Logic Families Intro
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Transcript of Logic Families Intro
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Logic FamiliesLogic Families
Introduction
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Logic Family DefinitionLogic Family Definition
y A circuit configuration or approach used to
produce a type of digital integrated circuit.
y 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.y the set of digital ICs belonging to the same
logic family are electrically compatible with
each other 2
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Common Characteristics of theCommon Characteristics of the
Same Logic FamilySame Logic Family
y Supply voltage range, speed of response,
power dissipation, input and output logic
levels, current sourcing and sinkingcapability, fan-out, noise margin, etc.
y 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. 3
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Types of Logic Family 1Types of Logic Family 1
y
The entire range of digital ICs is fabricatedusing either bipolar devices or MOS devices
or a combination of the two.
y Bipolar families:
Diode logic (DL). (obsolete)
Resistor transistor logic (RTL). (obsolete)
Diode transistor logic (DTL). (obsolete)
TransistorTransistor logic (TTL).
Emitter Coupled Logic (ECL), also known as
Current Mode Logic(CML).
Integrated Injection logic (I2L). (obsolete) 4
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Types of Logic Family 2Types of Logic Family 2
yMOS families:
PMOS family (using P-channel MOSFETs)
The NMOS family (using N-channel
MOSFETs)
The CMOS family (using both N- and P-
channel devices).
The Bi-MOS logic family uses both bipolar
and MOS devices.
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DL ExampleDL Example
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RTL ExampleRTL Example
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DTL ExampleDTL Example
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TTL SubfamiliesTTL Subfamilies
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CMOS Sub familiesCMOS Sub families
y 4000A
y 4000B, 4000UB,
y 54/74C, 54/74HC, 54/74HCT, 54/74AC
and 54/74ACT(TTL pin compatible)
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Characteristic Parameters 1Characteristic Parameters 1
y HIGH-level input current, IIH (current flowing into
(taken as positive) or out of (taken as negative) an input
when a HIGH-level input voltage equal to the minimum
HIGH-level output voltage specified for the family is applied.
y LOW-level input current, IIL. is the maximum current
flowing into (taken as positive) or out of (taken as negative)
the input of a logic function when the voltage applied at the
input equals the maximum LOW-level output voltage
specified for the family.y unit load(UL) HIGH-level and LOW-level input current or
loading typically found in data sheets (For devices of theTTL
family, 1 UL (HIGH)=40 A and 1 UL (LOW)=1.6 mA.
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Characteristic Parameters 2Characteristic Parameters 2
y HIGH-level output current, IOH. This is the
maximum current flowing out of an output
when the input conditions are such that
the output is in the logic HIGH state.Typically negative number.
y LOW-level output current, IOL.This is the
maximum current flowing into the outputpin of a logic function when the input
conditions are such that the output is in
the logic LOW state.
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Characteristic Parameters 3Characteristic Parameters 3
y HIGH-level off-state (high-impedance state) output
current, IOZH.This is the current flowing into
an output of a tristate logic function with the
ENABLE input chosen so as to establish ahigh-impedance state and a logic HIGH
voltage level applied at the output.The input
conditions are chosen so as to produce logic
LOW if the device is enabled.
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Input and output current specificationsInput and output current specifications
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Characteristic Parameters 4Characteristic Parameters 4
y LOW-level input voltage, VIL. This is the
maximum voltage level applied at the input
that is recognized as a legal LOW level for
the specified family.
y HIGH-level output voltage, VOH. This is the
minimum voltage on the output pin of a logic
function when the input conditions establishlogic HIGH at the output for the specified
family.
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Characteristic Parameters 5Characteristic Parameters 5
y
HIGH-level output voltage, VOH. T
his is theminimum voltage on the output pin of a
logic function when the input conditions
establish logic HIGH at the output for the
specified family.
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Characteristic Parameters 6Characteristic Parameters 6
y Supply current, ICC.The supply current when
the output is HIGH, LOW and in the high-
impedance state is respectively designated as
ICCH, ICCL and ICCZ.y Rise time, tr.This is the time that elapses
between 10 and 90 % of the final signal level
when the signal is making a transition from
logic LOW to logic HIGH.
y Fall time, tf.This is the time that elapses
between 90 and 10 % of the signal level
when it is making HIGH to LOW transition. 17
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Characteristic Parameters 7Characteristic Parameters 7
y Propagation delay tp. is the time delay betweenthe occurrence of change in the logical level atthe input and before it is reflected at theoutput. It is the time delay between the
specified voltage points on the input andoutput waveforms.
y Propagation delays are separately defined forLOW-to-HIGH and HIGH-to-LOW
transitions at the output. In addition, we alsodefine enable and disable time delays thatoccur during transition between the high-impedance state and defined logic LOW orHIGH states.
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Characteristic Parameters 8Characteristic Parameters 8
y Maximum clock frequency, fmax. This is the maximum
frequency at which the clock input of a flip-flop
can be driven through its required sequence
while maintaining stable transitions of logic levelat the output in accordance with the input
conditions and the product specification.
y Power dissipation. The power dissipation
parameter for a logic family is specified in terms
of power consumption per gate and is the
product of supply voltage VCC and supply
current ICC. 19
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Characteristic Parameters 9Characteristic Parameters 9
y Speedpower product. The speed of a logic circuitcan be increased, that is, the propagation delay can bereduced, at the expense of power dissipation.
y Fan-out. is the number of inputs of a logic function
that can be driven from a single output withoutcausing any false output.
y Noise margin.This is a quantitative measure of noiseimmunity offered by the logic family. When the outputof a logic device feeds the input of another device of
the same family, a legal HIGH logic state at the outputof the feeding device should be treated as a legalHIGH logic state by the input of the device being fed.Similarly, a legal LOW logic state of the feeding deviceshould be treated as a legal LOW logic state by the
device being fed. 20
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Noise MARGINNoise MARGIN
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