Asynchronous (ripple) counters — What major limitation affects their use at high frequencies?
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Alow-frequency applications are limited because of internal propagation delays
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Bhigh-frequency applications are limited because of internal propagation delays
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CAsynchronous counters do not have major drawbacks and are suitable for high- and low-frequency applications.
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DAsynchronous counters do not have propagation delays, which limits their use in high-frequency applications.
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EHigh-frequency operation is limited only by supply voltage, not timing delays.
Answer
Correct Answer: high-frequency applications are limited because of internal propagation delays
Explanation
Introduction:Ripple counters are simple and hardware-efficient, but their stage-to-stage triggering creates timing skew. Recognizing this limitation helps you decide when to choose synchronous counters instead.
Given Data / Assumptions:
- Asynchronous counter with cascaded flip-flops (output of one is clock for the next).
- Non-zero propagation delay for each flip-flop.
- Interest is in maximum reliable counting frequency.
Concept / Approach:
Because each stage toggles after the previous one, total response time accumulates roughly as the sum of propagation delays through multiple stages. At high input clock rates, the outputs may not settle before the next clock edge, causing miscounts or decoding errors.
Step-by-Step Solution:
Let t_pd be propagation delay per flip-flop.Worst-case count ripple traverses N stages → cumulative delay ≈ N * t_pd.Maximum usable clock frequency f_max is limited so that the next clock arrives after outputs have settled: f_max < 1 / (N * t_pd).Thus, higher frequencies are limited by propagation delays.Verification / Alternative check:
Timing diagrams show glitch windows during ripple transitions. Synchronous designs eliminate these by clocking all stages together, supporting much higher f_max.
Why Other Options Are Wrong:
- Low-frequency limitation: Delays are negligible at low frequency.
- No drawbacks / no delays: Physically inaccurate; every device has finite delay.
- Voltage-only limitation: Speed is fundamentally timing-limited, not just supply-limited.
Common Pitfalls:
- Decoding ripple outputs without registering them, which can capture transient states.
- Underestimating worst-case t_pd across PVT (process, voltage, temperature).
Final Answer:
high-frequency applications are limited because of internal propagation delays