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Timing analysis and CDC

The write side writes a 120-word burst at one word per cycle at 200 MHz. The read side, on a 150 MHz clock, reads one word in only 2 out of every 3 cycles. What is the minimum depth needed to avoid overflow (ignore synchronization delay)?

  1. A30 words
  2. B60 words
  3. C40 words
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Answer: B. 60 words

Writing 120 words takes 120 × 5 ns = 600 ns. The read side's effective rate is 150 MHz × 2/3 = 100 million words per second, so 60 words can be read in 600 ns. The maximum that accumulates in the FIFO is therefore 120 − 60 = 60 words. 30 words is calculated at the full 150 MHz as if the read side could read every cycle (90 words in 600 ns), and 40 words simply takes one third of the burst. The key is to compare effective rates including idle cycles, not clock frequencies.

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