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# A binary sequence signal detector design 3

Binary sequence signal detector can detect the input is a string of binary code, binary code when the code consistent with the pre-set, the detection circuit output high, or low output. The detection circuit can be widely used in everyday life and AV70-1 datasheet and production, and AV70-1 price and military. For example, can be used in password authentication, when a password consistent with the pre-set password, the authentication is successful. Another example is in the military field, is more concerned with enemy communications in some of the interesting content, and AV70-1 suppliers and mass communication data is encrypted, there is no need to decipher one by one, as long as some key words to decipher, the process can be in the listening only detect these key words, once the listener to immediately start the recorder to record the contents of key words to decrypt the communication time. Information in many electronic technology there are some sequences of signal detection circuit design, but design a single, extensibility is not strong. The following example is to illustrate the design method of 3 circuit.

Design task: Design a binary sequence signal detector, which has an input X, when the received sequence is 1001, then entered in the above sequence, while the last one, the circuit output Z = 1, otherwise the output is 0, input sequence can overlap. For example: when the input sequence X, 0100100101001 (top left), corresponding to the output Z = 0000100100001.

1 design with a separate trigger

Many different types of triggers, including double-ended input of the JK flip-flop and single-ended input of D flip-flop the most representative. As the circuit design with D flip-flop is more simple, so use it to design the circuit.

1.1 logical abstraction

Since 1001 the sequence to be detected, so the circuit has been located in the state of the input 0, S0, enter a 1 after the state S1, continuous input of 10 After the state S2, the state after continuous input for the S3 100 , continuous input state after the 1001 S4. So are the state transition shown in Figure 1.

state transition diagram in Figure 1 1,3

select the first line to explain its principles: S0 that received 0, when received on this basis, draw a line after a 0 to 00, while the need to detect the sequence is 1001, so the circuit state remains at S0 on; when the circuit on the basis of the S0, said after receiving a 1001 sequence received the first one, then the circuit state to S1. Similarly S2 said it had received 10, while on this basis, after receiving 0 becomes 100, the circuit to S3, but after receiving a 101 is changed, then lost the front to receive the role of the two codes, only the first 3 bit 1 can be used as the first 1001, so the reversal of the circuit state S1.

By observing the state transition table can be found, S1 and S4 have the same input under the same output, and the state get the same times after the transfer of state. Therefore they are equivalent can be combined, so you can simplify the state transition table shown in Figure 2.

Figure 2, the simplified conversion table

From the physical concept is not difficult to understand this situation. 1001, when the circuit is continuously received, the output is 1, but the sequence can overlap, so the last one as the next sequence of the first 1001, so the circuit in a row, after receiving the 1001 S4 is actually the state S1.

1.2 encoding

Simplified by the state transition Table 2 shows the circuit a total of 4 states (S0 ~ S3), and each flip-flop output Q can be 0 or 1 indicates that two states, and the two trigger 4 kinds of output Q1Q0 00,01,10,11 can represent these 4 states S0 ~ S3. This process is the encoding.

1.3 Lie truth table and write the equation of state

To the simplified form of the state to state, that is obtained by encoding truth table shown in Figure 3. One said the next state Q1Q0. Write on X, Q1, Q0 of the equation, that equation of state of the circuit, such as type (1) below.

Figure 3 truth table

1.4 for logic circuit The characteristics of the D flip-flop

equation Q *= D, which, according to the equation to draw the logic circuit can be shown in Figure 4.

Figure 4 serial code detection logic circuit

2 will be connected as shift register flip-flop design

Above mainly rely on the circuit design to achieve the state transition sequence code detection, although the income circuit is simple, but the design process more complicated, especially when the need to detect the sequence of code bits long, the larger workload. To this end, the shift register flip-flop connected as a way to simplify circuit design, but also easy to expand into the digit sequence code detector more. Connected with the four D flip-flop into the right shift register. Circuit shown in Figure 5.

Figure 5 sequence code shift register type of detector

Figure 5 shows. Pulse clk the shift under the action of the binary input code X input sequence to the right, whenever there is a complete 1001 sequence, the output Z will appear high. This realization of the sequence of code detection feature.

3 scale integrated circuits used in the design

Since sequences can be achieved with a shift register signal detection, then add a small amount with the integrated shift register gate can also be achieved, and the circuit reliability. Integrated with 4-bit shift register 74LS194, 1001, to achieve the sequence detector, shown in Figure 6.

Figure 6, constitute a shift register sequence with the integrated code detector

4 can not overlap when the sequence circuit design

Above 3 methods used to design the circuit, is the serial number can be overlapping sequence detector, if the request was not detected in the sequence of overlap in method 1, only the actual situation according to the state transition table can be modified . Design principles and steps behind the change. The problem with this design is still to be detected when the sequence of digits long, the design capacity, circuit reliability reduced. The adoption of Section 2, 3, design methods, an additional part of the control circuit, in order to ensure the synchronization of input and clock, you need to make every time it detects a sequence, direct location of the sequence into the register, the next lowest, while set the rest of you for the serial number register, the last anti-code of the code to eliminate duplication. In addition a latch enable output high and then more for 0.5 clock cycles, and its advantages are: the shift can be eliminated in the process of competition and adventure, making the output waveform more stable, more reliable circuit, the circuit shown in Figure 7 instructions.

Figure 7 with the shift register a sequence of non-overlapping sequence code detector

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