Fastest TTL-derived logic family (maximum clock frequency) Among the following logic families, which typically achieves the highest maximum clock frequency?

Difficulty: Easy

Correct Answer: AS-TTL

Explanation:


Introduction / Context:
Classic TTL families evolved to improve speed and reduce power. Schottky clamping prevents deep saturation, improving switching time; later “Advanced Schottky” refinements push speed further. This question compares well-known TTL derivatives and HCMOS on maximum practical clock frequency (ignoring ECL, which is not listed here).


Given Data / Assumptions:

  • S-TTL = Schottky TTL (74SXX).
  • AS-TTL = Advanced Schottky (74ASXX), generally faster than 74SXX.
  • HS-TTL = High-Speed TTL (74HXX), older and slower than Schottky variants.
  • HCMOS = High-speed CMOS; fast, but in classic comparisons AS-TTL leads over HS-TTL and S-TTL.


Concept / Approach:
Advanced Schottky TTL reduces storage delay and optimizes device structures/resistors for improved speed relative to earlier TTL lines. Although modern CMOS in deep-submicron processes is very fast, traditional family-to-family comparisons (same era) place 74ASXX at the top among the given TTL variants for max clock rate.


Step-by-Step Solution:

Order families by typical speed: HS-TTL < S-TTL < AS-TTL.Recognize HCMOS varies widely; classic HCMOS did not universally exceed 74ASXX in max clock speed across loads.Select AS-TTL as the fastest among listed choices.Note context: ECL would be faster but is not an option here.


Verification / Alternative check:
Historic datasheets show lower t_pd for 74AS vs. 74S and 74H series under comparable loading.


Why Other Options Are Wrong:

  • A/C: S-TTL and HS-TTL are slower than Advanced Schottky.
  • D: HCMOS speeds are application- and process-dependent; the classic comparison favors 74AS.


Common Pitfalls:
Confusing modern high-speed CMOS ASICs or FPGAs with classic HCMOS logic families.


Final Answer:
AS-TTL

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