What is Metastability in VLSI and How to Avoid it?
A race condition in the circuit’s input signals is typically the cause of metastability in VLSI (very large-scale integration), which is the phenomenon where a digital circuit may momentarily enter an undetermined state.
Reasons for Metastability in VLSI
In VLSI circuits, metastability can happen for a number of reasons:
- Noise
- Low VDD.
- Cross talk.
- Device aging
- High clock skew
- Process variation
- Temperature variations
- High parasitic capacitances.
- Excessive combinational delay
- If the input is an asynchronous signal
How metastability can be avoided or tolerated in a circuit.
The issue of metastability can be to some extent mitigated if input data adhere to setup and hold time limitations. Controlling metastability is challenging if the signals come from many clock domains.It’s important to understand that while we can’t entirely eliminate metastability, there are practical steps we can take to mitigate or tolerate it.
Adhering to Setup and Hold Time
You and I need to ensure that input data meets the setup and hold time requirements of the flip-flops. This significantly reduces the chances of metastability. However, this becomes challenging when signals originate from multiple clock domains.
Using Asynchronous Reset
We can use asynchronous resets to initialize flip-flops reliably, ensuring they start in a known state and reducing metastability risks.
Providing Settling Time
You should always allow adequate settling time for signals, which helps the system stabilize before moving to the next operation.
Metastable Hardened Flip-Flops
We can incorporate metastability-hardened flip-flops, designed to tolerate these issues better than standard flip-flops.
Precise Clock Period
Let’s make sure the clock period is precise, as avoiding delays in clock signals is crucial for minimizing metastability.
Metastability Filters and Synchronizers
You can also use metastability filters or add successive synchronizing flip-flops in the synchronizer. These solutions reduce risks but might introduce additional slack.
By implementing these measures, we can make circuits more robust against metastability.
