In SX Microcontrollers, SX/B Compiler and SX-Key Tool, Beau Schwabe wrote:

That's pretty good...  For stuff like this it's always good to use a crystal that is a multiple of the frequency you are looking for.  In this case you are doing just that, 5.12MHz is 128 times 40kHz.
If you don't like the idea of having a +/- 10Hz variance, you might try a crystal that's twice the value you are currently using or another multiple of 40kHz.  Doubling your crystal should at least
get you within +/-5Hz, but please realize that there are other factors that can cause this... i.e. the scope itself may not be able to resolve that particular frequency accurately due to a "picket fence"
effect creating a beat frequency in the least significant bit on the oscilloscope.

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