Digital Inegrated Circuits

8/6/2019 Digital Inegrated Circuits

1/22


2/22

IC 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 MetalOxide Semiconductor)


3/22

Characteristics of an

Ideal Logic Family The ideal logic family should have or be:

Low power

High speed Easy to use

Many different logic functions

Clear voltage levels for 0 (LOW) and 1 (HIGH)


4/22

Feature fan-out

The no. of standard loads can be connected

to the output of the gate without degradingits normal operation

Sometimes the term loading is used

Power dissipation

The power needed by the gate Expressed in mW


5/22


6/22

Computing fan-out

),min(IL

OL

IH

OH

I

I

I

IoutFan !


7/22

Power dissipation

CCCCD

CCLCCHCC

VavgIavgP

IIavgI

v!

!

)()(

2)(

?7400)(

?)(

3,1

TTLstandardFor

!

!

!!

ICinavgPTotal

avgP

mAImAI

D

D

CCLCCH


8/22

Propagation delay50% VH

50% VH

50% VH

50% VH

?)(

11,7

!

!!

avgt

nstnst

P

PLHPHL


9/22

Noise margin


10/22

Noise margin (Contd)


11/22

The Ideal Switching Action

of the BJT Consider a bipolar transistor in logic circuits

It is operated in either two states

produces the two logic levelsFully conducting state - saturated/turned on

or

Fully non-conducting state -cut-off state


12/22

Typical npn Transistor

ParametersRegion VBE (V) VCE (V) Current

Relation

Cutoff < 0.6 Opencircuit IB=IC=0

Active 0.6-0.7 > 0.8 IC =hFEIB

Saturation 0.7-0.8 0.2 IB IC/hFE


13/22

Transistor-Transistor Logic

(TTL) TTL was developed in 1965 The original basic TTL gate was a slight improvement over the

DTL gate. There are several TTL subfamilies or series of the TTL

technology. For example

Standard TTL 74 High Speed TTL 74H (twice as fast, twice as much

power)

Low Power TTL -74L (1/10 the speed, 1/10 the powerof standard" TTL)

Schottky TTL 74S(for high-frequency uses ) Low-power Schottky TTL 74LS Advanced Schottky TTL 74ALS


14/22

Part Numbers Part numbers for 7400 TTL series logic devices

often use the following naming convention

SN 74 ALS 245 A

manufacturer temperature range logic subfamily deviceLS - Low Power Schottky

AS - Advanced Schottky

ALS - Advanced Low

Power Schottky

SN = Texas Instruments

DM = Fairchild

74 - commercial

54 military (extended)

hundreds of different

devices in each family

package type, quality

grade, etc. (varies widely

by manufacturer)


15/22


16/22

Open-collector TTL NAND

Gate


17/22

Open Collector Outputs

(Cont.) An open-collector output can present a logic LO

output Since there is no internal path from the output Y

to the supply voltage VCC , the circuit cannotpresent a logic HIGH on its own To function properly an external pull-up resistor,

Rp is being used as shown

Use this symbolto Indicates opencollector output


18/22

Advantages of Open

Collector Outputs Why should we use open-collector

gates which require the addition of a

pull-up resistor in order to functionproperly when we could use a gatewith a totem-pole output instead?


19/22

1. Wired-ANDingOpen-collector outputs can be tieddirectly together which results in the

logical ANDing of the outputs. Thusthe equivalent of an AND gate can beformed by simply connecting theoutputs


20/22


21/22

Advantages (cont)2. Increased current levels - Standard TTL

gates with totem-pole outputs can onlyprovide a HIGH current output of 0.4 mA and

a LOW current of 1.6 mA. Many open-collector gates have increased currentratings

3. Different voltage levels - variety of outputHIGH voltages can be achieved. This is useful in

interfacing different logic families that havedifferent voltage and current levelrequirements


22/22

Disadvantages : The big disadvantage of open-

collector gates is their slow

switching speed. This is because thevalue of pull-up resistor is in k;,which results in a relatively long timeconstants

Documents

Digital Inegrated Circuits