Thanks for your comprehensive reply. How do you find time - do you ever sleep?
A lot more than I used to, unfortunately... until I'm fully back to normal health levels I sleep all I can!
2. What spec are the crystals you use? I looked at Farnell, RS, Rapid, Mouser and Digi-Key. Only Digi-Key have non-SMD in stock - all the others only have them with much higher load capacitance.
No particular spec. Just generic 27MHz fundamental-mode crystals.
I stopped believing in adding capacitors to crystals a few years back. The Ultimate3S kit still has pads for the originally supplied 22pF capacitors but they are no longer supplied or recommended. It was found that sometimes the 20MHz crystal wouldn't oscillate with the capacitors in place. But it always oscillates without them. So on the principles of the guy who designed TVs in ye olde days and used to remove one component at a time until it stopped working... same theory, I stopped putting capacitors in the circuit.
The fact the 27MHz Si5351A crystal tends to oscillate around 27.004 and the 20MHz Atmel AVR crystal at around 20.006MHz... indicates ideally a bit of capacitance would be needed. But stray PCB capacitance etc is enough to make sure everything always works, and we don't need them to be precisely on frequency either.
One way to have trimmed the reference oscillator would be a trimmer capacitor on one side of the crystal, commonly seen. But we just do it in software... as long as the software knows the oscillation frequency it just takes it into account when calculating Si5351A values and everything comes out on frequency. It saves physical parts (the trimmer capacitor) and it also means you can employ a GPS to let the software accurately measure the actual oscillation frequency itself. Just perfect all round.
So just buy any 27MHz crystal and proceed regardless. Do make sure it is a fundamental mode one though not 3rd harmonic type.
4. I now understand why level shifters are not needed - I will remove them from my project. The outputs are open-collector as it requires wired AND so, as long as the pull-ups are to the lower voltage, it is safe. The only consideration is if the lower logic levels are OK for the higher voltage device. I have a feeling there are a very large number of unnecessary I2C level shifters in the world. As long as I am in control of the hardware design I can guarantee there are no pull-ups to 5V.
Hahaha I like that thought, but the large number of unnecessary I2C level shifters in the world! I'd add, a lot of other logic level shifters also unnecessary. It's important not to design recklessly but it sometimes seems there's a tendency to be too conservative!
73 Hans G0UPL