1. DIGITAL ELECTRONICS
THEME 7: Register structures – with parallel input, with serial input. Shift registers – reversible, cycle.
Register structures are sequential logic circuits designed for intermediate memorizing of digital data. A part of this structures could be used also for converting the data format. The register structures are composed by connected flip-flops. The number of flip-flops (n) determines the bits number of digital data stored in the register structure.
The register structures are classified according to the type of their input and output:
Parallel registers
Serial registers
Parallel-serial registers
Parallel registers (usually called memory registers) are used only to memorize n – bits digital data. The inputs of all n flip-flops (Х1, Х2, ..Хn) are accessible and they are used to fed the digital data which will be stored in the register. The digital data are stored in all flip-flops simultaneously. For example, if all flip-flops are reset (inner state ‘0’) at a moment of time t, in the next moment of time (t+1) the flip-flops are set in new inners state in accordance with the data fed on their inputs:
Q1(t+1)=X1(t); Q2(t+1)=X2(t); Q3(t+1)=X3(t);……………………. Qn(t+1)=Xn(t).

2. DIGITAL ELECTRONICS
Parallel registers are divided on two groups depending on the way for fixing the data on the inputs in the flip-flops :
the first group are the registers composed by edge–controlled flip-flops, where the input data are written in the register during the active edge of the clock pulse (С) – these are “D-type registers”;
the other group are registers composed by clock-controlled flip-flops (gated latch), where the input data are written in the register during the active level of clock signal (С). This type of registers are called “with transparency fixations”.
An example of such register is IC 74373, which is 8-bits parallel register with transparency fixations. It has one additional input for control of tree-state of output buffers of the flip-flops.
Internal circuit of 8-bits parallel register IC (D-type register)

3. DIGITAL ELECTRONICS
Serial registers, also known as shift registers, except playing role of intermediate memory for n – bits digital data, are used for shifting the stored data on the left or on the right. The processes of Introducing the digital data to the registers and shifting that data inside are controlled by clock pulses. The shift registers are implemented by D, J-K or R-S flip-flops. Shift registers which can shift data in both directions – from right to the left and vise versa are called reversible.
The shift registers could be realized with parallel inputs or with serial input. The data which will be written in a register with serial input are fed in serial format only to the input of the first flip-flop. After n clock pulses the data will be written in the register.
In parallel registers the digital data fed to the parallel inputs are introduced at once simultaneously in all flip-flops.
Depending on the type of data format at the input and at the output of the registers they can convert the format of the stored information:
- From serial to parallel, when the data are introduced by the serial input and are taken from the parallel outputs of all flip-flops composing the register structure;
- From parallel to serial, when the data are introduced from the inputs of all flip-flops in the structure and are taken only from the output of the last flip-flop.

4. DIGITAL ELECTRONICS
Circuit and time-diagrams for introducing digital data 1101 in 4-bits shift register with serial input
Clock
Reset Q1 Q2 Q3 Q4 t
The shift registers are used for realization of cycle registers, in which the serial input is connected with the serial output of the register (the output of the last flip-flop in the structure). Here, the data stored in the register cycling inside under the control of clock pulses.

5. DIGITAL ELECTRONICS
Clock
Reset Q1 Q2 Q3 Q4 t
Circuit and time-diagrams of 4-bits cycle register with stored digital data 1000 inside.
In integral realization different types of registers are produced. This variety allows implementation of all possible configurations of register structures which are necessary in practice.