Help: Making the PIC18F252 work at 40 MHz

[fmartinezs]

Hi:

I've been working with the 18F252 for a while, always using a 20 MHz crystal. Now i need to speed up some routines, and besides improving the code, i want to use a higher clock. The manual states that this PIC can work up to 40 MHz; it implies that using a 10 MHz clock and a internal 4x PLL the PIC can work internally at 40 MHz. Is this the only way to make it work at this speed? Can it work directly with a 40 MHz resonator?

If you can give me a hand, i'll ve really grateful.

Best regards,
Fernando

[newguy]

It can't work directly with a 40 MHz crystal/resonator. You can only use crystals up to and including 20 MHz. For operation above 20 MHz you have two options: use a crystal whose frequency is 1/4th your desired frequency and use the PIC's internal x4 PLL or use an external clock souce at your desired frequency to directly drive the PIC's OSC1/CLKI pin. The #fuse for this option is EC or EC_IO (usually).

[fmartinezs]

Thanks!

I'll guess i'll try and find some 10 MHz xtals then.

Best regards,
Fernando