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Sequential logic circuit register

Last Update Time: 2018-08-28 17:30:45

Sequential logic circuit

The characteristic of sequential logic circuit is that the output signal is not only related to the input signal at any time, but also related to the original state of the circuit.

The main steps of reading sequential logic circuits are as follows:

1. Analyze the composition of the circuit, write the driving equation, and write the driving equation according to the given circuit. Asynchronous sequential circuits also need to write clock equations. If there are combinational circuits in the circuit, the output equations need to be written.

2. According to the driving equation, the state equation is written, and the state equation is obtained by replacing the state equation with the characteristic equation of the driving equation.

3. The state equation is transformed into a state transition table and a normal truth table. The values of all States are listed on the left side of the table, and the sub-values of each row and corresponding output values are calculated respectively. This method needs to draw the state transition diagram to observe the transformation relationship between states. In solving subproblems, we can get subproblems faster by using the concept of primitive variables and inverse variables.

4. When using a state transition table without a clock, the state transition table is converted to a state transition diagram, which you need to draw to observe the transition between states. When using the state transition table with clock, it is also recommended to use the state transition diagram to analyze the function of the circuit more intuitively.

5. According to the state transition diagram, the function of the circuit is explained according to several state periods in the state transition diagram to determine whether the circuit can be started. If there is only one state loop, it can be started, and if there are more than one state loop, it cannot be started. Indicates whether the number of valid cycles is an increment counter or a decrement counter, and the mode (count length) function of the state machine (counter), such as a decimal number.


Register is a kind of sequential logic circuit for storing data, which is widely used in various digital systems and computers. The memory circuit of a register consists of a latch or a flip-flop. Because a latch or a flip-flop can store binary numbers, N-bit registers can be formed from N latches or flip-flops. Triggers in registers only need to have settings 1 and 0, so you can use SR latches and D flip-flops or JK flip-flops to form registers. Because D flip-flops are simple and have a set of 1, the set of 0 functions, so registers are generally composed of D flip-flops. The register has a common input / output terminal, an enable control terminal and a clock signal input terminal. In general, enable control terminals are used as selection signals for register circuits, and clock signals are used as data input control signals.

For parallel digital input terminals, Q3~Q. Used for parallel digital output. The output Q3Q2Q, Qo = D3D2DjDo belong to the data update function when the clock signal CP is on the descending edge; the output Q3Q2Q1Qo = 0000 belong to the clearance function when CR = 0; except for these two cases, the output of the register Q3Q2Q1Q remains the same, which belongs to the storage function.

The shift register is a register with shift function. Registers only have the function of storing data or code. Sometimes, in order to process data, the data of registers need to be moved to higher or lower bit in turn under the shift control signal. The so-called serial refers to a signal line in order to transmit a variety of bits of data, this method has the advantage of saving wires, suitable for long-distance transmission, generally speaking, telephone lines and cables are transmitted in a serial manner. Parallelism refers to the simultaneous transmission of data using multiple signal lines. This method requires multiple lines and is only suitable for short-distance transmission. Both the data bus and address bus in the computer are transmitted in parallel. The data register function is only the basic function of the shift register. The shift register is often used for serial/parallel conversion. The shift register is also used for data multiplication and division because the shift of the binary data to the left equals 2 and the division of the right equals 2.