Logic GatesChapter 6

Subject : Digital SystemYear : 2009

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Overview

• CMOS and TTL IC• Delay time, Time propagation delay, Fan In, Fan

Out• Speed-Power Product• Programmable Logic

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CMOS and TTL IC• CMOS stands for Complementary Metal-Oxide

Semiconductor and is implemented with a type of field-effect transistor.

• TTL stands for Tansistor – transistor Logic and is implemented with bipolar jucntion transistor

• Keep in Mind that CMOS and TTL differ only in the type of circuit component and value of parameter and not in the basic logic operation

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CMOS and TTL IC

• There is a little disagreement about which circuit technology, CMOS or TTL, is the most widely used. It apperars that CMOS had become the dominant technology and may eventually replace TTL in small and medium scale ICs.Although TTL dominated for many years mainly because it had faster switching speeds and a greater selection of device type, CMOS always had advantage of much lower power dissipation although that parameter is frequency dependent.

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CMOS• The categories of CMOS in term of the DC supply

voltage are the 5 V CMOS, 3.3 V CMOS, the 2.5 CMOS and the 1.8 CMOS.

• Example of the basic CMOS series for 5 V category and their designations include :* 74HC and 74HCT High speed CMOS (the “T” indicates TTL compatibility)* 74AC and 74ACT Advance CMOS* 74ACH and 74 ACHT Advance High Speed CMOS

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TTL

• The basic TTL series and their designations are as follows :* 74 Standart* 74S Schottky TTL* 74 AS Advance Schottky TTL* 74 LS Low Power Schottky TTL* 74 F Fast TTL

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Propagation Delay time

• Propagation Delay time is a result of limitation on switching speed or frequency at which a logic circuit can operate

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Propagation Delay time con’t

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Fan Out• Fan out is the Maximum number of inputs of the

sane series in an IC family that can be connected to a gate’s output and still maintain the output voltage.

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Speed Power Product

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Programmable LogicProgrammable Link Process Technology• Fuse Technology

• Antifuse Technology

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Programmable Logic con’tProgrammable Link Process Technology• EPROM Technology

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Programmable Logic con’t

• SRAM Technology