14 Summary

As described, the dual crystal oscillator and single-ended input clock structure of the TCAN455x poses additional design requirements with regards to the component selection used in this circuit. It is common to see a crystal oscillator circuit that contains only a crystal and two external load capacitors but does not contain a series damping resistor. Omitting the series dampening resistor Rd may or may not be acceptable and increases the risk of causing the clock to stop, fail to start, or overdrive the crystal with excessive power reducing its lifetime due to mechanical stress and increasing the oscillation Vpp amplitude to unacceptable levels.

Relying solely on external load capacitors large enough to keep the oscillation Vpp amplitude away from the single ended clock input detection threshold is possible, but this could result in a small frequency shift if the total load capacitance seen by the crystal exceeds its typical specification

Therefore, it is recommended to add a series damping resistor into the design to limit the oscillation Vpp amplitude without requiring excessive load capacitance. By adding this extra series resistor and taking the time to determine the proper value for all components in the circuit, many problems can be avoided resulting in a reliable and stable circuit design.