AVR64E: 32kHz osc doesn't start without a prod

[nali]

I have an AVR64EA48 fitted with a 32.768kHz crystal for low-power sleep operation. But for some reson the 32k oscillator won't start unless it is loaded with a lot more capacitance then it should have: touching a x10 scope probe starts it, or increasing the loading caps to >20pF (currently 24pF as 2x12pF piggybacked as that's all I had in stock)

The XTAL I'm using is ECS-.327-7-34B-TR (7pF) and I'm using 6.8pF loading caps which as far as I can work out is the closest standard value. The Curiosity Nano dev board also has a 7pF xtal and uses asymmetric loading of 6.8pF & 5.6pF.

The traces between MCU & XTAL are only about 3mm, and they both sit above a solid ground plane on a 2-sided PCB.

Any suggestions where I should start looking?

[RoGeorge]

AVR series used to have fuse-bits to set the oscillator type for low-power/low-frequency, don't recall the exact name they were using.  And the fuse bits are active low.

Are the MCU fuses programmed for low-frequency oscillator?

[selcuk]

There is a XOSC32KCTRLA.LPMODE flag in the datasheet. Default is 0 and that expects a 18pF load capacitance. Do you set it to 1 before enabling 7pF crystal?

[nali]

Looks like setting LPMODE sorts it (well so far based on my sample of 1 anyway).

I missed the dual 8pF/18pF spec in Electrical Characteristics as I was looking at the Nano HW guide. I take it that means use a XTAL with a load spec of 18pF (which would result in load caps of 27-30pF)

Anyways working now thanks