Sequential Logic Design

Question 1

In general, what do the outputs of a digital system depend on?

Answer 1

The outputs of a digital system depend on the current inputs and the history of the input up to the current time.

Question 2

What is a defining attribute of sequential logic?

Answer 2

Sequential logic has storage (memory)

Question 3

What is the purpose of storage elements in sequential logic?

Answer 3

Storage elements maintain the state of the circuit

Question 4

List typical sequential circuits

Answer 4

• Counters
• Memories
• Shift registers
• Finite State Machines (FSM)

Question 5

What are the main two types of sequential circuits?

Answer 5

• Asynchronous
• Synchronous

Question 6

What is meant by an asychronous sequential circuit?

Answer 6

State transitions are triggered by primary input changes.

When the input changes, the circuit will react to the change without requiring a clock event.

Question 7

How is an asynchronous sequential circuit implemented?

Answer 7

An asychronous circuit require delays, this is implemented using special delay elements or simply relies on propagation delays in the combinational logic elements

Question 8

What is meant by a synchronous circuit?

Answer 8

State transitions are triggered by regular clock events.

We can say that changes in the state of memory elements are synchronised by a clock signal.

Question 9

What is a D-type latch?

Answer 9

A variation of the S-R latch which is provided a delay by the introduction of D.

–EDIT–

Question 10

What are the two models for a FSM?

Answer 10

• Mealy machine
• Moore machine

Question 11

What is a Mealy machine?

Answer 11

A FSM in which the outpu depends on inputs and the current state.

Question 12

What is a Moore machine?

Answer 12

A FSM in which the output depends only on the current state.

Question 13

Describe the layout of a Mealy machine.

Answer 13

• The states are contained in circles.
• The arrows show how the state changes.
• Each state will have arguments that cover all the possible arguments that may occur
• The numbers shown by the arrows represent inputs/outputs (e.g. 1/0 = input 1/output 0)

Question 14

How is a Mealy machine implemented in VHDL?

Answer 14

1. Input and output ports, and their types are defined in entity mealy_machine
2. In the architecture we define the two states as type state_type (state1, state2)
3. In the architecture we define the signals current_state and next_state: state_type
4. We define a sequential part which defines default value and the clock change-state change control
5. Begin architecture - a case statement is used to define what we want to happen in each state

Question 15

Describe the layout of a Moore machine

Answer 15

• The states and their outputs are defined in circles
• e.g. when a/0, b/1 = statea output0, stateb output1
• Arrows show the combinations of inputs but not the output unlike Mealy machine

Question 16

What is an important state that should be added to a CASE statement and why?

Answer 16

It is important to add the state:

when others => null;

This stops the circuit from going into an unknown state.