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+# HubAudio SPI Control Architecture
+
+- Status: Draft
+- Date: 2026-08-01
+- Document Type: Architecture Specification
+
+
+# 1. Overview
+
+The HubAudio system separates the control communication layer from the audio
+signal layer.
+
+SPI is used exclusively for:
+
+- device configuration
+- initialization
+- status monitoring
+- firmware loading procedures
+- parameter management
+
+SPI is not used for real-time audio transport.
+
+
+The control architecture is based on independent SPI domains.
+
+
+CONTROL PLANE
+
+             ESP32-S3
+
+          System Controller
+
+                |
+                |
+          SPI Control Bus
+
+    +-----------+-----------+
+    |                       |
+
+ADAU1467                Si4684
+
+Audio Processor Radio Receiver
+
+
+
+---
+
+# 2. ESP32-S3 Control Domain
+
+The ESP32-S3 is the system supervisor.
+
+Its responsibilities are:
+
+- system startup
+- peripheral initialization
+- configuration management
+- communication with external devices
+- firmware update coordination
+
+
+The ESP32-S3 owns the main control SPI interface.
+
+            ESP32-S3
+
+         SPI MASTER
+
+              |
+    +---------+---------+
+    |                   |
+
+ ADAU1467            Si4684
+
+SPI SLAVE          SPI SLAVE
+
+
+The control bus is independent from all audio data paths.
+
+
+---
+
+# 3. ADAU1467 SPI Domain
+
+The ADAU1467 contains its own SPI interface for external control.
+
+The ESP32-S3 uses this interface for:
+
+- DSP configuration
+- parameter updates
+- operational control
+- status reading
+
+
+The ADAU1467 also manages its external program memory.
+
+             ADAU1467
+
+    +----------------------+
+    |
+    | SPI MASTER
+    |
+    v
+
+          25AA1024
+
+   DSP Program Memory
+
+
+The ESP32-S3 does not directly access the EEPROM during normal operation.
+
+The ADAU1467 is responsible for loading its DSP configuration.
+
+
+---
+
+# 4. Si4684 SPI Domain
+
+The Si4684 is controlled by the ESP32-S3 through its SPI slave interface.
+
+The ESP32-S3 manages:
+
+- initialization sequence
+- command exchange
+- configuration
+- firmware loading procedure
+
+
+             ESP32-S3
+
+          SPI MASTER
+
+               |
+
+             Si4684
+
+               |
+
+    Internal Firmware Management
+
+               |
+
+          Internal RAM
+
+
+The Si4684 remains responsible for its internal operational memory.
+
+
+---
+
+# 5. SPI Bus Isolation Principle
+
+The HubAudio architecture intentionally avoids a single shared SPI bus.
+
+The design uses:
+
+          ESP32-S3
+
+      +-------------+
+
+      |             |
+
+   SPI-A          SPI-B
+
+      |             |
+
+  ADAU1467       Si4684
+
+
+Advantages:
+
+- no chip-select conflicts
+- independent timing
+- reduced electrical loading
+- easier firmware management
+- simpler debugging
+
+
+---
+
+# 6. Boot Sequence
+
+The expected startup sequence is:
+
+
+
+Power ON
+
+|
+
+ESP32-S3 boot
+
+|
+
+Initialize SPI buses
+
+|
+
+Configure ADAU1467
+
+|
+--> ADAU loads DSP program from 25AA1024
+
+|
+
+Configure Si4684
+
+|
+--> Firmware initialization
+
+|
+
+Enable Audio Domain
+
+|
+
+ADAU1467 starts audio processing
+
+
+
+---
+
+# 7. Separation Between Domains
+
+
+## Control Domain
+
+
+ESP32-S3
+
+|
+|
+
+SPI
+
+|
+
+Peripheral configuration
+
+
+
+## Audio Domain
+
+
+Audio Sources
+
+|
+|
+
+I2S
+
+|
+
+ADAU1467
+
+|
+
+Audio Outputs
+
+
+
+The two domains interact only through configuration and status information.
+
+
+---
+
+# 8. Design Rules
+
+SPI signals require:
+
+- controlled routing
+- clean reference plane
+- appropriate termination where required
+- separation from high-speed clock signals
+
+
+Critical signals:
+
+- SCLK
+- MOSI
+- MISO
+- CS
+
+
+---
+
+# 9. Design Philosophy
+
+The ESP32-S3 is the system coordinator.
+
+The ADAU1467 is the audio processor.
+
+The Si4684 is a specialized audio peripheral.
+
+Each component controls its own functional domain while remaining part of the
+complete HubAudio system.