Digital Circuit Implementation

Wafers and Chips

Integrated circuit (IC) chips are manufactured on silicon wafers

Transistors are placed on the wafers through a chemical etching process

Each wafer is cut into chips (dies)which are then packaged individually

Chip Manufacturing Process

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IC Cost

Cost of an integrated circuit (IC) can be modeled with three equations

Cost per die = (cost per wafer) / ((dies per wafer) * yield)

Yield = fraction of dies on a wafer that pass testing

Dies per wafer (wafer area) / (die area) Ignores border of the circular wafer that cannot accommodate

a rectangular die

Yield = 1 / (1 + (defects per area * die area) / 2)2

Based on many years of empirical observations

Basic IC Chip Types

Logic circuits may be implemented … on single chip, or using many chips interconnected on a printed circuit board

(PCB)

Main types of IC chips are: Standard chips Programmable Logic Devices (PLD) Custom chips

Standard Chips

Small number of transistors (< 100)

Simple and fixed functions

Logic designer must decide how to interconnect multiple chips for desired function

Agreed upon / standard functionality

Popular in the 1980s – too large in physical size for much industry use now (good for teaching though!)

7400 Series TTL Logic Chips

The 7400 NAND Chip: pin layout

Vcc = +5V

Gnd 7400

1

2

3

4

5

6

7

14

13

12

11

10

9

8

001

4

9

12

2

5

10

13

3

6

8

11

The equivalent logic layout

7400 Series Implementation

Implementing f = x1x2 + x2'x3 using 7400 series ICs

V DD

x 1 x 2 x 3

f

7404

7408 7432

Why TTL is Only Used For Small Systems

PLDs

Programmable chips – functionality determined by the designer Can even be reprogrammed

Can handle more complex functions than standard chips (approx 100 million transistors per PLD) FPGA: Field Programmable Gate Arrays CPLD: Complex Programmable Logic Devices PAL: Programmable Array Logic PLA: Programmable Logic Arrays

These are used very extensively in industry

Custom Chips

Programmable chips have two major drawbacks: Consume space due to large number of switches for

programmability Slow speed also limited by excessive switches

(resistance/capacitance)

Custom chips Logic designer builds a custom chip Manufactured by a special fabrication facility ($$$!)

ASIC: Application Specific Integrated Circuit Fast, small Expensive! And takes time to build and manufacture

Digital Design Process

Design Loop for Digital Hardware

The basic design loop:

Initial design takes creativity and experience

CAD tools are used for simulation and to work out details

Design concept

Successful design

Initial design

Simulation

Design correct?

Redesign

No

Yes

The Entire Development Process

Design is only one part

Verification and testingare also important – thisis called design verification

Errors may not be uncovereduntil after the prototype is made

Errors may not be uncovereduntil after “release”! Pentium bug

Required product

Design specifications

Initial design

Simulation

Design correct?

Redesign

Implement prototype

Testing

Meets specs?

Finished product

Minor errors?

Make corrections

No

Yes

No

Yes

Yes

No

Simulation Phase

Functional simulation Test the circuit to determine if it correctly performs all the

functions that are required

Timing simulation Test the circuit to determine if it meets the timing

requirements Correct functionality does not necessarily lead to fast speed

The physical design / layout will affect the timing Inherent gate delays Physical wiring leaves metal traces that have resistance

CAD Tools

There are a number of commonly used industry standard CAD tools CAD = Computer Aided Design Altera, Cadence, Mentor Graphics, Synopsys, Synplicity,

Xilinx We have Altera products

Tools are used for multiple purposes Synthesis, timing simulation, functional simulation, layout Can even download the design onto a PCB

CAD Tools vs Theory

Why learn any theory if the CAD tools do the work?

Initial design must be provided by the designer Quality of final design is a function of the quality of the initial

design

Tools implement the theory Designers need to understand how the tools work in order to

be effective in using them

Tools have many options Knowing which to select requires knowing what they do and

how they do it