Help:External oscillator with PLL

yousef

I noticed that almost all of the propeller boards uses an external crystal of 5 MHz with PLL*16,
is there a reason for the 5 MHz choice? can i use 40 MHz crystal with PLL*2 to achieve 80 MHz ?? or there are some limitations ??

Leon

The PLL won't work with a 40 MHz crystal! 40 MHz crystals operating in fundamental mode are very rare, anyway.

Phil Pilgrim (PhiPi)

If you use a PLL mode, the crystal has to be between 4 and 8 MHz, although 10 MHz crystals are used regularly. 40 MHz is way out of spec.

-Phil

yousef

Thanks, i thought it could handle any crystal

Phil Pilgrim (PhiPi)

It will probably handle a 40 MHz crystal okay with the correct settings, but you just can't use the PLL, since it will not lock. As a result, you won't get the system clock frequency you want.

-Phil