In digital design, what is the most suitable technique to detect the serial bit pattern \"1101\" arriving on a single input line?

Introduction / Context:
Detecting specific bit sequences in a serial data stream is a common task in digital communication, protocol design and pattern recognition. When bits arrive one at a time on a single line, the designer must remember recent bits and assert a detection signal when the desired pattern appears in the correct order. This question focuses on detecting the sequence 1101 and asks which design technique is most suitable for implementing such a detector.

Given Data / Assumptions:

The input is a serial stream of bits on a single line, sampled synchronously with a clock.
We need to detect the particular sequence 1, 1, 0, 1 in that order as it passes by.
Detection should occur even when the pattern appears overlapping within the stream, depending on design requirements.
We are limited to digital logic solutions and must ignore analog or mechanical approximations.
The goal is to assert an output signal when the pattern 1101 is recognised.

Concept / Approach:
The standard approach to pattern detection in a serial stream is to design a finite state machine (FSM), either Moore or Mealy type, that tracks the history of recently seen bits. Each state represents how much of the pattern has been matched so far, and transitions respond to incoming bits. When the machine reaches the state corresponding to having just seen the full pattern 1101, it asserts an output. The FSM can be implemented using flip flops to store the state and combinational logic to determine the next state and output. An alternative but related implementation uses a shift register to hold the last four bits and combinational logic to compare them with 1101.

Step-by-Step Solution:
Step 1: Identify the pattern to detect: the sequence 1, 1, 0, 1 appearing consecutively in the serial input. Step 2: Construct conceptual states such as: no bits matched yet, matched 1, matched 11, matched 110 and matched 1101. Step 3: Define transitions between these states based on the incoming bit. For example, if you have matched 11 and receive a 0, move to the state matched 110; if you receive a 1 instead, update the state to reflect the longest suffix that still matches the beginning of the pattern. Step 4: Implement this FSM using flip flops to store the current state and combinational logic that computes the next state and a detection output. Step 5: Alternatively, design a 4 bit shift register that shifts in each new bit and uses combinational logic to compare its contents with 1101; when equal, assert an output pulse. This is effectively a simple sequence detector built with register and logic.

Verification / Alternative check:
To verify the design, simulate the FSM or shift register version with a test sequence that includes 1101 at different positions and possibly overlapping occurrences. For example, feeding 11101 should cause detection at the correct clock cycle. Simulation waveforms should show the state transitions and the detection output going high exactly when the last bit of the pattern has arrived. This confirms that the FSM based sequence detector is recognising the pattern and not relying on any unrealistic behaviour.

Why Other Options Are Wrong:
Connecting the input to an adder and monitoring overflow has no direct relation to the specific sequence 1101; adders operate on numeric values, not on sliding bit patterns in this way.
A resistor capacitor network is an analog filter and cannot robustly distinguish a specific digital sequence like 1101 at logic levels, especially under varying data rates and noise.
A mechanical debouncer circuit is used for cleaning up contact bounce from switches, not for recognising complex bit sequences in high speed digital data.

Common Pitfalls:
One common mistake is to implement a simple equality check on a group of bits without considering how the data is aligned in time, leading to missed or misaligned detections. Another pitfall is forgetting to handle overlapping occurrences of the pattern, which requires careful definition of the FSM transitions. Designers should also ensure that the detector is properly synchronised to the data clock and that metastability is handled where asynchronous inputs are involved.

Final Answer:
The most suitable technique is to design a finite state machine based sequence detector using flip flops and combinational logic that asserts an output when the pattern 1101 is seen.