Metastability tolerant designs

We discussed, in our posts metastability and how a flip-flop goes metastable, the basics of metastability and what causes metastability failures in designs. We also discussed the impacts of metastability failures in our designs. So, we are bound to think of ways of preventing metastability in designs. The only way to do so is not to let the input toggle during setup-hold window. This can be done if we have completely synchronous designs and setup-hold timing is met for all timing paths. But, every design is bound to have asynchronous signals as everything in this world cannot run on a single clock. For example, when you press reset button on a device, this has to be an asynchronous event since the event is generated by your body, which runs on a different clock than the device. :-)

Thus, in reality, we cannot prevent metastability. We can only reduce its existence or make our designs such that the occurence of metastability does not affect the state machine.

• Avoid metastability to as much extent as possible: As discussed earlier, we can try to make the designs as such synchronous as possible. Thus, by virtue of setup-hold requirements being met, metastability will not be much of an issue. Another possible solution is to decrease the frequency of system. Less number of clock edges will mean less probability of data being captured during setup/hold window. However, do we really want to limit our designs' performance just for the sake of metastability? So, we must make our designs metastability tolerant.

• Make our designs metastability tolerant to as much extent as possible: The most common way to make designs metastability tolerant is to add synchronizer stages. Doing this, we are allowing certain flip-flops in the design to go metastable, but not allowing their metastability to impact the design by propagating to later stages. However, this also does not guarantee perfect immunity to design failures, but reduces the occurent of design failures due to metastability to almost nil, if carefully designed.