Concerto has two pins (X1 and XCLKIN) into which a single-ended clock can be driven from external oscillators or other clock sources. When connecting an external clock source through the X1 terminal, the X2 terminal should be left unconnected. Most internal clocks of this device are derived from the X1 clock input (or X1/X2 crystal) . The XCLKIN clock is only used by the USB PLL and CAN peripherals. Figure 6-7 shows how to connect external oscillators to the X1 and XCLKIN terminals.
Locate the external oscillator as close to the MCU as practical. Ideally, the return ground trace should be an isolated trace directly underneath the forward trace or run adjacent to the trace on the same layer. Spacing should be kept minimal, with any other nearby traces double-spaced away, so that the electromagnetic fields created by the two opposite currents cancel each other out as much as possible, thus reducing parasitic inductances that radiate EMI.