REGISTERS AND COUNTERS

Question 1

What are the classifications according to function of circuits that include flipflops

Answer 1

Registers
Counters

Question 2

Describe registers and give an example

Answer 2

A register is a group of binary cells.
An n ‐bit register consists of a group of n flip‐flops sharing a common clock and capable of storing n bits of binary information.
 Internal registers used in microprocessors to manipulate data

Question 3

Describe counters and give an example

Answer 3

Counters: A register that goes through a predetermined sequence of binary states.
 Counters are a special type of register
 The gates in the counter are connected in such a way as to produce the prescribed sequence of states.
 PC (program counter) in microprocessors

Question 4

What is the purpose of the flip-flops and gates in a register

Answer 4

A register consists a group of flip-flops and gates that
effect their transition.
 The flip-flops hold the binary information.
 The gates determine how the information is transferred into the register.

Question 5

What are registers used for

Answer 5

 simple data storage
 data movement
 processing operations

Question 6

Illustrate an 8-bit register

Answer 6

*See page 4 and 5

Question 7

Do the questions about 2-bit Register on page 6

Answer 7

**

Question 8

Describe and illustrate the making of a 4-bit register

Answer 8

The register here is constructed with four
D-type flip-flops.
*See page 7

Question 9

Explain the use of clear and clock in the 4-bit register

Answer 9

 “Clock” triggers all flip-flops on the positive edge of each pulse.

 “Clear” is useful for clearing the register to all 0’s prior to its clocked operation.
 The R inputs must be maintained at logic 1 (i.e., de-asserted) during normal clocked operation.

Question 10

Is it possible to use the diagram/state table model to design registers

Answer 10

Due to the large numbers of states and input combinations as n becomes large, the state diagram/state table model is not feasible!

Question 11

What are methods we can use to design registers?

Answer 11

 Add predefined combinational circuits to registers
 Example: To count up, connect the register flip-flops to an incrementer

 Design individual cells using the state diagram/state table model and combine them into a register
 A 1-bit cell has just two states
 Output is usually the state variable

Question 12

Discuss register storage in terms of expectations, the reality and possible ways to realize the expectations

Answer 12

 Expectations:
 A register can store information for multiple clock cycles
 To “store” or “load” information should be controlled by a signal

 Reality:
 A D flip-flop register loads information on every clock cycle

 Realizing expectations:
 Use a signal to block the clock to the register,
 Use a signal to control feedback of the output of the register back to its inputs, or
 Use other SR or JK flip-flops, that for (0,0) applied, store their state

Question 13

What is load in register storage

Answer 13

 Load is a frequent name for the signal that controls register storage and loading
 Load = 1: Load the values on the data inputs
 Load = 0: Store the values in the register

Question 14

Describe and illustrate Registers with Clock Gating and the logic needed for it

Answer 14

The Load signal enables the clock signal to pass through if 1 and prevents the clock signal from passing through if 0.

Logic:
Gated Clock = Clock + Load

*See page 10 for illustrations

Question 15

Describe Registers with Load-Controlled Feedback

Answer 15

 A clock edge applied to the C inputs of the register of will load all inputs in parallel.

 For synchronism, it is advisable to control the operation of the register with the D inputs rather than controlling the clock in the C inputs of the flip-flops.

 Loading a register: transfer new information into the register. Requires a load control input.

 Parallel loading: all bits are loaded simultaneously.

Question 16

What is a more reliable way to selectively load a register and give an example using a 2-bit register

Answer 16

 Run the clock continuously, and
 Selectively use a load control to change the register contents.

Example
 For Load = 0, loads register contents (hold current values)
 For Load = 1, loads input values (load new values)

 Hardware more complex than clock gating, but
free of timing problems

*See page 12 for illustration

Question 17

Describe and illustrate a 4-bit Register with Load Control:

Answer 17

 When the load input is 1, the data in the four inputs are transferred into the register with next positive
edge of the clock.

 When the load input is 0 ,the outputs of the flip-flops are connected to their respective inputs.

 The feedback connection from output to input is
necessary because the D flip-flops does not have a “no change” condition.

*See page 13

Question 18

Do the example on page 13 on Using Registers to Implement Sequential Circuits

Answer 18

*

Question 19

What are the different data formats of registers

Answer 19

 Serial
 Parallel

Question 20

Describe the classifications of registers

Answer 20

Input/Output
 SISO – Serial Input Serial Output
 SIPO – Serial Input Parallel Output
 PISO – Parallel Input Serial Output
 PIPO – Parallel Input Parallel Output

Application
 Shift Registers
 Storage Registers (e.g. PIPO)

Question 21

Describe shift registers

Answer 21

 Shift register is register capable of shifting its binary information in one or both direction
 Capable of shifting the binary information held in each cell to its neighboring cell, in a selected direction
 Consists of a chain of flip‐flops in cascade, with the output of one flip‐flop connected to the input of the next flip‐flop

Question 22

Illustrate and describe how shifting happens in shift registers

Answer 22

 All flip-flops receive common clock pulses, which activate the shift from one stage to the next.
 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.

*See page 17

Question 23

Illustrate the 7 basic data movements in shift registers

Answer 23

*See page 18

Question 24

Complete the table on page 19

Answer 24

*See page 19

Question 25

Describe and illustrate serial transfer

Answer 25

Information is transferred and manipulated one bit at a time.

*See page 20

Question 26

How is information loss prevented in serial transfer

Answer 26

To prevent the loss of information stored in the source register, the information in register A is made to circulate by connecting the serial output to its serial input.

Question 27

What is the function of the shift control input in serial transfer

Answer 27

The shift control input determines when and how many times the registers are shifted. This is done with an AND gate that allows clock pulses to pass into the CLK terminals only when the shift control is active.

Question 28

Perform the serial transfer from A(1011) to B(0010)

Answer 28

*See page 21

Question 29

Do the example on page 22

Answer 29

**

Question 30

Compare parallel and serial operations in digital computers

Answer 30

Operations in digital computer are usually done in parallel because this is a faster mode of operation.
 Serial operations are slower, but have the advantage of requiring less equipment.

Question 31

Describe serial addition

Answer 31

 The two binary numbers to be added serially are stored in two shift registers.

 Bits are added one pair at a time through a single full adder.