clifford changed the topic of #yosys to: Yosys Open SYnthesis Suite: http://www.clifford.at/yosys/ -- Channel Logs: https://irclog.whitequark.org/yosys
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<maikmerten> I have a question regarding the "est tns" output of nextpnr. For my design, the output startes highly negative, then creeps towards zero
<maikmerten> Info: at iteration #1: temp = 10000.000000, cost = 112265, est tns = -2366.69ns
<maikmerten> ...
<maikmerten> Info: at iteration #365: temp = 0.000001, cost = 9392, est tns = -0.96ns
<maikmerten> so is that the timing slack, with negative numbers meaning the timing target is not yet reached?
<daveshah> maikmerten: this is a very rough estimate of timing used by the placer
<daveshah> it's primarly there fore placer debugging than as a real indicator as to whether the design will pass timing after routing
<maikmerten> ah, thanks
<daveshah> it is the sum of all the estimated (budget - actual slack
<daveshah> *actual delay)
<daveshah> when that value is negative
<maikmerten> I noticed that with low values of --slack_redist_iter it appears to creep faster towards zero
<daveshah> That is also because slack_redist_iter will redistribute the budget
<daveshah> So it will allow a higher amount of delay in the slower parts of the paths
<maikmerten> so that's not simply a "try harder" button that trades of computational effort and quality
<maikmerten> so, in the end the routing ending up with
<maikmerten> Info: at iteration #385: temp = 0.000000, cost = 11258, est tns = -114.20ns
<maikmerten> Info: at iteration #386: temp = 0.000000, cost = 11258.000000
<maikmerten> is not troublesome, because that's basically a preliminary estimate
<maikmerten> with
<maikmerten> Info: Max frequency for clock 'uart_inst.CLK_I_$glb_clk': 29.83 MHz (PASS at 28.00 MHz)
<maikmerten> being the real deal
<daveshah> Yes, that is correct
<maikmerten> cool, thanks again for taking the time to answer my somewhat basic questions!
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