The difference between a driven reference and a crystal reference is that there needs to be enough gain in order for the crystal to startup correctly. With a driven reference we don't need to worry about this, since the oscillation is already started up by an external device.
Not only do we enable capacitors in order to match the Cload of the crystal, but we also change the crystal oscillator drive strength to guarantee startup. With an external driven reference, we minimize the capacitance and set the oscillator drive strength according to the input frequency.
CyberClocks software will automatically choose the oscillator drive strength level based on if you have a driven or crystal reference, the parallel load capacitance of the crystal, and the reference frequency. For best results, a fundamental mode, parallel resonant crystal should be used. We have our Crystal Parameters Recommendation for Cypress Frequency Synthesizers whitepaper available on our website that describes the recommended reference inputs for Cypress?s PLL-based frequency synthesizers, and concludes with an error budget analysis.
Following are the Crystal Specifications:
6pF-30pF capacitance (18pF typical).
drive level 500uW typical, max 2mW.
ESR < 25ohms.
Commercial or Industrial temperature depending on application.