Crystal load capacitor value selection

[FreshmanNewbie]

I'm using this USB2513 hub with this 24MHz oscillator.

By mistake I had placed 1Meg resistor across the crystal. In some boards, even with the resistor, it is working and in some boards it does not.

In one non-working board, I removed the 1Meg resistor and it started to work.
I just wanted to ensure whether my crystal circuit components are fine. Below are the component details:

1. Oscillator: ECS Part Number ECS-240-10-36-CKM-TR; This has a specified load capacitance of 10pF and shunt capacitance of 5pF.
2. C1=C2=18pF
3. Trace Length from XTALIN to C1 = 0.287"
4. Trace Length from XTALOUT to C2 = 0.351"
5. Both XTAL traces are routed over a ground plane on the next layer down. The thickness of the prepreg is 3.4 mils.

Please confirm if the values for C1 and C2 (18pF) would be expected to be correct for our application or whether we should consider slightly different values.

Edit:
Also, any idea how the Hub works even if some boards have the 1Meg resistor across the crystal?

 

[KlausST]

Hi,

7.2 gives the math and the explanation. Did you do the math?
If not: Why not? --> please do so.

Is it asking too much for your_schematic and your_PCB layout?

Klaus

 

[FreshmanNewbie]

I'm using this ECS-240-10-36-CKM-TR part with this IC

The load capacitance mentioned in the crystal datasheet is 10pF and the shunt capacitance is 5pF.

In the design, the load capacitors that I have placed with the IC are 18pF and the design works fine. A stray capacitance of 2pF is considered for the calculation.

I made this load capacitor calculation considering the below formula from the datasheet.

What I noticed is that, even without considering the shunt capacitance value of 5pF (as mentioned in the crystal datasheet), I calculated the value of load capacitors which turned out to be 18pF, and started working with this design.

Considering the shunt capacitance (5pF) of the crystal in the formula, the value of the crystal load capacitors comes to around 8pF. But I placed 18pF. There is a significant difference, right?

Now, my question is, the equation from the datasheet mentions including the shunt capacitance in the formula for load capacitor calculation. Whereas, I calculated the value of load capacitors without considering the shunt capacitance. But the design seems to be working.

Would there be any problem in the future?

 

[FvM]

An exact calculation of crystal load capacitance is important for applications with tight frequency tolerance. That's not the case for USB hub with huge 350 ppm tolerance. The frequency deviation with your capacitor values is 10 - 20 ppm maximal.