Universal Shift Register is a register which can be configured to load and/or retrieve the data in any mode (either serial or parallel) by shifting it either towards right or towards left. In other words, a combined design of unidirectional (either right- or left-shift of data bits as in case of SISO, SIPO, PISO, PIPO) and bidirectional shift register along with parallel load provision is referred to as universal shift register. Such a shift register capable of storing n input bits is shown by Figure 1.
The design shown by Figure 1 uses n 4×1 multiplexers to drive the input pins of n flip-flops in the register which are also connected to clock and clear inputs. All of the multiplexers in the circuit share the same select lines, S1 and S0 (pink lines in the figure), in order to select the mode in which the shift registers operates. It is also seen that the MUX driving a particular flip-flop has its
- First input (Pin Number 0) connected to the output pin of the same flip-flop i.e. zeroth pin of MUX1 is connected to Q1, zeroth pin of MUX2 is connected to Q2, … zeroth pin of MUXn is connected to Qn.
- Second input (Pin Number 1) connected to the output of the very-previous flip-flop (except the first flip-flop FF1 where it acts like an serial-input to the input data bits which are to be shifted towards right) i.e. first pin of MUX2 is connected to Q1, first pin of MUX3 is connected to Q2, … first pin of MUXn is connected to Qn-1.
- Third input (Pin Number 2) connected to the output of the very-next flip-flop (except the first flip-flop FFn where it acts like an serial-input to the input data bits which are to be shifted towards left) i.e. second pin of MUX1 is connected to Q2, second pin of MUX2 is connected to Q3,… second pin of MUXn-1 is connected to Qn.
- Fourth input (Pin Number 3) connected to the individual bits of the input data word which is to be stored into the register, thus providing the facility for parallel loading.
The working of this shift register is explained by the Table I. The corresponding truth table and the wave forms are given by Table II and Figure 2, respectively.