XTL 12Mhz CAP matching (revisit/check)

Community PCB Design
1

Hello all,

I posted a while back about matching my CAPS to my Crystal…

I wanted to repost the question to see I have thought about this correctly.

Overall I’m looking to match an LPC23768 to an ABM3B Crystal.

LPC Info from the NXP LPC Family Guide Info:

Now, in the NXP doc UM10160 LPC2101/02/03 System Ctrl (note this is also refered to when looking at the system docs on the LPC2368).It shows this data:

Fundamental oscillation frequency (Fosc)

Crystal load capacitance (Cl)

Max Crystal series resistance (Rs)

External load caps (C-X1,2)

NXP Chart 1:

(edited for only the Fosc of 10-15Mhz)

With Fosc: 10Mhz - 15Mhz

a → Cl 10 pF, Rs <300 Ohms, CX1=18pF, CX2=18pF

b → Cl 20 pF, Rs <220 Ohms, CX1=38pF, CX2=38pF

c → Cl 30 pF, Rs <140 Ohms, CX1=58pF, CX2=58pF

Pointer to the crystal:

http://www.abracon.com/Resonators/abm3b.pdf

My thoughs is that I should be looking at the either 38pF or 18pF…

I’m consdiering the 38pF CAP… will that work?

Or, can I pick something inbetween?

Any thoughts are greatly apprecaited.

Thanks,

Brett

2

The capacitor values depend on the rated load capacitance of the crystal. Basically, you subtract a few pF (to account for the stray capacitance of your PCB), then double it (to account for the fact that the two caps are in series from the point of view of the crystal). You cannot specify the capacitors until you’ve chosen a specific crystal.

Note that the exact capacitor values are only critical if you want to get the full rated accuracy of the crystal, which is probably only important if you’re using it for timekeeping. If you only need timing that’s accurate enough for something like RS-232 serial (which can tolerate several percent of error), then any value in low pF range will probably work. (If you really can live with only 0.5% accuracy, you might want to consider a ceramic resonator instead of a crystal - they’re generally cheaper and more rugged, and are available with the two caps built in.)

3

I like the idea ofthe Cermc Resinator but will the work for a USB connection?

Also, I did some closer reading… the max pF for the 12Mhz Crystal is 32pF… so I may not be able to use the 38pF CAP…

4

I don’t think a resonator is quite good enough to satisfy the USB spec - it’s the kind of thing that would usually work, but a crystal would be safer.