>I'm guessing you really mean an 18F452 as I don't think an 18F542 even exists?  
>
Right.

>In which case you cannot use a 40MHz crystal, as the 452 supports a maximum frequency of 25MHz in HS mode.  
>
I did not read the docs carefully enough. I see now that 18F452 does not 
support FOSC>25MHz, however it was working at 32MHz for me in another 
project.

:-D

  Les

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