Good evening all,
Just interested in how other interpret this section of the xmega E5 datasheet (Microchip RevA, p95)...
Do the table values C(Tosc1) & C(Tosc2) correspond to the circuit values C(L1) & C(L2)?
In the doc "AT01080: XMEGA E Schematic Checklist" the oscillator is claimed to have a typical C(load) of 3pF. It is not clear whether this includes any parasitic pin capacitance. A total C(load) of 3pF might imply C(L1) = C(L2) = 6pF.
Interestingly the 5.3 & 7.4 does sum to 3.08pF so this sounds plausible.
So do I include both values such that the total C(load) is 6pF or 3pF?
The motivation for this comes from a deployment of a 32.786kHz crystal with C(load) specified as 4pF. Working on a oscillator load of 3pF, I figured some trace/pin parasitic C would add at least 1pF to make it about right. If I add the 2 it can never be low enough to achieve 4pF. My current board appears to be running slow which might indicate too much C. With no external C included I cannot subtract any.
I don't suspect the layout has much room for improvement; the crystal is about 0603 size and mounted right square next to the xtal pins. There is 2 footprints for load caps next to that (unpopulated) and the entire section is surrounded by GND plain along with solid plain underneath with a number of via stitches.