From c09a2dbb5f7300c92e1f2c0f5da62f4d8ba0681c Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?blue=20=E2=80=94=20ESP32/PlatformIO=20firmware?=
 <blue@noreply.gitea.vayrette.com>
Date: Wed, 10 Jun 2026 12:10:04 -0400
Subject: [PATCH] Add 3-board relay wiring diagram

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 3-Board Relay Wiring.-.md | 63 +++++++++++++++++++++++++++++++++++++++
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 create mode 100644 3-Board Relay Wiring.-.md

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+# 3-Board Bluetooth Relay — Wiring
+
+Relays iPhone audio to **two** Bluetooth speakers (JBL Charge 5 + Cardo "Tangerine EDGE") at once.
+
+**Board A** (the A2DP sink — the board the iPhone connects to) is the I2S **master** and drives three signals. **Boards B and C** (the A2DP sources) both **listen** to those same three signals in parallel. Plus one shared ground.
+
+## Pin-to-pin
+
+| Signal | Board A (SINK) out | Board B (JBL) in | Board C (Cardo) in |
+|---|---|---|---|
+| Bit clock (BCK)      | **GPIO5**  | GPIO19 | GPIO19 |
+| Word select (WS/LRCK)| **GPIO25** | GPIO18 | GPIO18 |
+| Data                 | **GPIO23** | GPIO22 | GPIO22 |
+| Ground               | **GND**    | GND    | GND    |
+
+Each of A's three pins splits to **both** B and C:
+
+- A·**GPIO5**  -> B·GPIO19 **and** C·GPIO19
+- A·**GPIO25** -> B·GPIO18 **and** C·GPIO18
+- A·**GPIO23** -> B·GPIO22 **and** C·GPIO22
+- A·**GND**    -> B·GND    **and** C·GND
+
+## Diagram
+
+```
+        BOARD A  (SINK / I2S MASTER)            iPhone connects here
+        +---------------------------+
+        | GPIO5  (BCK) o------------+----------+-------------+
+        | GPIO25 (WS)  o----------+ |          |             |
+        | GPIO23 (DAT) o--------+ | |          |             |
+        | GND          o------+ | | |          |             |
+        +---------------------|-|-|-|----------+             |
+                              | | | |                        |
+              +---------------+ | | +------+   +-------------+ (BCK to C)
+              |   +-------------+ +-----+  |   |   +--------- (WS  to C)
+              |   |   +---------------+--+-+---+---+--------- (DAT to C)
+              v   v   v               v  v   v   v
+        BOARD B (JBL)              BOARD C (Cardo)
+        +-----------------+        +-----------------+
+        | GPIO19 (BCK in) |        | GPIO19 (BCK in) |
+        | GPIO18 (WS  in) |        | GPIO18 (WS  in) |
+        | GPIO22 (DAT in) |        | GPIO22 (DAT in) |
+        | GND             |        | GND             |
+        +-----------------+        +-----------------+
+       sends to JBL               sends to Cardo
+```
+
+## Important
+
+- **Only share GND** between boards — do **not** tie their 3.3V/5V together. Each board runs off its **own USB**.
+- The GND link is **mandatory** (common clock reference).
+- Keep wires short (< ~20 cm); it's a ~1.4 MHz clock.
+- One-way: A **outputs**, B and C **only listen**.
+
+## Power-on order
+
+Bring up **Board A first** so the I2S bus is clocking before B and C read it.
+
+## Limitations
+
+- The two speakers are **not** sample-synchronized (independent BT buffering -> tens of ms skew). Fine for music/intercom; not for tight stereo L/R.
+- Audio is fixed at SBC **44.1 kHz / 16-bit / stereo**.
+- This relay carries **music (A2DP) only** — no microphone, no phone-call (HFP) audio.