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Hardware Abstraction Layer (HAL)

Overview

The HAL provides a clean separation between hardware-specific implementations and application logic, enabling:

  • Easy hardware variant support
  • Configuration-driven hardware selection
  • Testable interfaces
  • Resource-constrained embedded systems

Architecture

Layer Structure

Application Layer
    ↓
HAL Interface Layer
    ↓
Hardware Drivers
    ↓
Physical Hardware

Key Components

1. Core (HAL/Core/)

  • hal_types.h: Common types and status codes
  • hal_errors.h: Error handling and logging

2. Display (HAL/Display/)

  • display_interface.h: Abstract display interface
  • Drivers/: Hardware-specific implementations
    • ssd1306_driver: SSD1306 OLED display driver

3. Input (HAL/Input/)

  • input_interface.h: Abstract input interface
  • Drivers/: Hardware-specific implementations
    • shift_register_74hc165: 74HC165 shift register driver

4. Board (HAL/Board/)

  • Board Support Packages (BSP) for each hardware platform
  • handheld_bsp: Handheld controller BSP

Usage

Basic Initialization

#include "HAL/Board/handheld_bsp.h"

handheld_hardware_t hardware;

void setup() {
    hal_status_t status = handheld_bsp_init(&hardware);
    if (status != HAL_OK) {
        // Handle error
    }
}

Using Display Interface

const display_interface_t* display = hardware.display->interface;

// Clear display
display->clear(hardware.display);

// Draw text
display_point_t cursor = {10, 20};
display->set_text_cursor(hardware.display, &cursor);
display->write_text(hardware.display, "Hello", 1);

// Refresh display
display->refresh(hardware.display);

Using Input Interface

const input_interface_t* input = hardware.input->interface;

// Update input state
input->update(hardware.input);

// Read button state
bool pressed;
input->read_channel(hardware.input, 0, &pressed);

// Get events
input_event_data_t events[8];
uint8_t num_events;
input->get_events(hardware.input, events, 8, &num_events);

Hardware Configuration

Hardware profiles are defined in Config/handheld_config.h:

static const hardware_profile_t handheld_v1_profile = {
    .display = {
        .driver_type = DISPLAY_DRIVER_SSD1306,
        .params = {
            .ssd1306 = {
                .width = 128,
                .height = 64,
                .i2c_address = 0x3C,
                // ...
            }
        }
    },
    .input = {
        .driver_type = INPUT_DRIVER_SHIFT_REGISTER,
        .params = {
            .shift_register = {
                .load_pin = 25,
                .clock_pin = 26,
                .data_pin = 27,
                // ...
            }
        }
    }
};

Adding New Hardware

1. Create Driver

Implement the appropriate interface:

  • For displays: display_interface_t
  • For inputs: input_interface_t

2. Add Configuration

Define hardware parameters in configuration profile.

3. Update BSP

Modify BSP to instantiate new driver based on configuration.

Benefits

  • Portability: Same application code works with different hardware
  • Testability: Interfaces can be mocked for unit testing
  • Scalability: Easy to add new hardware variants
  • Maintainability: Clear separation of concerns
  • MISRA-C Compliance: Follows embedded best practices