Ring Around the Rosie

Today, set up to do a direct test of the ring oscillator frequency/period. That seems to work fine. Here's the scope trace showing a ~1V logic swing "very rounded square wave" output measured via a 75ohm cable (I couldn't find any 50ohm ones lying around). The period is about 1.7ns, frequency about 580 MHz. Possibly it's smeared out in part because the board trace to the CLK_OUT connector isn't rated for signals at that high a frequency - there might not be enough impedance control. Or maybe the problem is the cable.

Let's try a 5-stage ring oscillator, and see if the longer period means the wave shape and amplitude will fare better. Ideally, the period should be 67% longer (5/3x). It turns out to be 2.5 ns, which is only about 50% longer, interesting, though possibly the discrepancy is due to measurement error. Oh and the amplitude is larger, as expected: 1.35V. The wave shape is different, too, and in an interesting way:

The lows are flatter than the highs, which makes sense since nFETs are faster than pFETs (since electron mobility. Frequency is now 395 MHz, and meanwhile the period of bit 20 of the counter is 5.3 ms, which is consistent with the 2.5 ns for the ring oscillator period. I don't think the counter was working at all with the 3-stage ring oscillator on the last test, but I should probably check again to make sure. Nope, it wasn't. OK, now let's try 7-stage:


At 7 stages, the period is 3.42 ns (still only about 2x that of the 3-stage RO, instead of 2.5x) and frequency is 293MHz. Interesting. Amplitude is now about 1.6V (peak-to-peak). Period of bit 20 is 7.16 ms. So, if I did dual-edge-triggered registers on that signal, it would be about the same period as single-edge triggered with the original 1.7ns 3-stage period, and it would probably work more reliably due to the larger signal swing and more flat-topped waveform. So, that is probably in fact the best approach. However, I'm still baffled by why I was able to get *faster* ring oscillators on the DE2 board than on the DE3. I probably need to do some more experiments on the DE2 at home. And study the datasheets for both devices some more.