Re: Understanding DFF behaviour

Posted by cadet1620 on Apr 19, 2016; 3:01am
URL: http://nand2tetris-questions-and-answers-forum.52.s1.nabble.com/Understanding-DFF-behaviour-tp4029740p4029750.html

[I'm just a volunteer here so all my posts are just my opinion...]

jbloggz wrote

Thank you! That's exactly the answer I was hoping for. So it's actually a bug in the built-in dff implementation? I can understand it would be annoying to fix, since all the test scripts rely on it. However it would be nice if this was mentioned somewhere (or did I miss it?).

To my knowledge There is no bug list.

I might fix it and see how this affects anything.

That would be interesting to know.

Is the reason that it's harmless because the input never changes between the rising and falling edges in the simulator?

Correct.  In normal use the test scripts make input changes and then do "tick, tock;".  Any input changes cause by output changes in the parts in the HDL will (bugs not withstanding 8^) only happen on "tock".

It's an interesting bug, because it seems like it was done on purpose. I'd love to here the reasons behind it if there were any.

Again, I'm guessing.  I suspect that a programmer who was not a hardware specialist was told to implement a master-slave DFF and that's how he thought they worked.

--Mark