Today, we will have a look at the crystal interface configuration of the VersaClock 5.
This is a VersaClock 5 oscillator with its XIN and XOUT pins. Virtually any fundamental note crystal can be connected to the XIN and XOUT pins as shown.
The VersaClock 5 features internal tuning capacitors whose capacitance can be programmed to a value matching the crystal. To determine the tuning capacitance value, first look for the CL value in your crystal data sheet. In this example, CL is 10 pF. The internal tuning capacitors of the VersaClock 5 have to match this value. The tuning capacitors consist of fixed capacitors of 9 pF each plus variable capacitors in the range of 0 to 16 pF. The fixed capacitors being parallel have an equivalent value of 4.5 pF.
To match our 10 pF CL, the variable capacitors need to be set to a value of 5.5 pF. Each internal variable capacitor of the VersaClock 5 will have an actual value of 11 pF.
If you use the VersaClock 5 custom part configuration utility, just enter this 5.5 pF value into the relevant field as shown here.
Sometimes, the CL of the crystal exceeds the tuning capabilities of the VersaClock 5 internal capacitors. This is the case when CL is greater than 12.5 pF. In this case, external capacitors need to be added to the board as shown. For maximum tuning range, choose the value of the external tuning capacitor with the programmable internal capacitor set to the middle of the range, which is an equivalent capacitance of 4 pF.
Let's consider the following example, CL is 18 pF, the internal fixed capacitors of the VersaClock 5 are as we know 4.5 pF equivalent, and the internal variable capacitors have been set to an equivalent value of 4 pF. The external capacitor need an equivalent value of 18 minus 4.5 minus 4 which is 9.5 pF. This translates to an individual capacitance of 19 pF for each capacitor.
We hope this training has been helpful. Don't hesitate to contact IDT for any further questions you might have.