This controlCARD is required to support a broad range of TI's baseboards. Several designs rely on GPIO18 and GPIO19 for SPIA , while others require these GPIO to be utilized as a precision clock input source. To accommodate both of these systems a switch (S3) has been added to the design. This methodology should not be used in a final system as it increases EMI emissions and creates robustness susceptibilities. It is up to the system designer to choose the best way to implement the clocking circuity for a given system.