HubAudio/docs/architecture/HubAudio-SPI-Architecture.md

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

         System Controller

      System Controller

            |
            |
      SPI Control Bus

+-----------+-----------+
|                       |

Audio Processor Radio Receiver

Audio Processor Radio Receiver

---

2. System Controller Control Domain

The System Controller is the system supervisor.

Its responsibilities are:

• system startup
• peripheral initialization
• configuration management
• communication with external devices
• firmware update coordination

The System Controller owns the main control SPI interface.

        System Controller

     SPI MASTER

          |
+---------+---------+
|                   |

Audio Processor Radio Receiver

SPI SLAVE SPI SLAVE

The control bus is independent from all audio data paths.

---

3. Audio Processor SPI Domain

The Audio Processor contains its own SPI interface for external control.

The System Controller uses this interface for:

• DSP configuration
• parameter updates
• operational control
• status reading

The Audio Processor also manages its external program memory.

         Audio Processor

+----------------------+
|
| SPI MASTER
|
v

      Audio EEPROM

DSP Program Memory

The System Controller does not directly access the EEPROM during normal operation.

The Audio Processor is responsible for loading its DSP configuration.

---

4. Radio Receiver SPI Domain

The Radio Receiver is controlled by the System Controller through its SPI slave interface.

The System Controller manages:

• initialization sequence

• command exchange

• configuration

• firmware loading procedure

         System Controller
  
      SPI MASTER
  
           |
  
         Radio Receiver
  
           |
  

  Internal Firmware Management

           |
  
      Internal RAM
  

The Radio Receiver 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:

      System Controller

  +-------------+

  |             |

SPI-A SPI-B

  |             |

Audio Processor Radio Receiver

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

|

System Controller boot

|

Initialize SPI buses

|

Configure Audio Processor

| +--> ADAU loads DSP program from Audio EEPROM

|

Configure Radio Receiver

| +--> Firmware initialization

|

Enable Audio Domain

|

Audio Processor starts audio processing

---

7. Separation Between Domains

Control Domain

System Controller

| |

SPI

|

Peripheral configuration

Audio Domain

Audio Sources

| |

I2S

|

Audio Processor

|

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 System Controller is the system coordinator.

The Audio Processor is the audio processor.

The Radio Receiver is a specialized audio peripheral.

Each component controls its own functional domain while remaining part of the complete HubAudio system.