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Overview

The DInputBackend class provides DirectInput (HID gamepad) protocol support. This backend offers maximum compatibility with PC applications, older games, and emulators that don’t support XInput.
DInput is ideal for applications that need standard HID gamepad support or when XInput is not available.

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Platform Support

  • Raspberry Pi Pico: ✅ Supported
  • Arduino (AVR with USB): ✅ Supported
The AVR implementation uses the Arduino Joystick.h library, while the Pico implementation uses TUGamepad from TinyUSB.

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Constructor

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Raspberry Pi Pico

DInputBackend(
    InputState &inputs,
    InputSource **input_sources,
    size_t input_source_count
)

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Arduino (AVR)

DInputBackend(
    InputState &inputs,
    InputSource **input_sources,
    size_t input_source_count
)
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inputs
InputState&
required
Reference to the global input state structure that tracks all button and analog input values.
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input_sources
InputSource**
required
Array of pointers to input source objects (GPIO buttons, analog sticks, etc.) that provide input data.
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input_source_count
size_t
required
Number of elements in the input_sources array.

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Key Features

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Button Mapping (Pico)

The Pico implementation maps 13 buttons:
  • Button 0: B
  • Button 1: A
  • Button 2: Y
  • Button 3: X
  • Button 4: R (right shoulder)
  • Button 5: RT (right trigger digital)
  • Button 6: L (left shoulder)
  • Button 7: LT (left trigger digital)
  • Button 8: Select/Back
  • Button 9: Start
  • Button 10: Right stick click
  • Button 11: Left stick click
  • Button 12: Home

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Analog Inputs

  • Left Stick: X/Y axes (8-bit, 0-255)
  • Right Stick: X/Y axes (8-bit, 0-255)
  • Triggers: L/R analog (8-bit, 0-255)
  • D-pad: Hat switch (8 directions + center)

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Y-Axis Inversion

The Pico implementation inverts Y-axes to match standard HID conventions: 
_gamepad.leftYAxis(255 - _outputs.leftStickY);
_gamepad.rightYAxis(255 - _outputs.rightStickY);

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Configuration

To use DInput as your primary backend: 
CommunicationBackendConfig backend_config = {
    .backend_id = COMMS_BACKEND_DINPUT,
};

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Backend ID

CommunicationBackendId BackendId() {
    return COMMS_BACKEND_DINPUT;
}
Returns COMMS_BACKEND_DINPUT to identify this backend type.

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SendReport Method

void SendReport();
The SendReport() method handles the complete input processing pipeline:
  1. Scans inputs at three different speeds (slow, medium, fast)
  2. Waits for HID device to be ready
  3. Updates output state based on game mode logic
  4. Maps outputs to HID gamepad report format
  5. Sends the report via USB

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Input Scanning (Pico)

The Pico backend uses a three-stage scanning approach for optimal latency: 
ScanInputs(InputScanSpeed::SLOW);
ScanInputs(InputScanSpeed::MEDIUM);

while (!_gamepad.ready()) {
    tight_loop_contents();
}

ScanInputs(InputScanSpeed::FAST);

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AVR Implementation Notes

The AVR implementation (for Arduino Leonardo, Pro Micro, etc.) uses a single-stage input scan: 
ScanInputs();
UpdateOutputs();
// Map to Joystick_ object and send

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Usage Example

// In your config file
#include "comms/DInputBackend.hpp"

static InputState inputs;
static InputSource *input_sources[] = { &gpio_input };
size_t input_source_count = 1;

// Create the backend
DInputBackend dinput_backend(
    inputs,
    input_sources,
    input_source_count
);

void loop() {
    // Send controller state to PC
    dinput_backend.SendReport();
}

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Destructor

~DInputBackend();
The destructor resets all gamepad inputs to their default state, ensuring clean disconnection.

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Platform Differences

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Raspberry Pi Pico

  • Uses TUGamepad class from TinyUSB
  • Supports hat switch for D-pad
  • Includes trigger analog values
  • Y-axis inversion for HID compliance

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Arduino (AVR)

  • Uses Joystick_ library
  • Standard HID gamepad descriptor
  • Compatible with Leonardo, Pro Micro, etc.
The DInput backend is only available on platforms with native USB support. Traditional Arduino boards (Uno, Mega, etc.) without USB HID capability cannot use this backend.

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Advantages

  • Universal compatibility: Works with virtually all PC applications that accept gamepad input
  • No driver required: Uses standard HID protocol
  • Multi-platform: Supported on both Pico and Arduino
  • Simple integration: No special host-side configuration needed

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Limitations

  • No native rumble support (XInput exclusive feature)
  • Less optimized for modern games compared to XInput
  • Some games may not properly detect analog triggers

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See Also

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