears the WROOM can). Pass-through tails let identical boards self-nest into a vertical bus column with no orientation flip. (Samtec Flex-Stacking pass-through / SSQ-elevated, or Amphenol BergStik stack-through.) THT over fine-pitch SMT for vibration pull-out strength; bus speeds are low so no controlled-impedance needed.
+
+**Pinout (grounds bracket the I2S clock/data for return current):**
+| Pin | Sig | Pin | Sig |
+|--|--|--|--|
+|1|5V|2|5V|
+|3|GND|4|GND|
+|5|3V3|6|3V3|
+|7|GND|8|I2S_BCK|
+|9|GND|10|I2S_WS|
+|11|GND|12|I2S_DATA|
+|13|GND|14|GND|
+|15|I2C_SDA|16|I2C_SCL|
+|17|UART1_TX|18|UART1_RX|
+|19|UART2_TX|20|UART2_RX|
+|21|LED_DAT|22|LED_CLK (APA102)|
+|23|ADDR_CHAIN_IN|24|ADDR_CHAIN_OUT|
+|25|SPARE0|26|GND|
+
+(LED is APA102 -> two lines DATA+CLK, per the LED page; if a board's bus pins 23/24 are reused for chain auto-enum they must not be the I2C pins.)
+
+## 2. Mechanical stack
+WROOM-up identical boards; **M3 corner standoffs carry the vibration load** (connector contacts aren't structural). SINK at bottom (owns I2S master + power entry); broadcasters stack above; **HUD ribbon-attached on top** facing the rider (needs only I2C+5V+GND). Each board has a **local 3V3 buck/LDO** — don't bus regulated 3V3 across 6-12 boards (IR drop). 5V bussed on the two paralleled pins (~6 A).
+
+**Scale to 6/12 via a passive backplane** carrying the bus to several vertical sockets (e.g. 4x3 = 12), with the **ADDR chain routed serially** stack->stack for deterministic addressing, and **I2S distributed as a buffered star** (one branch per stack column) — never one tall column past ~4 boards.
+
+## 3. I2C addressing (identical boards/firmware)
+SINK fixed (0x08); broadcasters 0x10..0x1B (up to 12).
+- **Recommended v1: 4-bit straps** on input-only/safe GPIOs read at boot -> addr = 0x10 | nibble. Vibration-proof, supports 16. (NOTE: this system's I2C is on GPIO32/33 per firmware, so straps live on other safe pins e.g. 13/14/27/26; the connector's ADDR_CHAIN pins must avoid 32/33.)
+- **Scale-up: GPIO daisy-chain auto-enumeration** — each board claims the next address by physical position (zero config). Firmware: hold next board's CHAIN_OUT low, wait for CHAIN_IN, claim next index from SINK over I2C, release upward. Elegant for many identical boards; needs a boot handshake + timeout fallback to straps for bench use.
+
+## 4. Signal integrity
+- **I2S (1.41 MHz @44.1k, 2.82 @48k):** don't drive up to 12 inputs from one GPIO. **Buffer at the SINK (74LVC125/2G34) + 22-33 R series term per line**, **star fan-out** (one branch/stack). Medium GPIO drive (not max — high drive into a capacitive stub network increases ringing). Grounds bracket BCK/WS/DATA through every connector.
+- **I2C:** up to ~14-16 nodes; keep bus < 400 pF. **Run 100 kHz with a single 2.2 kOhm pull-up pair** at the HUD/SINK (Fast-mode 400 kHz isn't safely achievable across a fully-loaded 12-board bus on one pull-up — the R_min/R_max window closes). For 12 boards/higher speed add a **PCA9548A mux or P82B96 buffer** per stack. One pull-up set only; every other board taps open-drain.
+
+## Sources
+Samtec Flex-Stacking pass-through + design guide + SSQ; Amphenol BergStik 2.54 stacking; TE board-to-board / Micro-MaTch; ESP-IDF I2S (clock, drive strength); FluidNC signal-quality (edges/termination); AllAboutCircuits + TotalPhase I2C pull-up math; ESP32 GPIO/strapping reference.