Recommendations for alternative crystals

[MANGO]

Hi, I use the nRF52832 board (PCA10040 v1.1).

Before creating a custom PCB that uses nRF52832-QFAA, I chose

Q22FA12800025 FA-128 32 MHZ 8.0PF (load cap = 8pF) and

FC-135 32.7680KA-AC (load cap = 9pF); the ones that are shown in the PCA10040's BOM.

However, due to my local Epson distributor's lead time,

the distributor recommended me an alternative crystal that has different load cap values.

The recommendation was to use

FA-128 32.0000MF20X-K5 (load cap = 10pF)

FC-135 32.7680KA-A5 (load cap = 12.5pF)

So my questions are,

0. Is it fine choosing these alternatives when using nRF52832-QFAA based 1.6mm thick, FR-4, and 2 layered PCB?

1. What would be the cap value when using the alternatives?

One confusing thing was the PCA10040 used 12pF cap when using Q22FA12800025.

Why was the cap value 12pF and what value should I use when I choose the alternatives?

-Best Regards

[bjorn-spockeli]

Hi Mango922,

Yes, as long as the crystals meet the +/- 40ppm accuracy requirement then the recommended crystals should be fine.

The capacitor values are calculated according to the formula in this answer.

Best regards

Bjørn

[MANGO]

I see. C1 = C2 = C_L * 2 - 4pF

Then in case of

0. FA-128 32.0000MF20X-K5 (load cap = 10pF) -> C1 = C2 = 16pF

1. FC-135 32.7680KA-A5 (load cap = 12.5pF) -> C1 = C2 = 21pF

Is this correct?

I guess that 4pF value is explained in this blog (General PCB design guidelines for nRF52).

However, does this equation works regardless of what layers of PCB (2 or 4), PCB thickness, or dielectric (in my case FR-4)?

I'm trying to create a custom PCB that has nRF52832-QFAA, 2 layers, 1.6mm thickness, and FR-4 dielectric.

[bjorn-spockeli]

Yes, the formula should work regardless of how many layers, which thickness and dielectric you have. Using the formula with 2 layers, 1.6mm thickness, and FR-4 dielectric should not be an issue.

[MANGO]

Hi, Bjørn. Just in case, the Q22FA12800025 FA-128 32 MHZ 8.0PF, I have one more to check.

That 32MHz crystal's load capacitor is 8pF. The PCA10040 schematic populated 12pF due to the formula, right?

8*2 - 4 = 12pF

I just want to double check whether the formula works on both high/low crystal products.

[bjorn-spockeli]

@mango: Yes, that is correct.