[layout] 32KHz crystal opposite side of the nrf (via)

[Tizana]

hi,

i'm using the 32KHZ external crystal and i didn't find any free space in my PCB to put the crystal in the same side with the nrf chip, so the only solution that i found is to put the crystal in the opposite side and use VIA to connect it to the nrf chip.

is this bad practice to connect the crystal like this way ? if yes what are the consequences ?

above is a screen shot of the crystal connection through VIA ( the layout is not finished yet and no ground panel created yet)

thx,

[RK]

The vias will add extra load capacitance, so you'll have to experiment to adjust the load caps (probably downwards) to get the right frequency. But as long as the traces are short and the vias large enough, it shouldn't be that bad.

[Tizana]

adding the load capacitance mean the Cstray will increase ? is that what you mean ?

so i have to adjust the cap C9 and C8 ? how can i determine the Cstray ?

[RK]

trial and error after you've built one.

[butch]

RK: you mean use a frequency counter while running, and swap out various capacitances ?

Tizana: move your vias to the left to shorten the traces. I don't think there is anything inherently wrong with using vias. Using vias and the back side xtal, you could make your traces shorter than a front side design.

Doesn't the stray or parasitic capacitance depend on the length of traces and distance to ground? I think standard advice is to keep the area around the crystal free of ground plane. You don't show your ground connections or ground plane, which could be important.

Unless you are intending to keep real time (same as someone else's clock) it is not critical. If your design only works with itself, then the fact that all your boards are identical means their RTC clocks will have the same error.