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Backplane

The OpenDriveHub backplane is an I²C multiplexer bus that connects plug-in input modules to the transmitter. It is built around a TCA9548A 8-channel I²C switch.

How It Works

The TCA9548A sits on the ESP32's main I²C bus (SDA = GPIO21, SCL = GPIO22) at address 0x70 (configurable in odh::config::tx::kI2cMuxAddr).

Each module is connected to one of the mux's eight downstream channel pairs (SD0/SC0 through SD7/SC7). Because only one channel is active at a time, modules on different slots may use the same I²C address without conflict.

ESP32 (SDA, SCL)
   │
   └── TCA9548A  (0x70)
         ├── CH0 → Slot 0 module (e.g. PotModule at 0x48)
         ├── CH1 → Slot 1 module (e.g. SwitchModule at 0x20)
         ├── CH2 → Slot 2 module …
         ├── CH3 → (empty)
         ├── CH4 → …
         ├── CH5 → …
         ├── CH6 → (not populated)
         └── CH7 → (not populated)

The default slot count is 6 (odh::config::tx::kModuleSlotCount). This is a software limit; the hardware supports up to 8.

Firmware Interface

The backplane is managed by the odh::Backplane class (firmware/src/transmitter/backplane/Backplane.h).

Key API

namespace odh {

class Backplane {
public:
    explicit Backplane(uint8_t muxAddress = 0x70, uint8_t slotCount = 8);

    bool begin();                   // Initialise and verify TCA9548A
    bool selectSlot(uint8_t slot);  // Activate one channel
    void deselectAll();             // Deactivate all channels
    uint8_t activeSlot() const;     // Current active slot (kNoSlot if none)
    uint8_t scanSlots(uint8_t addr);// Probe each slot for a device
    uint8_t slotCount() const;
    bool isReady() const;
};

} // namespace odh

Usage Pattern

odh::Backplane bp(odh::config::tx::kI2cMuxAddr, odh::config::tx::kModuleSlotCount);
bp.begin();

for (uint8_t i = 0; i < bp.slotCount(); i++) {
    bp.selectSlot(i);
    // probe / read from whatever device is on this channel
}
bp.deselectAll();

Module Detection

The ModuleManager uses Backplane::selectSlot() to iterate over each slot and probes a fixed set of I²C addresses to identify the module type:

Probe Address Chip Module Type Class
0x48 ADS1115 ModuleType::Potentiometer PotModule
0x20 PCF8574 ModuleType::Switch SwitchModule
0x21 PCF8574 ModuleType::Button ButtonModule
0x36 AS5600 ModuleType::Encoder EncoderModule

If no known device responds at any of the probed addresses, the slot is marked empty. Detection runs once at startup.

The module type enum is defined in firmware/src/transmitter/modules/IModule.h:

enum class ModuleType : uint8_t {
    Unknown       = 0x00,
    Switch        = 0x01,
    Button        = 0x02,
    Potentiometer = 0x03,
    Encoder       = 0x04,
};

Adding a New Module Type

  1. Create the header and source in firmware/src/transmitter/modules/:

    // MyModule.h
#pragma once
#include "IModule.h"

namespace odh {

class MyModule : public IModule {
public:
    explicit MyModule(uint8_t slot);
    bool begin() override;
    void update() override;
    uint8_t inputCount() const override;
    uint16_t inputValue(uint8_t index) const override;
};

} // namespace odh

  2. Add a new enum value to ModuleType:

    enum class ModuleType : uint8_t {
    // ... existing types ...
    MyDevice = 0x05,
};

  3. Register the probe in ModuleManager – add your I²C address and instantiation logic to the detection loop.

  4. Design the hardware module – create a directory under hardware/modules/your_module/ with a README.md describing:

    • Chip / breakout board used
    • I²C address (must not conflict with existing modules)
    • Schematic / wiring table
    • BOM

Hardware Notes

I²C Pull-ups

The TCA9548A requires pull-up resistors on the upstream bus (ESP32 side):

  • 4.7 kΩ from 3V3 to SDA
  • 4.7 kΩ from 3V3 to SCL

Module breakout boards (ADS1115, PCF8574, AS5600) typically include their own pull-ups on the downstream side. If using a bare chip, add 4.7 kΩ pull-ups on each downstream channel pair as well.

Clock Speed

The bus runs at 400 kHz fast-mode (odh::config::kI2cFreqHz). All currently supported chips handle this speed.

Power

All modules and the mux are powered from the transmitter's 3.3 V rail. Current draw per slot is typically < 5 mA, but analog modules with potentiometers connected to 3.3 V may draw more through the wiper.

Slot Connector

See hardware/transmitter/README.md for the physical connector pinout. Each slot header exposes:

Pin Signal
1 3V3
2 GND
3 SDAn (TCA9548A channel N)
4 SCLn (TCA9548A channel N)