What is Metastability in an FPGA?

How digital designers handle asynchronous events

If you have ever tried to sample some input to your FPGA, such as a button press, or if you have had to cross clock domains, you have had to deal with Metastability. A metastable state is one in which the output of a Flip-Flop inside of your FPGA is unknown, or non-deterministic. When a metastable condition occurs, there is no way to tell if the output of your Flip-Flop is going to be a 1 or a 0. A metastable condition occurs when setup or hold times are violated.

Metastability is bad. It can cause your FPGA to exhibit very strange behavior. The figure on the right demonstrates a metastable event. The red area represents the tsu or Setup Time. As you can see, the data input to the Flip-Flop has gone from low to high during the setup time of the Flip-Flop. This causes the output to be metastable. The output is metastable for some amount of time, after which it settles out to either a 0 or a 1. However we still do not know which state the output ended in. Sometimes it might be a 0, other times this situation occurs it might be a 1. Again, this is not desired behavior. You must always know what your FPGA is doing.

How to Prevent Metastability

Most metastable conditions occur in one of two ways:

  1. You are sampling a signal external to the FPGA
  2. You are crossing clock domains

Both of these situations can be fixed the same way. Whenever you are encountering a situation that might introduce Metastability you can simply "double-flop" your data.

In the figure to the left, a signal that is asynchronous to the clock is being sampled by the first Flip-Flop. This will create a metastable condition at the output. If you again sample this output, you can now fix your metastable event. The output of the second Flip-Flop will be stable.

For further reading about Metastability, including the science behind it (and a lot of technical information about failure rates and such) Altera wrote a very detailed paper about it.