Propagation Delay Comparison: Among S-TTL, AS-TTL, HS-TTL, and HCMOS, which family has the shortest typical gate delay?

Introduction / Context:
Propagation delay (tpd) is the time a logic family takes to respond at its outputs after its inputs change. Selecting a family with a shorter delay can raise maximum clock frequency and reduce skew in synchronous systems.

Given Data / Assumptions:

• S-TTL (Schottky TTL) improves speed over standard TTL by preventing deep saturation.
• AS-TTL (Advanced Schottky TTL) further optimizes device structures and interconnect to lower delay beyond S-TTL.
• HS-TTL (often used loosely) typically refers to faster TTL but generally not faster than AS-TTL.
• HCMOS focuses on low power; many HCMOS parts are fast, but classic AS-TTL remains extremely quick in simple gate delays.

Concept / Approach:
Within the classic TTL lineage, Advanced Schottky (AS-TTL) parts were designed to minimize internal storage effects and interconnect parasitics even more than S-TTL, yielding the lowest delays among the listed families.

Step-by-Step Solution:
Rank typical TTL variants by speed: AS-TTL faster than S-TTL.Interpret HS-TTL as high-speed TTL class but not surpassing AS-TTL.HCMOS is efficient and versatile, but standard gate delays do not necessarily beat AS-TTL under like-for-like conditions.Select the family with shortest delay: AS-TTL.

Verification / Alternative check:
Historic data books list AS-TTL with lower typical tpd than S-TTL. Families like 74F and 74AS were known for very small gate delays in their era.

Why Other Options Are Wrong:

• S-TTL: Fast, but AS-TTL is generally faster.
• HS-TTL: Generic label; does not exceed AS-TTL delays in standard references.
• HCMOS: Often slower than the fastest bipolar TTL in pure small-signal delay, though highly competitive in power and integration.

Common Pitfalls:
Assuming newer CMOS always beats older bipolar; at the gate-delay level, advanced bipolar families can still be exceptionally fast.

Final Answer:
AS-TTL