7.1.3 USS Oscillator (USSXT)

Depending on the device variant (see Section 3), the device with USS module supports a high-frequency crystal on the USSXT pins. External bypass capacitors for the crystal oscillator pins are required. Serially connect a 22-Ω resistor close to the USSXTOUT pin (see Figure 7-4). The USSXT does not support bypass mode, so it is not possible to apply digital clock signals to the USSXTIN pin. Never connect the USSXTIN pin to a power supply line (AVCC, DVCC, or PVCC). If the USSXT pins are not used, terminate them according to Section 4.6.

Figure 7-4 shows a typical connection diagram.

Figure 7-4 Typical Crystal Connection

Consider the following items for the USSXT layout:

• Keep the trace of USSXTIN and USSXTOUT as short as possible. If one must be longer than the other, keep USSXTIN shorter, because USSXTIN is more sensitive to EMI.
• Make the ground shield open ended without making a loop.
• Use a ground plane to reduce the impedance of the ground trace.
• If USSXT_BOUT is used, keep coupling to USSXTIN and CH0_IN to a minimum.
• If USSXT_BOUT is feeding other clock or device inputs, apply a small capacitor (10 pF) as the line termination load at the end of the line. This avoids reflection artifacts on sensitive inputs (for example, HFXTIN).

Figure 7-5 shows the recommended PCB layout.

Figure 7-5 USSXT PCB Layout Recommendation