Download Lecture 9. Logic Families and Their Characteristics

EE 280
Introduction to Digital Logic Design
Lecture 9.
Logic Families and Their Characteristics
Logic Families
TTL - Transistor-Transistor Logic
– standard logic family; used for the longest time.
ECL - Emitter Coupled Logic
– suitable for systems requiring high-speed operations
MOS - Metal Oxide Semiconductor Logic
– suitable for systems with high component density
CMOS - Complementary Metal Oxide Semiconductor Logic
– suitable for systems with low power consumption (VLSI circuits)
• gradually becomes the dominant logic family
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Logic Families
Some subfamilies within the TTL family of gates.
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Logic Families
Some subfamilies within the CMOS family of gates.
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Fan-out and Fan-in
Definitions:
Fan-out:
The maximum fan-out is that number of gates
which still can be driven while still maintaining the
required voltage levels for a logic 0 & 1.
Fan-in:
Standard Load:
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Fan-out
Load of 1 gate
Load of 2 gates
Load of 3 gates
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CMOS Input/Output (I/O) Characteristics
Typical I/O Stages
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CMOS Input/Output (I/O) Characteristics
Equivalent circuit for the analysis of logic gate connections
in CMOS technology
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TTL Input/Output (I/O) Characteristics
Typical I/O Stages
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TTL Input/Output (I/O) Characteristics
Equivalent Circuit
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Speed and Propagation Delay
Limitations on speed in logic circuits arise from two sources:
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Causes of Propagation Delay
1.
In TTL, transistors require a nonzero time to switch from on to off
of off to on.
2.
Two component causes:
Time to switch the transistor
Time to charge or discharge the load capacitance
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Propagation Delay
Input Pulse
Logic Threshold
90%
Logic Threshold
10%
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