The Hardware Proxy Pattern in Embedded Systems

Introduction

Accessing hardware in embedded systems is notoriously tricky. Beyond the raw registers and memory-mapped I/O, developers often face additional complexities: compression, encoding, DMA orchestration, concurrency, power management, and vendor-specific quirks. Without discipline, code quickly devolves into spaghetti tied tightly to silicon details.

Enter the Hardware Proxy Pattern: a structured design approach that creates a dedicated software proxy layer between the rest of the system and the hardware device. This proxy encapsulates access, transformation, and resource management in a way that makes code more portable, testable, and reliable.

When working with sustainable, low-power embedded systems, following established design patterns is especially valuable. These patterns simplify hardware/software interactions, reduce power consumption, and make the system more maintainable. The Hardware Proxy is just one of several key patterns. Others include:

• Hardware Adapter Pattern – Adapt between mismatched interfaces.
• Mediator Pattern – Coordinate multiple modules without tangled dependencies.
• Observer Pattern – Distribute sensor data efficiently to subscribers.
• Debouncing Pattern – Filter noisy signals like button presses.
• Interrupt Pattern – Respond quickly and efficiently to urgent events.
• Polling Pattern – Periodically check sensors when interrupts aren’t practical.

This post focuses on the Hardware Proxy Pattern. Future posts in this series will cover the other patterns with detailed explanations, C/C++ code examples, power optimization tips, and real-world use cases.

What is the Hardware Proxy Pattern?

A Hardware Proxy is a software element responsible for all access to a hardware block. It provides a stable, high-level API to clients, while internally handling:

• Register access and low-level protocols
• Data transformations (compression, encoding, framing)
• Power and clock management
• Concurrency control and request serialization
• Error handling, retries, and recovery
• Capability discovery for multiple hardware variants

Instead of exposing hardware registers or DMA buffers directly, the proxy mediates all interactions, ensuring the rest of the system only deals with clean, abstracted operations.

Responsibilities

1. Encapsulation – hide registers, interrupts, and vendor quirks.
2. Transformation – perform pre/post-processing (e.g., compression, encryption, packetization).
3. Lifecycle – own initialization, power-up/down, and shutdown.
4. Concurrency – serialize access and enforce safe multi-client sharing.
5. Error Recovery – retries, timeouts, and watchdog integration.
6. Testing Hooks – provide mocks and simulations for unit testing.

When to Use It

The Hardware Proxy Pattern shines in cases where:

• Complex hardware pipelines exist (e.g., codecs, encryption engines).
• Multiple software clients compete for the same device.
• Transformations (compression, encoding, framing) are mandatory.
• Platform portability is required — one driver must run across multiple SoCs.
• Safety-critical or testable design is essential.

It is less necessary in trivial cases (e.g., a simple GPIO toggle), where a thin Hardware Abstraction Layer (HAL) may suffice.

Benefits

• Encapsulation of complexity – one point of truth for hardware access.
• Improved testability – mock backends replace real hardware.
• Centralized power/resource management – no duplicate clock toggles scattered in the code.
• Cleaner client code – clients issue high-level operations without worrying about low-level details.
• Portability – proxy hides vendor-specific details.

Drawbacks

• Extra indirection can add latency.
• Larger code size in very constrained environments.
• If poorly designed, the proxy can become a bottleneck or a “god object.”

Design Variants

1. Synchronous Proxy (Blocking API)

In this variant, the proxy exposes simple blocking functions. When a client calls the API, the proxy immediately performs any required data transformation, configures the hardware, and waits until the operation completes or times out. This is easiest to implement and use but ties up the calling thread until completion.

• Use Case: Simple transactions like reading a register or writing a short buffer.
• Pros: Straightforward logic, predictable flow.
• Cons: Can waste CPU cycles and increase energy use if waiting on slow hardware.

2. Asynchronous Proxy (Callback / Event Driven)

Here, the proxy initiates the hardware transaction but returns immediately. Completion is reported via callbacks, events, or futures/promises. This avoids blocking the caller and allows the system to sleep or perform other work while the hardware is busy.

• Use Case: Long data transfers over UART, SPI, or radio.
• Pros: Efficient CPU and power usage, scales well to concurrent clients.
• Cons: More complex state management; requires careful concurrency handling.

3. Command Queue + Worker Task

In this design, the proxy maintains a queue of pending requests. A dedicated worker task or thread dequeues items, applies transformations, and services the hardware. Interrupts only signal task wake-ups or enqueue events. This design is common in RTOS-based systems.

• Use Case: Multiple clients sharing the same peripheral.
• Pros: Centralized arbitration, avoids conflicts, keeps ISRs short.
• Cons: Extra RAM for queue; adds scheduling latency.

4. Transform Strategy Plug-in

This variant separates hardware access from data transformation by allowing a pluggable strategy object or function table. The proxy can use different compression, encoding, or security algorithms without changing its own logic.

• Use Case: A sensor hub that may need different encoding schemes for different networks.
• Pros: High flexibility, easy testing of alternative algorithms.
• Cons: Slight overhead of indirection; requires clear interface contracts.

5. Capability Discovery

The proxy exposes a standardized way to query supported features (e.g., max transfer size, supported encodings, hardware version). This allows higher layers to adapt dynamically.

• Use Case: Supporting multiple revisions of hardware where features may differ.
• Pros: Improves portability and forward compatibility; reduces hardcoding.
• Cons: Slightly more code; must ensure capability queries remain accurate and efficient.

Concurrency & ISR Rules

• Keep ISRs short: acknowledge interrupt, enqueue event, exit.
• Proxy APIs must document context: ISR-safe vs task-only.
• Use lock-free queues: SPSC ring buffers for ISR → task communication.
• Avoid blocking in ISR context.
• Worker tasks handle transformations and callbacks.

Error Handling

• Explicit error codes (TIMEOUT, HW_ERR, BAD_PARAM).
• Retries with backoff for transient failures (e.g., I²C NAKs).
• Timeouts for blocking APIs.
• Integration with watchdog timers.

Testing & Mockability

• Backend interface: swap hardware backend for mock in unit tests.
• Simulated backends: loopback or host-simulated devices.
• Fault injection: simulate timeouts, corrupted frames, bus errors.
• CI integration: run driver logic on host without hardware.

Anti-Patterns

• Heavy computation in ISR context.
• Leaking hardware details beyond the proxy.
• Unbounded queues with potential memory exhaustion.
• Priority inversion (high-priority task waiting on low-priority worker).

Example Workflow (Asynchronous Proxy)

1. Caller submits a request via hw_proxy_submit().
2. Proxy validates and applies transformation.
3. Request is enqueued.
4. Worker task dequeues and starts DMA transfer.
5. DMA ISR signals completion.
6. Worker finalizes and invokes callback in task context.

Caller -> Proxy.submit() -> Enqueue -> Worker Task
Worker -> Transform.encode() -> Start HW DMA
DMA ISR -> Signal Worker
Worker -> Callback to User

Checklist for Implementation

• Public API with clear ISR/task context rules.
• Backend interface for real + mock hardware.
• Transform strategy plug-in system.
• Bounded command queue.
• Worker task for heavy lifting.
• Error handling with retries and timeouts.
• Power/resource management.
• Capability query API.
• Unit tests with mocks and simulations.
• Watchdog and health-check integration.

Scope of These Patterns

All of the design variants and approaches discussed above — synchronous proxy, asynchronous proxy, command queue with worker, transform plug-ins, and capability discovery — are specifically focused on how software should access and interact with hardware. They do not describe higher-level business logic or application state management, but instead concentrate solely on the safe, efficient, and portable use of physical peripherals. By treating these patterns as hardware-access mechanisms, you ensure that the rest of your system remains decoupled from low-level details while still taking advantage of optimized, sustainable interactions with the device.

Hardware Proxy: design variants

Comparison Table: When to Use Each Hardware-Access Pattern

Reference:

Design Patterns for Embedded Systems in C by Bruce Powel Douglass

Conclusion

The Hardware Proxy Pattern offers a disciplined way to manage hardware access in embedded systems. By centralizing access, transformations, and concurrency handling, it makes systems more robust, portable, and testable. While it introduces some indirection, the trade-off is well worth it for non-trivial hardware blocks.

In practice, you’ll want to choose between synchronous or asynchronous proxies, decide whether to use pluggable transforms, and carefully define context rules. Combined with strong testing and error-handling practices, the Hardware Proxy Pattern can significantly improve the reliability of your embedded systems.

This article is part of a series on Common Embedded Design Patterns for Sustainable Systems. In the upcoming posts, we’ll dive deeper into the Adapter, Mediator, Observer, Debouncing, Interrupt, and Polling patterns — complete with code examples, power optimization tips, and real-world use cases.