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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 EE280 Lecture 8 9-2 1 Logic Families Some subfamilies within the TTL family of gates. EE280 Lecture 8 9-3 Logic Families Some subfamilies within the CMOS family of gates. EE280 Lecture 8 9-4 2 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: EE280 Lecture 8 9-5 Fan-out Load of 1 gate Load of 2 gates Load of 3 gates EE280 Lecture 8 9-6 3 CMOS Input/Output (I/O) Characteristics Typical I/O Stages EE280 Lecture 8 9-7 CMOS Input/Output (I/O) Characteristics Equivalent circuit for the analysis of logic gate connections in CMOS technology EE280 Lecture 8 9-8 4 TTL Input/Output (I/O) Characteristics Typical I/O Stages EE280 Lecture 8 9-9 TTL Input/Output (I/O) Characteristics Equivalent Circuit EE280 Lecture 8 9 - 10 5 Speed and Propagation Delay Limitations on speed in logic circuits arise from two sources: EE280 Lecture 8 9 - 11 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 EE280 Lecture 8 9 - 12 6 Propagation Delay Input Pulse Logic Threshold 90% Logic Threshold 10% EE280 Lecture 8 9 - 13 7