Register-transfer-level (RTL) code, formal analysis, RTL simulation, and logic synthesis have all raised the abstraction level of electronic design and verification. Today’s designers operate very differently than their predecessors who drew circuit-level schematics and ran only SPICE. However, underneath all this abstraction the physical properties of electronic devices remain unchanged, and these must be considered during design. One well-known example is metastability, which can occur wherever a signal crosses between flip-flops running on asynchronous clocks, known as a clock domain crossing (CDC).
The most common example of metastability happens when the output value of a flip-flop on the sending clock changes during the setup and hold time of the clock for the receiving flip-flop. The output of the flip-flop on the receiving clock can take on an indeterminate value that requires some time to settle to a one or zero. If the output of the receiving flip-flop is used immediately, an invalid value may be fed into the downstream logic and produce incorrect results. Unfortunately, there is no way to design a flip-flop that does not have a risk of metastability.
