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Clock Domain Crossing (CDC) Protocol

A set of design rules and synchronizers (e.g., two-flip-flop synchronizer) required when signals pass between circuits operating at different clock frequencies to prevent metastability.

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The statement of the theorem

When a signal

S

crosses from a domain clocked by

\text{CLK}_1

to a domain clocked by

\text{CLK}_2

, a synchronizer is required. The standard two-flip-flop synchronizer uses the following structure:

S_{sync, 1} = D \text{ (on } \text{CLK}_2 \text{ edge)}

and

S_{sync, 2} = D \text{ (on } \text{CLK}_2 \text{ edge)}

. The output

S_{out}

is taken from

S_{sync, 2}

. This structure mitigates metastability by allowing the signal to settle over two clock cycles.