DIGITAL LOGIC FAMILIES

Walter Joseph,

Asst. Professor,

DEC, RSET.

DIGITAL IC SPECIFICATIONS

Threshold Voltage

Propagation Delay

Power Dissipation

Fan-in Fan-in

Fan-out (loading factor)

Voltage & Current parameters

Noise Margin

Operating Temperature

Speed Power Product

• Propagation Delay:- time required for a pulse to propagate from

input to output.

• Fan in:- no: of inputs that the gate is designed to handle

• Fan out:- no; of std. loads that the output of a gate can drive

without impairing its normal operation.

• Noise margin:- Noise immunity is the ability of a ckt to tolerate • Noise margin:- Noise immunity is the ability of a ckt to tolerate

noise voltages at its inputs. Quantitative measure of noise

immunity is Noise margin.

• Speed Power Product:- for measuring the overall performance of

an IC family. (propagation delay * gate power dissipation)(figure

of merit of an IC family)

Propagation delay

where

tpLH = signal delay time when o/p goes from logic 0

to logic 1

tpHL = signal delay time when o/p goes from logic 1

to logic 0.

Power Dissipation

( )*

cc avg

D cc

IP V

n=

where,

VCC = gate supply voltage

ICC(avg) = avg ct. drawn from supply by the

entire IC

n = no: of gates in IC

( )2

CCH CCLcc avg

n

I II

+=

n = no: of gates in IC

ICCH = ct. drawn by the IC when all the

gates are in HIGH state

ICCL = ct. drawn by the IC when all the

gates are in LOW state

Fan out (loading factor)Figure 1: Current sourcing in HIGH stateFigure 1: Current sourcing in HIGH stateFigure 1: Current sourcing in HIGH stateFigure 1: Current sourcing in HIGH state

IH

OH(max) I

High state fan out =I

Figure 1Figure 1Figure 1Figure 1

Figure 2Figure 2Figure 2Figure 2

Figure 2: Current sinking in LOW stateFigure 2: Current sinking in LOW stateFigure 2: Current sinking in LOW stateFigure 2: Current sinking in LOW state

IL

OL(max) I

LOW state fan out =I

Voltage & current parameters

• VIH(min) : min vge level required at i/p of a gate for that i/p to be

treated as logic 1.

• VOH(min) :

• VIL(max) : max vge level that can be treated as logic 0 at the i/p of a

gate.

• VOL(max) :

• IIH : ct. that flows into an i/p when a specified HIGH level vge is

applied to that input.

• IIL :

• IOH : ct. that flows from an o/p in a logic 1 state under speified

load conditions.

• IOL :

Noise Margin (VN)

VNH= VOH(min) – VIH(min)

VNL= VIL(max) – VOL(max)

LOGIC FAMILIES

• RTL

• DCTL

• DTL

• HTL

(Resistor-Transistor logic)

(Direct Coupled Transistor logic)

(Diode-Transistor logic)

(High Threshold logic)

Obsolete

• TTL - suitable for SSI & MSI.

• ECL - used in superfast computers.

• IIL - suitable for VLSI & ULSI.

• MOS - suitable for LSI.

• CMOS - suitable for SSI, MSI & LSI.

(Transistor-Transistor logic)

(Emitter Coupled logic)

(Integrated Injection logic)

Bipolar Logic families

• Saturated Logic - RTL,DCTL,DTL,HTL,TTL,IIL

• Non saturated Logic - Schottky TTL,ECL

MOS familiesMOS families

• PMOS- P channel MOSFETs

• NMOS- N channel MOSFETs

• CMOS- Complementary MOSFETs

Comparison of Logic families

Logic

Family

Propagation

delay time

(ns)

Power

dissipation

per gate

(mW)

Noise

margin

(V)

Fan-in Fan-out Cost

TTL 9 10 0.4 8 10 Low

ECL 1 50 0.25 5 10 HighECL 1 50 0.25 5 10 High

MOS 50 0.1 1.5 - 10 Low

CMOS < 50 0.01 5 10 50 Low

IIL 1 0.1 0.35 5 8 Very Low

TTL Uses transistors in saturation mode.

Widely used bipolar family.

Fastest of the saturated logic families.

Basic TTL logic ckt is NAND gate.

Sub-families Standard TTL

High Speed TTL

•TTL with passive pull-up

•TTL with totempole output

•TTL with open collector High Speed TTL

Low Power TTL

Schottky TTL

Advanced Schottky TTL

Low Power Schottky TTL

Advanced Low Power Schottky TTL

Fast TTL.

•TTL with open collector

• Tristate TTL

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 levels

Fully conducting state - saturated/turned on (or)

Fully non-conducting state - cut-off state

Part Numbers

Two input TTL NAND gateTotempole

arrangement

Diode : Ensures TC

and TD do not conduct

simultaneously.

Figure 1: A 2Figure 1: A 2Figure 1: A 2Figure 1: A 2----input TTL NAND Gate with a Totem Pole Output Stageinput TTL NAND Gate with a Totem Pole Output Stageinput TTL NAND Gate with a Totem Pole Output Stageinput TTL NAND Gate with a Totem Pole Output Stage

Input StagePhase Splitter

Stage

Output Stage

NAND Gate Circuit Redrawn with at least One Input LOWNAND Gate Circuit Redrawn with at least One Input LOWNAND Gate Circuit Redrawn with at least One Input LOWNAND Gate Circuit Redrawn with at least One Input LOW

Operation of TTL NAND gate

Inputs Transistor TA

Transistors

TB & TD

Transistor

TCOutput

1 2Emitter

junction 1

Emitter

junction 2

Logic 0 Logic 0Forward

Biased

Forward

BiasedCut off Saturation Logic 1

Logic 0 Logic 1Forward

Biased

Reverse

BiasedCut off Saturation Logic 1

Logic 1 Logic 0Reverse

Biased

Forward

BiasedCut off Saturation Logic 1

Logic 1 Logic 1Reverse

Biased

Reverse

BiasedSaturation Cut off Logic 0

Disadvantages of Totempole

• Inclusion of TC and D keeps the circuit power dissipation

low.

• In the o/p HIGH state, TC acts as emitter follower with its

associated low impedence. Small time constant for

charging up any capacitive load on the o/p. This action is

called active pull-up & provides very fast rise time

waveforms at the o/p.

Advantages of Totempole

Disadvantages of Totempole• During 0 to 1 transition at o/p, TD turns OFF more slowly

than TC turns ON and relatively large currents will be drawn

from the supply.(as both are on for a few ns). So TTL ckts

suffer from current transients/spikes because of totempole

connection.

• Totempole o/ps cannot be wired ANDed.

Wired AND concept

The Destruction Effect if Totem Pole Outputs

are Tied Together(Wired-AND)

Totem pole outputs tied

together can produce

harmful current through

TC1 and TD2ON OFF

ONOFF

TTL with open collector

An open-collector output can present a logic LOW output.

Since there is no internal path from the output Y to the supply

voltage VCC , the circuit cannot present a logic HIGH on its own.

To function properly an external pull-up resistor, R is being

used as shown.

TTL with open collector (cont.)

Use this symbol toUse this symbol toUse this symbol toUse this symbol to

Indicates openIndicates openIndicates openIndicates open

collector outputcollector outputcollector outputcollector output

Symbol : Underlined diamond

Open collector NAND with external resistor.

Advantages of Open Collector Outputs

1. Wired-ANDing - Open-collector outputs can be tied directly

together which results in the logical ANDing of the outputs. Thus

the equivalent of an AND gate can be formed by simply

connecting the outputs.

2. Increased current levels - Standard TTL gates with totem-pole outputs

can only provide a HIGH current output of 0.4 mA and a LOW current

of 1.6 mA. Many open-collector gates have increased current ratings

3. Different voltage levels - A wide variety of output HIGH voltages can

be achieved using open-collector gates. This is useful in interfacing

different logic families that have different voltage and current level

requirements

Advantages of Open Collector Outputs (cont.)

requirements

The big disadvantage of open-collector gates is their slow switching

speed. This is because the value of pull-up resistor is in kΩ, which

results in a relatively long time constants.

Comparison of Totem Pole and Open Collector

Output

The major advantage of using a totem-pole connection is that it

offers low-output impedance in both the HIGH and LOW output

states.

Tri-state TTL

combines the advantages of the totem-pole and

open collector circuits

Three output states are HIGH, LOW, and high

impedance (Hi-Z).

IN-data input ; EN- enable input for control.

For EN = 0, regardless of the value on IN (denoted For EN = 0, regardless of the value on IN (denoted

by X), the output value is Hi-Z.

For EN = 1, the output value follows the input value

Variations:Data input, IN, can be inverted

Control input, EN, can be inverted by addition of "bubbles" to signals.

Hi-Impedance Outputs

Tristate gate utilize the high-speed

operation of the totem-pole arrangement

when input enabled.

Permit outputs to be connected together.

What is a Hi-Z value?

Both transistor are turned off in the totempole Both transistor are turned off in the totempole

arrangement.

This means that, looking back into the

circuit, the output appears to be disconnected

(open circuit).

An equivalent circuit for the

tristate output in the high-Z state

Tri-state Inverter

Standard TTL Transistor Switching

Problem

• Transistors are driven into deep saturation to fully conduct, or cut

off to switch off.

• The result of deep saturation is that the two junctions are now

forward biased.

• The forward biasing of the BC junction forces a large number of • The forward biasing of the BC junction forces a large number of

minority carriers to the collector region.

• When the transistor switches off, these minority carriers needs to

be removed. This takes a finite amount of time called the storage

time (major component of the propagation delay) and thus

increases the switch off time.

31

Solution • Prevent the transistor from going deep in saturation. This

accomplished by preventing the BC junction from becoming

forward biased.

• The Schottky diode is used to do the above by placing it across

the BC junction. Because of its lower barrier potential, it will

conduct current from the base directly to the collector before the

BC is forward biased. Thus less carriers are stored in the BC is forward biased. Thus less carriers are stored in the

collector area and the switching becomes much faster.

32

33

Normal Transistor in saturation Transistor with Schottky diode in saturation

Schottky TTL (74 series)

• Transistors never go to full saturation and thus

increases speed.

• Operates in active or cut off region alone.

• Accomplished by using Schottky barrier Diode(SBD)

b/w base and collector.

• So the collector junction cannot get forward biased.• So the collector junction cannot get forward biased.

• Forward voltage : 0.25V

• 54S/74S series have the highest speed among TTL

gates.

Darlington pair

ECL

• Current-Mode Logic (CML)/ Current-Steering Logic(CSL)

• Operates on the principle of current switching.

• Fastest of all logic families. (tp=1ns)

– Non saturated digital logic family

– Eliminates turn off delay of saturated transistors by operating in active

mode.mode.

– Currents are kept high, o/p impedence is low. So ckt and stray

capacitances can be quickly charged and discharged.

– Has limited voltage swing.

• Consists of difference amplifiers and emitter followers.

• Emitter terminals of 2 transistors are tied together & hence

called ECL.

• Logic LOW : -1.7V ; Logic HIGH : -0.9V

ECL InverterDifferential

Amplifier

ECL OR/NOR

Drawbacks of ECL• High cost

• Low noise margin

• High power dissipation

• Its –ive supply vge and logic levels are not compatible with other

logic families.

• Problem of cooling

Imp. characteristics of ECLImp. characteristics of ECL

• Transistor never saturate. So high speed

• Logic levels are –ive

• Noise margin is less

• Used as inverter/buffer

• Large fan out (25)

• Large power dissipation

• Total current flow is constant. So no noise spikes will be internally

generated.

MOS Families vs Bipolar Families• Simpler and inexpensive to fabricate

• Require much less power (PD=0.1mW)

• Better noise margin (1.5V for +5V supply)

• Greater supply voltage range

• Higher fan out (50 for freq > 100Hz & unlimited for low freq)

• Require much less chip area• Require much less chip area

• Higher reliability

• Slower in operating speed (tp=50ns)

• Susceptible to static charge discharge damage

MOSFET

Deletion Type

Enhancement Type

NMOS NMOS

PMOS

EQUIVALENTS OF PMOS & NMOS

NMOS INVERTER

Q1 : Load MOSFET (resistor) (Enhancement/Depletion)

Q2 : Switching MOSFET (Enhancement type only)

NMOS NAND

NMOS NORNMOS NOR

CMOS• p channel and n channel MOS devices are fabricated on the same

chip.

• Faster

• Consumes less power (suited for battery operated systems)

• Operated at higher voltages(better noise immunity)

• Very high i/p &(o/p) resistance. so draws almost zero ct. from the

driving gate.

• Very high fan-out• Very high fan-out

• Noise margin is same for both states. (30% of VDD)

• Increase in VDD results in increase in PD.

• Increased complexity

• Lower packing density

CMOS INVERTER

CMOS NAND

CMOS NOR

Self study

• Subfamilies of TTL & CMOS

References

• Fundamentals of Digital Circuits– Anand• Fundamentals of Digital Circuits– Anand

Kumar

• Digital Electronics- G K Kharate