CyClocksRT Crystal Capacitance Calculation for CY22381, CY22392, CY22393, CY22394 and CY22395

Question: How does CyClocksRT calculate cyrstal capacitance for the CY22381, CY22392, CY22393, CY22394 and CY22395?  Should I account for board capacitance when entering a value?

 

Answer:

When configuring clock generators for use with a crystal, CyClocksRT asks the user to enter a value for the crystal load capacitance rating (CL).  It then calculates the appropriate device settings so that the crystal sees the correct load.
 

For the devices listed here, CyClocksRT does not assume any board capacitance.  It configures the chip to provide exactly the capacitance value entered by the user.  (CL is the "net" capacitance the crystal wants to see.  The capacitance on each individual pin of the crystal would be 2*CL).

Because the software does not adjust the chip settings to account for board capacitance, the user must subtract the board capacitance from the crystal CL spec and enter this adjusted value into the software.

Example: If you will use a crystal with CL = 18 pF (36 pF per pin), and you estimate (or measure) your net board capacitance to be 2 pF (4 pF per pin), you should enter 16 pF into CyClocksRT as the crystal capacitance value.

Note that assumptions about board capacitance are not the same for all clock generator products.