1. Introduction to Digital Design

2. Text Book Digital Design by - John F. Wakerly
- you will find some solutions at this site.
- Xlinix Web site

3. Digital Design Digital Design is also know as “Logical Design”.
The purpose is to build Digital Systems.

4. Analog versus Digital
Analog systems process time-varying signals that can take on any value across a continuous range of voltages (in electrical/electronics systems).
Digital systems process time-varying signals that can take on only one of two discrete values of voltages (in electrical/electronics systems).
Discrete values are called 1 and 0 (ON and OFF, HIGH and LOW, TRUE and FALSE, etc.)

5. Digital Revolution Digital systems started back in 1940s.
Digital systems cover all areas of life:
still pictures
digital video
digital audio
telephone
traffic lights
Animation

6. Benefits of Digital over Analog
Reproducibility
Not effected by noise means quality
Ease of design
Data protection
Programmable
Speed
Economy

7. Digital Devices
Gates
Flip-Flops
PLDs
FPGAs

8. Gates The most basic digital devices are called gates.
Gates got their name from their function of allowing or blocking (gating) the flow of digital information.
A gate has one or more inputs and produces an output depending on the input(s).
A gate is called a combinational circuit.
Three most important gates are: AND, OR, NOT

9. Digital Logic
Binary system -- 0 & 1, LOW & HIGH, negated and asserted.
Basic building blocks -- AND, OR, NOT

10. AND, OR, NOT Gates

12. Electronic Aspects of Digital Design
How we represent digital information in electronic devices?
By discrete voltages.

13. What is the Basic Digital Element in Electronics ?
a Switch

14. Using Switch to represent digital information

15. Digital Abstraction
It is difficult to make ideal switches means a switch is completely ON or completely OFF.
So, we impose some rules that allow analog behavior to be ignored in most cases, so circuits can be modeled as if they really did process 0s and 1s, known as digital abstraction.
Digital abstraction allows us to associate a noise margin with each logic values (0 and 1).

16. Real Switches to represent digital information5v 5v
1k
Output
Output 5v
4.5v
10k

17. Logic levels Undefined region is inherent digital, not analog
amplification, weak => strong
Switching threshold varies with voltage, temp, process, phase of the moon
need “noise margin”
The more you push the technology, the more “analog” it becomes.
Logic voltage levels decreasing with new processors.
5 -> 3.3 -> 2.5 -> 1.8 V

18. MOS Transistors
Voltage-controlled resistance
PMOS
NMOS

19. CMOS Inverter

20. Switch model

21. Alternate transistor symbols

22. CMOS Gate Characteristics
No DC current flow into MOS gate terminal
However gate has capacitance ==> current required for switching (CV2f power)
No current in output structure, except during switching
Both transistors partially on
Power consumption related to frequency
Slow input-signal rise times ==> more power
Symmetric output structure ==> equally strong drive in LOW and HIGH states

23. Flip-flops A device that stores either a 0 or 1.
Stored value can be changed only at certain times determined by a clock input.
New value depend on the current state and it’s control inputs
A digital circuit that contains filp-flops is called a sequential circuit

24. Flip-flops
S-R latch symbols
D flip-flop
J-K flip-flops

25. Integrated Circuits
A collection of one or more gates fabricated on a single silicon chip is called an integrated circuit (IC).
ICs were classified by size:
SSI - small scale integration - 1~20 gates
MSI - medium scale integration - 20~200 gates
LSI - large scale integration - 200~200,000 gates
VLSI - very large scale integration - over 1M transistors
Pentium-III - 40 million transistors

26. DIP Packages

27. Gates in ICs

28. Programmable Logic Devices
PLDs allow the function to be programmed into them after they are manufactured.
Complex PLDs (CPLD) are a collection of PLDs on the same chip.
Another programmable logic chip is FPGA - field-programmable gate arrays.

29. CPLDs and FPGAs
CPLD
FPGA

30. Application Specific Ics (ASICs)
Chips designed for a particular application are called semicustom ICs or application-specific ICs (ASICs).
ASICs generally reduce the total component and manufacturing cost of a product by reducing chip count, physical size, and power consumption, and they often provide higher performance.
But costly if not produced in bulk.

31. Printed-Circuit Boards
An IC is normally mounted on a printed-circuit board (PCB) that connects it to other ICs in a system.
Individual wire connection or traces can be as narrow as 4 mils with 4 mils spacing (one-thousandth of an inch)
Now a days, most of the components use surface mount technology.
They are normally layered.

32. Software Aspects of Digital Design
Today software tools are an essential part of digital design.
Software tools improve productivity, correctness and quality of designs
Software tools are:
Schematic entry
HDL (Hardware Description Language) Editors
Simulators - to verify the behaviour of the design
Synthesis tools - circuit design
Timing analyzers and verifiers

33. Digital Design Levels
the lowest level of design is device physics and IC manufacturing processes.
design at the transistor level
level of functional building blocks
level of logic design using HDLs
computer design and overall system design.

34. Different Design Levels
Consider a simple design example:
Build a multiplexer with two data inputs A and B, a control input S, and an output Z.
Switch model for the example multiplexer

35. Designing at the transistor level
Transistor-level circuit diagrams
Gate symbols (for simple elements)

36. Logic design using Truth tables

37. Logic design using boolean algebra
Equations: Z = S¢ × A + S × B
Logic diagrams

38. Prepackaged building blocks, e.g. multiplexer

39. Various hardware description languages
ABEL
VHDL
We’ll start with gates and work our way up

40. Name of the program module
the type of PLD
pin numbers
ABEL statement to achieve the multiplexer
Standard library
and a set of definitions
Inputs and outputs
functions behaviour

41. Structural VHDL program for the multiplexer

42. Summery Design to minimize cost.
Rule of thumb is to minimize the number of ICs.
Though PLDs costs more but uses less PCB area.
Unless mass production avoid ASIC design.
Design to solve real life problems.