Countermeasures for Metastability

Countermeasures for metastability

The output of a synchronous sequential circuit can potentially persist in an unstable equilibrium called a metastable state, depending on the timing of a data signal to be latched relative to the clock signal.
A sequential circuit enters a metastable state when its input setup and hold time (ts and th) requirements shown in the datasheet are not satisfied.

Countermeasures for metastability

Metastability potentially occurs when an active input (e.g., a clock signal) and a passive input (e.g., a data signal) are asynchronous to each other. To prevent sequential circuits from entering a metastable state, the recommended timing conditions shown in the datasheet must be satisfied.
For example, when the CK and D inputs are asynchronous, they can be synchronized as shown below.
In this case, however, care should be exercised as to the cycle period and propagation delay of CK.

If they are close, the data signal might not propagate to the second flip-flop.
The synchronizer shown in Figure 5.3 consists of two flip-flops. The first flip-flop prevents an increase in tpd and a hazard from being transferred to the output of the second flip-flop.
Even in this case, care is needed when the phase difference between CK1 and CK2 is close to the CK-to-Q delay (tpd) of the first flip-flop.

Note: If the two flip-flops cannot operate from the same clock, metastability is avoidable by creating an inverted clock synchronous to CK1 and using it as CK2 (e.g., CK2 = /CK1).

Usage Considerations of CMOS Logic ICs

Handling of Unused Input Pins
Input Rise and Fall Time Specifications
Multiple Outputs from a General-Purpose CMOS Logic IC Come Into Conflict (Short-Circuiting)
Connecting a Load Capacitance to a CMOS Output Pin
Calculating the Operating Supply Current and Power Dissipation
Level Shifting Using an Input-Tolerant Function
Example of Application of the Power-Down Protection Function (Partial Power-Down)
Input-Tolerant and Output Power-Down Protection Functions Available with Each Series
Types of Noise to be Noted
Countermeasures for Reducing Switching Noise
Countermeasures for Signal Reflection
Countermeasures for Crosstalk
Countermeasures for Hazards
Countermeasures for Latch-Up
Countermeasures for ESD Protection


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