Interpreting a low speed–power product — what does “lower is better” mean? As a general rule, a logic family with a lower speed–power product is preferable because it offers which combination of characteristics?

Introduction / Context:
The speed–power product (SPP) combines two core parameters into one metric. It is especially useful for comparing families across technology nodes or architectural styles. Understanding what a “low” value signifies helps engineers pick gates that deliver both performance and efficiency.

Given Data / Assumptions:

• SPP = propagation delay (ns) * power dissipation (mW).
• Lower SPP is better, all else equal.
• Comparisons should be made at similar voltage and temperature conditions.

Concept / Approach:
Since the two terms multiply, a small product occurs when both factors are small. A logic family with very short delays but very high power may have a larger SPP than a moderately fast, low-power family. Conversely, a slow but ultra-low-power family can also end up with a larger SPP. The best families combine low delay and low power to minimize the product.

Step-by-Step Reasoning:

Verification / Alternative check:
Example: Family A, tp = 5 ns, PD = 20 mW ⇒ SPP = 100. Family B, tp = 10 ns, PD = 5 mW ⇒ SPP = 50. Family B is slower but much lower power, yielding a lower SPP and greater energy efficiency per operation.

Why Other Options Are Wrong:

• Long delays (options a, b) cannot yield small SPP regardless of power.
• “Short delay and high power” (option c) may still produce a large product.
• “No advantages” contradicts the definition of SPP.

Common Pitfalls:

• Comparing values without noting test frequency; average power can vary with toggle rate.

Final Answer:
short propagation delay and low power consumption