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I recently started working with PIC micro controllers, on a project to make my own joystick. Since PIC18F2550 controllers support a USB protocol they were my first choice, but what fascinated me more than the PIC is that, USB 1.0 communication requires a clock rate of 48MHz which to my knowledge crystal oscillators cannot provide, instead the crystal oscillator output of 16MHz is scaled up to 48MHz, using a circuit called A phase locked loop. This circuit combines a VCO, frequency divider to have multiple frequency locks, a phase comparator using OpAmps (I hate EC's Logic gates) and a low pass filter.
The VCO frequency is varied until the phase difference is nullified this is called a frequency lock, after this point the PLL maintains this frequency.
One problem I'm having with this circuit is when the frequency of the VCO output (from the asynchronous counter) fed to the phase comparative is changed the frequency was supposed to change, don't know why. Any suggestions ???
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