View Full Version : Help:External oscillator with PLL

10-28-2011, 07:04 PM
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 ??

10-28-2011, 07:24 PM
The PLL won't work with a 40 MHz crystal! 40 MHz crystals operating in fundamental mode are very rare, anyway.

Phil Pilgrim (PhiPi)
10-28-2011, 07:25 PM
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.


10-28-2011, 07:54 PM
Thanks, i thought it could handle any crystal :tongue:

Phil Pilgrim (PhiPi)
10-28-2011, 08:01 PM
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.