Re: Understanding DFF behaviour

Posted by cadet1620 on Apr 19, 2016; 1:56am
URL: http://nand2tetris-questions-and-answers-forum.52.s1.nabble.com/Understanding-DFF-behaviour-tp4029740p4029748.html

jbloggz wrote

Is the inactive edge the falling edge? So that means that for a proper edge triggered dff, the input is copied to the output at the rising edge?

The nand2tetris sequential logic is documented to be falling edge triggered (fig 3.5).   Therefore the rising edge is the inactive edge because nothing is supposed to happen when it occurs.  

I still don't understand why they didn't just make the dff behave realistically... There doesn't appear to be any benefit to the way it was implemented.

If I was to change the built-in implementation to work like a proper edge triggered dff, would that cause issues?

As I said, it's a bug.  And it doesn't cause any problems with the normal usage of the simulator—running the test scripts and verifying chip outputs.

The potential problem with fixing this bug is that since it affects how DFFs work, all test scripts that test circuits that might use DFFs will need to be retested and may need updating.

--Mark