HDL & Verification

Which statement about tri-state descriptions that output Z (high impedance) in an HDL is generally correct?

  1. AAll internal signals are normally built with tri-states
  2. BThey are used for bidirectional pins connecting off-chip, while internal signals normally use multiplexers
  3. CZ means the same as 0, so it can be used anywhere
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Answer: B. They are used for bidirectional pins connecting off-chip, while internal signals normally use multiplexers

A tri-state lets several circuits share one wire by disconnecting their outputs, and is used for bidirectional pins such as external bus or memory data lines. Inside a chip it risks two drivers colliding or no one driving the line so it floats, and it complicates timing analysis. So internal signals are normally selected with multiplexers instead.

  • A. Multiplexers are the norm internally.
  • B. Correct. Internal tri-states complicate timing and verification.
  • C. Z means undriven, which is different from 0.
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