What is hierarchical state machine in embedded systems?

Embedded system design revolves around efficiency and precision, demanding sophisticated structures like Hierarchical state machines (HSM) to manage complexity while ensuring optimal functionality. 

At its core, Hierarchical State Machines is a powerful model used to organize complex systems by breaking them into manageable states and transitions. 

In Embedded System, HSMs provide a clear visualization of the system’s behavior and facilitate efficient decision-making. They consist of multiple layers of states, enabling a hierarchical arrangement that simplifies intricate system structures.

In this article, we’ll delve deep into Hierarchical State Machines, discussing their significance, implementation, and the role they play in Embedded System.

State Transition Explosion 

One primary challenge in embedded systems is the State Transition Explosion, where the number of states and transitions grows exponentially with the system’s complexity. Hierarchical State Machines addresses this issue by organizing states hierarchically, thereby reducing the complexity of transition management.

The DRY principle 

The DRY (Don’t Repeat Yourself) principle is fundamental in Hierarchical State Machines. It emphasizes the need to eliminate redundancy by defining each state and its transitions only once, promoting code reusability and maintainability. In Embedded System Design, adhering to the DRY principle within Hierarchical State Machines ensures streamlined and concise code.

State Nesting Hierarchy 

Hierarchical State Machines (HSMs) are a way to organize states within a system in a nested structure. Imagine states as different modes or conditions that a system can be in—like “idle,” “processing,” or “error handling.” 

With Hierarchical State Machines, these states can have sub-states or child states within them, creating a hierarchy. The benefit of this hierarchy is that you can group similar behaviors or functionalities within a parent state. 

For instance, if you have several states that share a common set of actions or operations, you can encapsulate these shared behaviors in a parent state. This encapsulation promotes code modularity, meaning you can compartmentalize and manage the code for specific functionalities more easily.

Now, in Reactive Systems used in IoT solutions and services, this modular approach becomes highly beneficial. IoT systems often deal with numerous states and complex interactions between them. By using Hierarchical State Machines and encapsulating common behaviors in parent states, you create reusable components. These reusable components or functionalities can then be applied across different states or even different parts of the system.

This reusability aspect is crucial for system adaptability and scalability in the context of IoT and Hierarchical State Machines. When new functionalities or states need to be added or when the system needs to scale to accommodate more devices or processes, having these encapsulated and reusable components makes it easier to adapt and expand the system. Instead of re-creating functionalities from scratch, you can leverage these modular components, saving time and effort while ensuring consistency and reliability across the system.

In short, Hierarchical State Machines facilitate a structured way of organizing states, promoting code modularity, and enabling the reusability of functionalities. In the realm of Reactive Systems in IoT, this reusability greatly contributes to system adaptability and scalability, allowing for more efficient development, maintenance, and growth of complex interconnected systems.


Reuse Behavior in Reactive Systems 

Reuse of behavior in reactive systems refers to the practice of utilizing or repurposing existing functionalities, actions, or patterns within systems that respond to stimuli from their environment, known as reactive systems. These systems react to changes or events in real-time and adapt their behavior accordingly.

There are a few ways behavior reuse occurs in reactive systems:

  1. Component Reusability
  2. Design Patterns
  3. Functional Composition
  4. Event Handling and Handlers

By emphasizing behavior reuse, developers can streamline development, reduce redundancy, improve maintainability, and create more scalable and flexible reactive systems. This approach also promotes consistency in behavior across different parts of the system, enhancing overall system reliability and predictability.

For example, the concept of “Ultimate Hook,” explains how Graphical User Interfaces (GUIs) maintain a consistent appearance across applications. It involves a hierarchical event processing system where events are sent to the application first for customization. If unhandled, these events then default to the system’s standard appearance. This setup allows applications to personalize their behavior while ensuring a unified look-and-feel across the GUI.

This showcases programming-by-difference, where the application developer is required to write code solely for the variations from the standard system behavior.

Liskov Substitution Principle (LSP) 

The Liskov Substitution Principle (LSP), a key tenet of object-oriented programming, holds importance in HSM design. It asserts that objects of a superclass should be replaceable with objects of their subclasses without affecting the system’s functionality. This principle ensures the smooth integration of different state hierarchies within Hierarchical State Machines, preserving system integrity.

Implementation State Hierarchy in C

Implementing Hierarchical State Machines in C demands meticulous attention to detail. Leveraging pointers to functions and structures allows for the construction of state machines. Each state, transition, and hierarchy needs careful representation in code to ensure the system behaves as intended. This implementation in Embedded System Design demands both precision and efficiency.


In the realm of Embedded System, Hierarchical State Machines stand as a pivotal tool for managing complexity, promoting code modularity, and ensuring efficient system behavior. 

Their role in facilitating scalable, adaptable, and maintainable systems in IoT solutions and services cannot be overstated.

Embracing Hierarchical State Machines aligns with the demands of modern embedded systems, enabling engineers to navigate complexity effectively while adhering to essential design principles like DRY, LSP, and state nesting hierarchies.

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