chapter 7 - ic logic family

8/3/2019 Chapter 7 - IC Logic Family

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BEE1213 Digital ElectronicsINTEGRATED CIRCUIT

LOGIC FAMILY

CHAPTER 7


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ICs have made digital systems more reliable byreducing the number of external interconnection fromone device to another.

ICs have reduced the amount of electrical powerneeded to perform a given function.

IC cannot handle very large currents or voltagesbecause the heat generated in such small spaceswould cause temperature to rise beyond acceptablelimits

ICs are principally used to perform low-power circuitoperations that are commonly called informationprocessing.

Introduction


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Introduction

Rapidly growth from SSI, with fewerthan 12 gates per chip; through MSI,

with 12 to 99 equivalent gates perchip

Others LSI, VLSI, ULSI and GSI

There are some things IC cannot dowhen deal with very large current


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Digital IC Terminology

VIH (min) High level input voltage. Theminimum level required for a logical 1 at aninput. Any voltage below this level will notbe accepted as a HIGH by the logic circuit

VIL (max) The maximum input voltage forlogic zero

VOH (min) The minimum voltage level at alogic circuit output in the logic 1 state underdefined load conditions


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Digital IC Terminology(cont.)

VOL (max) Low level output voltage. Themaximum voltage level at a logic circuitoutput in the logical 0 state under definedload conditions

IIH High level input current. The currentthat flows into an input when a specifiedhigh level voltage is applied to that input

IIL Low level input current. The current thatflows into an input when a specified lowlevel voltage is applied to that input


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Digital IC Terminology(cont.)

IOH High level output current

IOL Low level output current


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Exercise

Describe the input and output logic for IC7442


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Fan out

Also known as loading factor

Defined as the maximum number of

logic inputs that an output can drivereliably

A logic circuit that specify to have 10

fan out can drive 10 logic inputs


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Propagation delay

Two types of propagation delay tPLH , delay time ingoing from logical 0 to 1; tPHL delay from 1 to 0


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Noise Immunity

Stray electric and magnetic fields can induce voltages

on the connecting wires between logic circuits thisunwanted signal called noise

These cause the input signal to a logic circuit dropbelow VIH (min) or rise above VIL (max)

Noise immunity refers to the circuits ability to tolerate

noise without causing spurious changes in the output

voltage


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Noise Immunity (cont.)

Figure 8-4(a) is a diagram showingthe range of voltages that can occur at

a logic circuit output. Any voltage greater than VOH(min)

considered a logic 1

Voltages lower than VOL (max) logic0


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The high state noise margin VNH isdefined as

VNH = VOH (min) VIH (min) The low state noise margin VNL is

defined as

VNL = VIL (max) VOL (max)


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Example 8-1

The I/O voltage specification for the standard TTL family are listedin the following table. Use these values to determine the following

The maximum amplitude noise spike that can be tolerated when aHIGH output is driving an input

The maximum amplitude noise spike that can be tolerated when aLOW output is driving an input

Parameter Min (V) Typical (V) Max (V)

VOH

2.4 3.4

VOL 0.2 0.4

VIH 2.0

VIL 0.8


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Power Requirements

Every IC need a certain powerrequirement to operate

This power supply is come from thevoltage supply that connected to thepin on the chip labeled VCC(TTL) orVDD(MOS)

The amount of power require by ICs isdetermined by the current that itdraws from the VCC

The actual power is ICCxVCC


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Cont.

ICC(avg) = (ICCH + ICCL)/2

PD(avg) = ICC(avg)Xvcc


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The TTL Logic Family

FIGURE 8-7 (a) Basic TTL NAND gate; (b) diode equivalent for Q1.


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TTL NOR gate


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TTL Data sheet

In 1964, Texas Instruments corporation introduced the first

line of standard TTL ICs

The 54/74 series, most widely used IC logic families

The difference between 54 and 74 series is a range oftemperatures

IC number is the same with all series produce by different

manufactures

Each manufacturer however usually used the prefix that

represent the special words Texas Instrument uses theprefix SN, National semiconductor uses DM etc


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Supply Voltage andTemperature Range

74ALS series and the 54ALS series usenominal supply voltage (VCC) of 5V, but cantolerate a supply variation of 4.5 to 5.5V.

74ALS series can operate properly inambient temperatures ranging from 0 to 70degrees C, while the 54ALS series canhandle -55 to +124 degree C.


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Voltage Levels

Input and output voltage levels can befound on the data sheet.

The min and max values shown arefor worst case conditions of powersupply, temperature and loadingconditions


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TTL Series Characteristics

We have found the type of ICs 74,74LS, 74ALS before

LS low power Schottky, ALSadvance low power Schottky

The function is same, but the

difference is on the characteristic


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TTL Data Sheet


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TTL Series Characteristics

74 series of TTL offers a wide variety ofgates and flip flops

Consist of:

a. Standard TTL, 74 series no longer beuse

b. Schottky TTL, 74S series

c. Low-power Schottky TTL, 74LS Series

d. 74AS

e. 74ALS and 74F


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Schottky TTL, 74S Series

7400 series operates using saturated switchingin which many of the transistors, whenconducting will be in saturated condition

This can causes a storage time delay ts whenthe transistors switch from ON to OFF andeffect the speed

74S series come to solve the speed problem

It accomplishes this by using a Schottky barrierdiode (SBD)


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Schottky TTL, 74S Series(cont.)


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Advances Schottky TTL, 74ASseries

74AS give moreadvance on speedswitching of TTL

ICs at much lowerconsumption

The comparison isshown in the

following table for aNAND gate in eachseries

74S 74A

SPropagationdelay

Powerdissipation

Speed-powerproduct

3ns

20mW

60

pJ

1.7ns

8mW

13.6pJ


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Advanced Low Power SchottkyTTL, 74ALS Series

Improved on bothspeed and powerdissipation

74S 74AS

Propagationdelay

Powerdissipation

Speed-powerproduct

9.5ns

2mW

19pJ

4 ns

1.2mW

4.8 pJ


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Example

Use table 8-6 to calculate the dc noise marginsfor a typical 74LS IC. How does this comparewith the standard TTL noise margins ?

Solution 74LS

VNH = VOH(min) VIH(min)

= 2.7 2.0= 0.7 V


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Example 2

Use table 8-6 to calculate the dc noisemargin for all types of 74


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TTL Loading and Fan Out


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Determining the Fan Out

How many inputs an IC output candrive

We need to know capability of theoutput

Example How many 74ALS00NAND gate inputs can be driven by a74ALS00 NAND gate output


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Example 8-6

Refer to the data sheet on the TI CDROM and determine how many

74AS20 NAND gates can be driven bythe output of another 74AS20

chapter 7 - ic logic family

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