Applications of an FSM with Output Toggle When Detecting Specific Patterns

In digital electronics, Finite State Machines (FSMs) form the backbone of most control oriented logic. From communication protocols, processors to embedded controllers, FSMs  enable systems to react to sequences of events in a reliable and deterministic way. One  particularly powerful variant is the pattern-detection FSM with an output toggle. 

Unlike conventional pattern detectors, where the output pulses high for one clock cycle upon  detection, this FSM toggles the output bit each time a specific pattern appears in the input  stream. This subtle difference gives rise to a wider range of applications where mode  persistence, state switching or event counting is required. 

In this article, we explore why toggle-based pattern detectors are used and how real-world  digital systems benefit from them. 

What is an FSM with Output Toggle? 

A toggle-based output simply means every time the target input pattern is recognized, the FSM  flips the output bit (0 → 1 or 1 → 0). This makes the output function like a 1-bit memory or  mode flag, capturing alternating occurrences of the pattern. The output remains stable until the  pattern is detected again. This makes the design self-contained, lightweight and ideal for mode  switching.

Example: Design a sequence detector that receives a binary data stream at its input and signals
when a combination of 1011 arrives at the input by toggling its output. Consider the first
received bit is MSB, without overlapping.

How Output Toggle Works

The FSM transitions through states corresponding to partial matches of the target sequence.
When the full sequence is detected, the FSM toggles its output (from 0 to 1 or 1 to 0). The
output toggle is achieved using a flip-flop that stores the current output state, updated on each
detection event.

Real-World Applications of Output Toggle FSMs

Here are some of the most practical and impactful uses across digital and embedded systems.

1. Event Counting: Even–Odd Tracking
An output toggle FSM can be used to track whether the number of packet arrivals is odd or even by toggling its output each time a packet is detected. When a packet arrives, the FSM detects the event (such as a specific sequence or a packet delimiter) and flips its output state. If the output starts at 0, the first packet arrival toggles it to 1, the second toggles it back to 0, and so on. This way, the current state of the output directly represents whether an odd or even number of packets have arrived: a value of 1 means an odd count, and 0 means an even count. This approach is especially useful in communication protocols and diagnostic systems where it’s important to know if the number of events (like packets or errors) is odd or even, for example, to trigger actions on every other event or to implement simple parity checks for error detection. The simplicity of the toggle FSM makes it reliable and area-efficient, ideal for
systems with limited hardware resources.

2. Communication Protocols
In communication protocols, FSMs are used to detect framing or synchronization sequences in data packets, which helps identify the start and end of a valid message. When a receiver encounters a specific sequence (often called a preamble or sync word) in the incoming data stream, the FSM transitions to a state indicating that a new frame is beginning. This transition triggers an output toggle, signalling the start of a message to the higher layers of the protocol stack.
For example, in wireless communication systems, the FSM might have states for searching, checking, locking, and protecting. When the FSM detects the correct framing sequence, it toggles its output to indicate that the receiver is synchronized and ready to process the payload data. This toggle ensures that the system can reliably distinguish between valid packets and noise or corrupted data, minimizing errors and improving communication efficiency. This technique is crucial for protocols like Ethernet, Wi-Fi, and cellular networks, where precise frame detection is necessary for reliable data transfer and error-free communication.

3. Lightweight Security, Access Control and Secret-Key Triggers
Toggle FSMs are widely used for security and control in embedded systems, especially for lock/unlock mechanisms, authentication, and enabling secure logic. When a secret bit sequence is detected, the FSM toggles its output, which can unlock a device or enable a secure function. This method is common in hardware security, where only the correct sequence grants access, and any incorrect sequence keeps the system locked, reducing the risk of brute-force attacks. In one-wire authentication systems, a single data line is used for both communication and authentication. The FSM monitors the line for a predefined sequence and upon detection, toggles an output to authenticate the device or user. This is efficient and secure, as it minimizes wiring and attack vectors. Toggle FSMs are also used to generate secure enable signals for hidden logic or to activate hardware test routines. By requiring a coded toggle sequence, only authorized users or processes can enable sensitive features, which enhances reliability and reduces the attack surface. The simplicity of the FSM design means fewer bugs and vulnerabilities, making it ideal for safety-critical applications.

4. Instrumentation and Debugging (Triggering)
Pattern Triggers: Users define a binary pattern, such as 1011, across multiple channels. The FSM continuously monitors incoming digital signals, advancing through matching states (e.g., state1 waits for ‘1’, state2 for ‘0’, etc.) until the full sequence aligns, then toggles from Idle to Recording state to capture subsequent data. This sequential logic ensures precise synchronization, ignoring noise or partial matches and often supports advanced qualifiers like “don’t care” bits or edge conditions.
Event Counting: In high-speed physics experiments, sensors generate particle signatures as unique bit patterns amid continuous data floods. A toggle FSM detects each occurrence, flipping between Buffer A and Buffer B on every match to alternate storage without overwriting active captures.

Conclusion

FSMs with toggle outputs may appear simple, but they bring powerful capabilities to pattern- driven digital systems. Whether it’s switching modes, event counting, secret key triggers etc. These FSMs enable flexible, persistent state behaviour without needing extra counters or memory.