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Just one question. You mentioned that cliques are only available on ACEX 1K, FLEX 10K, FLEX 6000, or Mercury devices, so I was just wondering how can this trick be implemented on the newer devices?
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To be honest I don't know. But if you were to try this on say a Cyclone (1,2 or 3) then you'd probably find that Quartus had no problem meeting the timing just because the device is faster.
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Also I was wondering why Quartus would not implement something like this automatically, to guide the fitter when it sees certain paths not making timing?
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Quartus does try to place logic to meet timing but I think what you're doing here is possibly a bit more of a human kind of thinking and possibly not that easy to turn into an algorithm. Or of course it may be that Quartus does do this on newer devices which is why you don't have the option of using cliques.
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Would this be an advantage of using a 3rd party synthesis tool? that perhaps they would synthesis the design differently and obtain better results?
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Quite possibly. I used to use Leonardo (Mentor Graphics) in my previous job. I found that for the cyclone designs that we were doing at the time, using Leonardo for synthesis gave better results than using the Quartus synthesis capability and weirdly better than Precision (Mentor's newer tool). Leonardo also seemed to give better results than Synplify.
Interestingly at that time, Quartus was better at the microscopic synthesis - i.e. if you carefully coded up exotic registers (clock enables and synchronous sets/clears asynchronous sets/clears/loads) then Quartus was better than Leonardo. However take a huge design and Leonardo was just better at optimising huge lumps of logic.
A word of caution here though - don't take any of this as a recommendation to buy that particular tool. There were a few designs where Synplify was better than Leonardo. some Lattice designs that I worked on were better in Precision than either Synplify or Leonardo. It's been a couple of years since I did any serious comparison and this may all have changed and may not have been valid for the sort of designs that you're doing. Also I think Mentor have been winding Leonardo down and trying to replace it with Precision.
Try your design(s) and see. If you get in touch with the vendors of the various tools then then they usually give you a trial licence.
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Anyway although this particular project has been a real head-wrecker, I´ve certainly learned a lot of new stuff. I never would have known about cliques for sure and probably wouldn´t have have learned as much about timing issues
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You've also learned how vitally important it is to document your designs properly! from what you've said I seriously doubt your man would be able to explain all of his timing constraints himself. Although these sorts of jobs are a pain I do believe they can make you a better engineer in the long run if you decide you don't want to leave a similar mess as part of your legacy.
Cheers
batfink
Honored Contributor
