can you explain how we design a digital sequence detector
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A sequence detector is a sequential circuit that outputs 1 when a particular pattern of bits sequentially arrives at its data input. Figure 1 shows a block diagram of a sequence detector. It has two inputs and one output. The inputs are the clock used to synchronize the functionality of the circuit and the data input.
The data input receives the input sequence. In Figure 1, the input sequence and the output sequence of the circuit are a sample of a 0111 sequence detector. If you follow the input and output sequences, you can see that only when the last four bits of the input sequence are 0111 does the output turn to 1 during one clock cycle. It then turns back to 0.
The sequence detectors that we cover in this lesson do not reset their states after each detection. In other words, they memorize the input sequence before the detection of the required pattern and use it to redetect the pattern. For example, if the input of a 1111 sequence detector is 11111111, the output will be 00011111. The sequence detector keeps the previously detected 1s to use in the following detections of 1111.
Design of a Sequence Detector
In this lesson, we will use Moore state machines. Moore machines are state machines where the outputs are states and are not directly determined by the inputs. In other words, inputs only cause a state transfer, which might or might not be an output state. In a Moore state diagram, a state is assigned the following values
the state's namethe states value, which is 0 or 1the state's code
Let's design a sequence detector that would detect the sequence 0111. We start with an initial state, called Init as shown in Figure 2. At this point, you can consider this state as the state of the circuit when it starts. The circuit at this point has not gotten any values on its data input. The name of the state is Init, its value is 0 and its code is 000. We are coding the state using 3 bits because we will need five states to design this circuit. You can leave the coding phase until the end of the design, if you do not know the number of required states.
Figure 2: Step 1 of the design of the state diagram for the sequence detector 0111
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