I have an issue with Altera FIFOs. It seems that the full signal wrfull gets asserted even when the FIFO is not full. My FIFOs are of size 8. The SignalTap traces below show the read levels of my FIFOs (rdlevel) as well as the wrfull signals:
What could explain that the wrfull signals are not behaving properly?
Best Answer
I haven't used Altera, or the FIFO you are using, and the signals you have in your picture makes no sense to me. But I have designed many FIFO's and maybe I can still give some insight into what you are seeing...
Many FIFO's have a strange notion of "full". Specifically, "full" is when there is one less words than what you think there should be. A 16-word deep FIFO can only hold 15 words. Or a 64 word FIFO can really only hold 63 words. In these FIFO's, the RAM used is 16 or 64 words but the logic used to calculate the full-ness will signal FULL at one less. Attempting to put 16 or 64 words into that FIFO will result in bad stuff. Not every FIFO is like this, but most of them are.
Some FIFO's, particularly ones with a separate read and write clock, will take time for data written to it to appear on the read port. Sometimes a surprising amount of time. To put it a different way... Let's say that the FIFO is completely empty, and you start writing to the FIFO as fast as possible. You might be able to write several words to the FIFO before the "FIFO Not Empty" signal goes active. This means that you can't start emptying the FIFO until later than you might expect. Which means that you might have more words in the FIFO than you anticipated. Which could cause it to fill up.
The length of time for a written word to appear on the read port varies depending on the internal architecture of the FIFO. Sometimes it is within a clock cycle. Other FIFO's could require several write-clocks plus several read-clocks.
Requiring several write-clocks and/or several read-clocks brings up an interesting issue. There are many FIFO's which don't behave well with discontinuous clocks. That is, a clock that just isn't continuously running. If it takes 3 read-clocks to bring the write data to the read port, and you only give it 2 clocks then the data is not going to get there. The data will accumulate in the FIFO and never get read. You can actually get into a state where the FIFO claims to be both empty and full at the same time! Empty, because the data has not gotten to the read port yet. And full, because there is no more room in the RAM to write new data.
So, those are some reasons why the full flag might be going active when you don't expect it to. But you must also consider that you are just doing something else wrong. There's a bug in your code, or you have a noisy clock, or you have incorrect timing constraints. Those are real possibilities that I can't diagnose remotely.