timing constraint is impossible to meet

Hi Wojtek, In the ISE GUI (I'm using 8.2 at the moment; I hope your version is similar), find the 'Processes' window. Expand the section 'Implement Design', then expand 'Map', then 'Generate Post-Map Static Timing'. If you double-click 'Analyze Post-Map Static Timing', it'll churn away and eventually open the timing analyser. You can then find what path is failing. The key insight is to run the timing analyser on the map results, before you enter the P&R phase. HTH, Syms.

The problem is, that I can not 'Generate Post-Map Static Timing', because the map fails!

-- Wojtek

Relax the timing until it maps, then you can find the slowest paths (at PAR if necessary). Fix these and retry. Then tighten the timings a bit. Repeat until done.

It's frustrating, I agree. Map should have completed, just to let you run the report.

Alternatively, what did synthesis report as the longest paths? Did they exceed your timing constraints? It may be worth fixing those first. (However, synthesis may not see the longest path if you are using black box components)

- Brian

OK, soz that didn't help you, ISE 8.2 doesn't seem to behave like that, I've only experienced this problem when it does the timing stuff in P&R. Out of interest, what version of the tools are you using? It sounds like a big backwards step from what I'm used to! Maybe there's a switch to turn off timing driven mapping? Cheers, Syms. p.s. What Brian posted! :-)

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Here one point is not clear to me. How a timing constraint forces the map to fail.If every thing (in the design) is correct then it passes the map with timing errors.I think there is some problem in the design which is forcing the map to fail.

I've seen a similar issue, but it wasn't related to clock period constraints. The offset in constraint was applied to input pins that had some combinatorial logic before getting latched by a flop. The offset in wasn't too tight by itself, but the extra combinatorial cloud was making it impossible to meet, hence a similar error. Start looking at offset in/out constraints, since it sounds like your clock constraints are relaxed enough.

HTH,

-P@

Try this environmental variable before running P&R

set XIL_TIMING_ALLOW_IMPOSSIBLE=1

Hans

Can I mention the deep and abiding love I have for whoever decided that the options to MAP needed to be passed through a combination of command line arguments, settings files, and nearly undocumented environment variables, with any given setting only accessible through one of those? That hasn't complicated my build chain one tiny bit, nosireebob.

-- Rob Gaddi, Highland Technology Email address is currently out of order

Hey, that's not fair... When I want to do a "never ifdef" by testing an env var or macro that is sure to be absent, I usually use IMPOSSIBLE_THINGS_BEFORE_BREAKFAST - and that one is getting awfully, awfully close :-)

-- Jonathan Bromley, Consultant

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Seconded!

Martin

I think the OP's problem is that he has set the timing driven packing and placement option for Map. Turning this off allows map to complete and generate the timing so you can find the failing path. In essence, turning on timing driven packing and placement allows map to do the first half of place & route. This is where the timing error prevents map completion.

Regards, Gabor