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This is the last in the series of Constraint Construction blogs! Today we're going to go over DESIGN RULES and MODES OF OPERATION.
DESIGN RULES: Follow them, or else...
Often times, these rules are indeed set in the timing library. But perhaps you want sharper transitions in your design to reduce noise issues. Or maybe you want to give yourself some margin of safety with minimum capacitance. Let's go over the main design rules to live by.
MODES OF OPERATION: There's more than one way to peel an apple.
Well, I hope this has been informative for everyone! The goal here is, whenever you get some constraints from someone, and you are not sure how well they are 'constructed', reference back to this and ask these simple questions and you just might catch problems early enough!
To read up some more on this topic, check out this section in the Encounter docs (please note, to access the doc requires and account and password): Setting Constraints and Performing Timing Analysis in Encounter RTL Compiler
Hi Aditya, the author of this blog is no longer with Cadence. You can try posting your question to the forums. Thanks!
would like to know how to define constraints if i am writting code for a ripple carry adder where o/p of a FF is i/p clk to other???
My opinion is to ignore max_fanout and what REALLY matters is fixing that max_capacitance value. In a way, they are doing the same thing, but the set_max_fanout has no idea the distances away any instances the driving pin are. You could meet max_fanout but break max_cap because of a large net capacitance.
I don't often see max_fanout actually in libraries and only see the max_cap. Within a liberty model (.lib), there are lookup tables which reference input slew and output capacitance. The max_capacitance is simply the maximum that table goes. It's basically saying beyond that cap value, we have no idea what the delay will be! For max_fanout, usually they'll determine some input capacitance for a certain fanout that try to match that value up with max_cap.
Hope that helps explain it!
I often wonder why there are 2 attributes: max_fanout and max_capacitance, in the library files. Aren't they both doing the same thing? What is the difference in the way these two values are determined for entry in the library?