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<title>iCE40 Ring Oscillation Test Notes</title>
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<h1>iCE40 Ring Oscillation Test Notes</h1>
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<h2>Dependencies</h2>
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<li><a href="https://github.com/YosysHQ/icestorm">IceStorm tools</a></li>
<li><a href="http://www.clifford.at/yosys/download.html">yosys</a></li>
<li><a href="https://github.com/YosysHQ/nextpnr">nextpnr</a></li>
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Install instructions for all of the above tools can be found on the
<a href="http://bygone.clairexen.net/icestorm/">IceStorm website</a>.
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<h2>Building</h2>
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The default make target synthesizes the design and runs a timing analysis. The target prog programs
an available iCEBreaker via USB.
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<h2>Results</h2>
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When clocking the FPGA at 24MHz, the resulting ring oscillation is still identifiable as a square
wave.
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<img src="./img/ring_12mhz.png" alt="12MHz">
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By 111MHz, it's more of a triangle wave. But still stable and nearly rail to rail. This is the
fastest clock speed that `icetime` approves.
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<img src="./img/ring_55mhz.png" alt="55.5MHz">
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Despite not passing the timing analysis, when the FPGA is clocked at 120MHz no problems seems to
occur.
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<img src="./img/ring_60mhz.png" alt="60MHz">
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Even at 129MHz, stable ring oscillation can occur. I (Erik) thought that the first time I ran it I
got a chaotic, aperiodic signal. But I haven't been able to reproduce this so I might have made a
mistake. Still, I'll hold off on declaring this result official.
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<img src="./img/ring_64mhz.png" alt="64.5MHz">
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At faster clock speeds things start to go wrong. Usually there's still a periodic ring oscillation,
just much slower than one would hope.
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