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| dpiegdon | hi there! | 14:18 |
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| dpiegdon | i have been playing around with ringoscillators, and found a working example from clifford, at http://svn.clifford.at/handicraft/2015/ringosc . | 14:25 |
| tpb | Title: handicraft - Revision 2994: /2015/ringosc (at svn.clifford.at) | 14:25 |
| dpiegdon | I was able to get this running with arachnepnr easily, | 14:25 |
| dpiegdon | but on nextpnr, which is timing driven, I always get the error: ERROR: timing analysis failed due to presence of combinatorial loops, incomplete specification of timing ports, etc. | 14:25 |
| dpiegdon | which makes sense, I guess. but is there a way to tell nextpnr to ignore timing here? or alltogether? | 14:26 |
| dpiegdon | (I have tried --no-tmdriv) | 14:26 |
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| daveshah | dpiegdon: use --force | 14:43 |
| dpiegdon | daveshah: ok, then the error is reduced to a warning, but no output file is generated. | 14:48 |
| daveshah | What is your full command line? | 14:49 |
| dpiegdon | nextpnr-ice40 --no-tmdriv --force --hx1k --package tq144 --pcf icestick.pcf --json $< | 14:50 |
| dpiegdon | i have uploaded my example to https://github.com/dpiegdon/ringoscillator | 14:50 |
| tpb | Title: GitHub - dpiegdon/ringoscillator: FPGA ringoscillator example. Originally taken from http://svn.clifford.at/handicraft/2015/ringosc/ (at github.com) | 14:50 |
| daveshah | You need --asc output.asc to create any output | 14:52 |
| dpiegdon | damn - copy and paste error | 14:56 |
| dpiegdon | thanks! | 14:56 |
| dpiegdon | yes, that works. ringing at 322MHz :) | 14:58 |
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| MoeIcenowy | by the way is it a way to evalulate the frequency of such a ring oscillator? | 15:35 |
| MoeIcenowy | can icetime do it? | 15:35 |
| sorear | with the icetime database it is possible to produce a conservative lower limit for the frequency (upper bound on delay of the closed-loop path) | 15:37 |
| sorear | ring oscillators are very sensitive to things like temperature (and are frequently used as sensors) | 15:37 |
| sorear | a more sophisticated timing analysis (for hold times) would also give you a lower bound on the delay → upper limit for the frequency over the operating voltage/temperature range | 15:38 |
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| dpiegdon | but icetime does not produce valid results here. neither for nextpnr nor for arachnepnr. it *did* produce some valid results for an earlier example. i am going to look into this a bit. | 15:55 |
| dpiegdon | so for the original version from clifford, icetime shows the (~correct) value of 8.92 MHz. but only when using arachnepnr, not for nextpnr. also when reducing the delay line, the value is rougly the lower bound, as described by sorear. | 15:59 |
| dpiegdon | but when i strip the example to only the ring oscillator (remove freq counter and control), icetime is clueless and shows 0MHz. | 16:00 |
| dpiegdon | the oscillator *does* work, though | 16:00 |
| dpiegdon | so i assume icetime does somehow uses the freq-counter logic loop to count the generated frequency. | 16:04 |
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