Registers - Group of Flip-Flops

We know that a flip-flop can be used to store one bit of information. If we want to store more bits at a place then we use registers. A register is a group of flip-flops each capable of storing one bit of information. An n-bit register has n flip-flops and so it can store any binary information of n bits. In addition, a register may have combinational gates that perform certain data processing tasks. In its broadest definition a register consists of a group of flip-flops and gates that effect their transition. The flip-flops hold the binary information and the gates control when and how new information is transferred into the register. The common clock input triggers all flip-flops on the rising edge of each pulse and the binary data available are transferred. The clear input goes to a special terminal in each flip-flop. When this input goes to 0, all flip-flops are reset synchronously. We use the clear input to clear the register to 0s prior to its clocked operation. The clear input remains at logic 1 during normal clocked operation. Note that the clock signal enables the D input but that the clear input is independent of the clock.

Loading is the transfer of new information into a register. In Parallel Loading all bits of the register are loaded simultaneously with a common clock pulse transition.

Register with Parallel Load

In most digital systems a master clock generator supplies continuous pulses. The clock pulses are applied to all flip-flops and registers in the system. The master clock is used as a pump that supplies a constant beat to all parts of the system. A separate control signal must be used to decide which specific clock pulse will have an effect on a particular register.

Shift Registers

A shift register can shift its binary information in one or both directions. The logical diagram of a shift register consists of a chain of flip-flops in cascade. The output of one flip-flop is connected to the input of the next flip-flop. All flip-flops receive common clock pulses that initiate the shift from one stage to the next.

Bisectional Shift Register with Parallel Load

A unidirectional shift register is capable of shifting its contents in one direction only. A bi-directional shift register can shift in both directions. Some shift registers provide the necessary input and output terminals for parallel transfer. The most general shift register has all the capabilities listed below. Others may have some of these capabilities, with at least one shift operation.