The crystal circuit must also be designed such that it does not exceed the maximum shunt capacitance for OSC1 operating conditions defined in Table 7-29. Shunt capacitance, Cshunt, of the crystal circuit is a combination of crystal shunt capacitance and parasitic contributions. PCB signal traces which connect crystal circuit components to OSC1 have mutual parasitic capacitance to each other, CPCBXIXO, where the PCB designer should be able to extract mutual parasitic capacitance between these signal traces. The device package also has mutual parasitic capacitance, CXIXO, where this mutual parasitic capacitance value is defined in Table 7-30.
PCB routing should be designed to minimize mutual capacitance between XI and XO signal traces. This is typically done by keeping signal traces short and not routing them in close proximity. Mutual capacitance can also be minimized by placing a ground trace between these signals when the layout requires them to be routed in close proximity. It is important to minimize the mutual capacitance on the PCB to provide as much margin as possible when selecting a crystal.
A crystal should be chosen such that the below equation is satisfied. CO in the equation is the maximum shunt capacitance specified by the crystal manufacturer.
Cshunt ≥ CO + CPCBXIXO + CXIXO
For example, the equation would be satisfied when the crystal being used is 25 MHz with an ESR = 30 Ω, CPCBXIXO = 0.04 pF, CXIXO = 0.01 pF, and shunt capacitance of the crystal is less than or equal to 6.95 pF.