REGISTERS, COUNTERS, and the MEMORY UNIT

• Registers, counters and memories are extensively used in the design of digital systems in general and digital computers in particular. Registers can also be used to facilitate the design of sequential circuits. Counters are useful for generating timing variables to sequence and control operations in a digital system. Memories are essential for storage of programs and data in the digital computer.

Register

• A group of binary storage cells suitable for holding binary information.

• Constitute by a group of flip-flops since each flip-flop is a binary cell capable of storing one bit of information.

• An n-bit register has a group of n flip-flops and is capable of storing any binary information containing n bits.

• May have combinational gates that perform certain data-processing tasks.

• Consists of a group of flip-flops and gates that effect their transition. The flip-flops hold binary information and gates control when and how new information is transferred into the register.

COUNTERS

• Essentially a register that goes through a predetermined sequence of states upon the application of input pulses.

• The gates a connected in such a way as to produce a prescribed sequence of binary states in a register.

• A special type of register, it is common to differentiate them by giving them a special name.

Memory Unit

• A collection of storage cells together with associated circuits needed to transfer information in and out of storage.

• RAM differs from ROM in that RAM can transfer the stored information out (read) and is also capable of receiving new information in for storage (write). A more appropriate name for such a memory would be read-write memory.

REGISTERS

4 bit register

Exercise : Design a 4 bit-register with a parallel load using D flip-flops.

REGISTER WITH PARALLEL LOAD

Exercise: Convert the four-bit register with a parallel load using

a) JKb) T

Sequential Logic Implementation

• Since register are readily available as MSI circuits, it becomes convenient at times to employ a register as part of the sequential circuit.

• Block diagram shows a sequential circuit that uses a register. The present state of the register and the external inputs determine the next state of the register and the values of external output.

Example

• Design the sequential circuit whose state table is listed in table below.

Present State Input Next State Output

A1 A2 x A1 A2 y

0 0 0 0 0 0

0 0 1 0 1 0

0 1 0 0 1 0

0 1 1 0 0 1

1 0 0 1 0 0

1 0 1 0 1 0

1 1 0 1 1 0

1 1 1 0 0 1

Solution

Shift Register

• A register capable of shifting its binary information either to the right or to the left.

• Consists of a chain of flip-flops connected in cascade, with the output of one flip-flop connected to the input of the next flip-flop. All flip-flop receive a common clock pulse which causes the shift from one stage to the next.

Simple Shift Register

• One that uses only flip-flop. The Q output of a given flip-flop is connected to the D input of the flip-flop at its right. Each clock pulse shifts the contents of the register one bit position to the right. The serial input determines what goes into the leftmost flip-flop during the shift. The serial output is taken from the output of the rightmost flip-flop prior to the application of a pulse. Although this shift register shift its contents to the right, if we turn the page upside down, we find that the register shifts its content to the left. Thus a unidirectional shift register can function either as a shift-right or as a shift-left register.

Serial Transfer

• A digital system is said to operate in a serial mode when information is transferred and manipulated one bit at a time. The content of one register is transferred by shifting the bits from one register to the other. The information is transferred one bit at a time by shifting the bits out of the source register into the destination register.

Serial VS Parallel modes of Operation

• In the parallel mode, information is available form all bits of a register and all bits can be transferred simultaneously during one clock pulse.

• In the serial mode, the registers have a single serial input and a single serial output. The information is transferred one bit at a time while the registers shifted in the same direction.

• Computers may operate in a serial mode, a parallel mode, or in a combination of both. Serial operations are slower because of the time it takes to transfer information in and out of shift registers. Serial computers, however, require less hardware to perform operations because one common circuit can be used over and over again to manipulate the bits coming out of shift registers in a sequential manner. The time interval between

Bidirectional Shift Register with Parallel Load