logic family

Chap.10 Digital Integrated Circuits

Content10-1 Introduction10-2 Feature 10-3 Feature of BJT 10-4 RTL and DTL10-5 TTL 10-6 ECL 10-7 MOS 10-8 CMOS10-9 MOS transmission gate

Integration Levels

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• Gate/transistor ratio is roughly 1/10– SSI < 12 gates/chip– MSI < 100 gates/chip– LSI …1K gates/chip– VLSI …10K gates/chip– ULSI …100K gates/chip– GSI …1Meg gates/chip

4

Logic Family DefinitionA circuit configuration or approach used

to produce a type of digital integrated circuit.

Consequence: different logic functions, when fabricated in the form of an IC with the same approach, or in other words belonging to the same logic family, will have identical electrical characteristics.

the set of digital ICs belonging to the same logic family are electrically compatible with each other

10-1 IntroductionIC digital logic families

◦RTL （ Resistor-transistor logic）◦DTL （ Diode-transistor logic）◦TTL （ Transistor -transistor logic）◦ECL （ Emitter-coupled logic）◦MOS（Metal-oxide semiconductor）◦CMOS （ Complementary Metal-

oxide semiconductor）

Moore’s law

6

• A prediction made by Moore (a co-founder of Intel) in 1965: “… a number of transistors to double every 2 years.”

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Common Characteristics of the Same Logic Family

Supply voltage range, speed of response, power dissipation, input and output logic levels, current sourcing and sinking capability, fan-out, noise margin, etc.

Consequence: choosing digital ICs from the same logic family guarantees that these ICs are compatible with respect to each other and that the system as a whole performs the intended logic function.

Positve logic and Negative logic

Positive logic: H is set to be binary 1

Negative logic: L is set to be binary 1

10-2 Feature

The feature to be concerned of IC logic families:◦ fan-out

The no. of standard loads can be connected to the output of the gate without degrading its normal operation

Sometimes the term loading is used◦ Power dissipation

The power needed by the gate Expressed in mW

◦ Propagation delay The average transition-delay time for the signal to

propagate from input to output when the binary signal changes in value

◦ Noise margin The unwanted signals are referred to as noise Noise margin is the maximum noise added to an input

signal of a digital circuit that does not cause an undesirable change in the circuit output

Computing fan-out

),min(IL

OL

IH

OH

I

I

I

IoutFan

William KleitzDigital Electronics with VHDL, Quartus® II Version

Copyright ©2006 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458

All rights reserved.

Computing fan-out ( High-level output)

12

Input and output current specifications

William KleitzDigital Electronics with VHDL, Quartus® II Version

Copyright ©2006 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458

All rights reserved.

Computing fan-out ( Low-level output)

Power dissipation

CCCCD

CCLCCHCC

VavgIavgP

IIavgI

)()(2

)(

?7400)(

?)(

3,1

TTL standardFor

ICinavgPTotal

avgP

mAImAI

D

D

CCLCCH

Propagation delay

50% VH

50% VH

50% VH

50% VH

For standard TTL

?)(

11,7

avgt

nstnst

P

PLHPHL

Other delay times

◦Rise Time from 10% up to 90% level

◦Fall Time from 90% down to 10% level

Noise margin

Noise margin

High-state noise margin=0.4

Low-state noise margin=0.4

In the beginning…

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Diode Logic (DL)• simplest; does not scale• NOT not possible (need

an active element)=

=

Resistor-Transistor Logic (RTL) • replace diode switch

with a transistor switch• can be cascaded• large power draw

was…

20

=

Diode-Transistor Logic (DTL) • essentially diode logic with transistor amplification• reduced power consumption• faster than RTL

DL AND gate Saturating inverter

10-3 Feature of BJT BJT

◦ npn or pnp ◦Si or Ge◦Si is used mainly◦npn is most popular

Table 10-1 Typical npn Transistor Parameters

Region VBE (V) VCE (V) Current Relation

Cutoff < 0.6 Open circuit

IB=IC=0

Active 0.6-0.7 > 0.8 IC =hFEIB

Saturation 0.7-0.8 0.2 IB ≥IC/hFE

Feature of npn-type BJT

HVLV

V

VLh

VHkR

VVkR

ii

o

FE

B

CCC

and for

? Find

2.0,50

5,22

5,1

Diode – symbol and characteristic

10-4 RTL and DTL circuits RTL

◦ Resistor TL◦ L: 0.2V, H: 1~3.6V

DTL◦ Diode TL◦ L: 0.2V, H: 4~5V

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DL Example

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RTL Example

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DTL Example

RTL--NOR

DTL--NAND

Modified DTL

10-5 Transistor-Transistor Logic (TTL)

The original basic TTL gate was a slight improvement over the DTL gate.

There are several TTL subfamilies or series of the TTL technology.

Eight TTL series appear in Table 10-2.Has a number start with 74 and follows

with a suffix that identifies the series type, e.g 7404, 74S86, 74ALS161.

Three different types of output configurations:◦ 1. open-collector output◦ 2. Totem-pole output◦ 3. Three-state (or tristate) output

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TTL Subfamilies

TTL family evolution

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Legacy: don’t use in new designs

Widely used today

Open-collector TTL Gate

Wired-AND of Two Open-Collector

Open-Collector Gates Forming a Common Bus Line

In this case

Y = ?

TTL Gate with Totem-Pole Output

Schottky TTL Gate

Three-state TTL Gate

10-6 Emitter-Coupled Logic (ECL) Nonsaturated digital logic familyPropagation rate as low as 1-2nsUsed mostly in high speed circuitsNoise immunity and power dissipation

is the worst of all logic families. High level -0.8V, Low level -1.8VIncluding

◦ Differential input amplifier◦ Internal temperature and voltage

compensated bias network◦ Emitter-follower outputs

ECL Basic Gate

Graphic Symbols of ECL Gates