iCE40 Ring Oscillation Test Notes


Install instructions for all of the above tools can be found on the IceStorm website.


The default make target synthesizes the design and runs a timing analysis. The target prog programs an available iCEBreaker via USB.


When clocking the FPGA at 24MHz, the resulting ring oscillation is still identifiable as a square wave.


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.


Despite not passing the timing analysis, when the FPGA is clocked at 120MHz no problems seems to occur.


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.


At faster clock speeds things start to go wrong. Usually there's still a periodic ring oscillation, just much slower than one would hope.