Gisselquist Technology recently posted a good blog article about metastability and common solutions. If you are trying to learn FPGAs, you’ll want to read it. If you know a lot about FPGAs already, you might still pick up some interesting tidbits in the post.
Don’t let the word metastability scare you. It is just a fancy way of saying that a flip flop can go crazy if the inputs are not stable for a certain amount of time before the clock edge and remain stable for a certain amount of time after the clock edge. These times are the setup and hold times, respectively.
Normally, your design tool will warn you about possible problems if you are using a single clock. However, any time your design generates a signal with one clock and then uses it somewhere with another clock, metastability is a possible problem. Even if you only have one clock, any inputs from the outside world that don’t reference your clock — or, perhaps, any clock at all — introduce the possibility of metastability. Continue reading “FPGA Metastability Solutions”