EC-21.5 Crossing Clock Domains, and Getting Reset Right

Work through these

• Metastability: what a flip-flop does when setup and hold are violated

  A flip-flop clocked while its input is changing can sit between the two logic levels for an unbounded time before settling arbitrarily. The behaviour is statistical rather than deterministic, which is exactly why simulation never shows it.

  NPTEL: VLSI Physical Design with Timing Analysis (IIT Roorkee) · Course

• The two-flip-flop synchroniser, and the mean time between failures it buys

  Chaining a second register gives the first one time to settle, and the improvement is quantified as a mean time between failures that depends on the clock rate and the data rate. Three stages are used where that figure is not long enough.

  NPTEL: VLSI Design Verification and Test (IIT Guwahati) · Course

• Why a multi-bit value cannot be synchronised bit by bit

  Independent synchronisers settle at independent moments, so a counter crossed bit by bit can be sampled as a value it never held. This is the single most common serious mistake in multi-clock design.

• Handshakes, Gray codes and asynchronous first-in-first-out memories

  The three standard answers are to pass one bit and hold the data still, to encode so that only one bit changes at a time, or to use a memory built for the purpose. Each has a throughput and latency cost worth knowing.

  NPTEL: System Design through Verilog (IIT Guwahati) · Course

• Reset: asynchronous assertion, synchronous release, and reset domains

  A reset that is removed at an arbitrary moment can violate recovery timing and leave part of a design in a different state from the rest. The standard shape asserts immediately and releases in step with each clock.

  NPTEL: System Design through Verilog (IIT Guwahati) · Course

• Checking crossings automatically, because review will not find them all

  Tools exist that identify every signal crossing between clocks and verify that a recognised structure protects it. On a design with several clocks this is not optional, because the number of crossings exceeds what anybody can enumerate by hand.