The oscillator is mentioned in this commit
From that commit:
<pre>
- // Implement nasty ring oscillator for fallback use when main system clock is offline
- // Thanks to Clifford Wolf for the idea and basic code!
</pre>
Wow, I'm surprised they have to use a LUT-based ring oscillator here. All the Xilinx and Altera FPGAs I've ever dealt with had an internal "hard" (not made of LUTs) ring oscillator that could be routed into the fabric. Still not great, but more predictable than LUTs. Don't the Lattice ICE chips have this? Can't the PLLs be used for this (use the slowest setting, ignore the lock signal)?
Anyways, if your board has the external oscillator you should definitely use it.
