An external crystal is connected to the device pins. Figure 6-10 describes the crystal implementation.
The load capacitors, Cf1 and Cf2 in Figure 6-10, should be chosen such that the below equation is satisfied. CL in the equation is the load specified by the crystal manufacturer. All discrete components used to implement the oscillator circuit should be placed as close as possible to the associated oscillator rtc_osc_xi_clkin32 and rtc_osc_xo pins.
The crystal must be in the fundamental mode of operation and parallel resonant. Table 6-9 summarizes the required electrical constraints.
|fp||Parallel resonance crystal frequency||32.768||kHz|
|Cf1||Cf1 load capacitance for crystal parallel resonance with Cf1 = Cf2||12||24||pF|
|Cf2||Cf2 load capacitance for crystal parallel resonance with Cf1 = Cf2||12||24||pF|
|CO||Crystal shunt capacitance||5||pF|
|LM||Crystal motional inductance for fp = 32.768 kHz||10.7||mH|
|CM||Crystal motional capacitance||2.2||fF|
|tj(rtc_osc_xi_clkin32)||Frequency accuracy, rtc_osc_xi_clkin32||±200||ppm|
When selecting a crystal, the system design must take into account the temperature and aging characteristics of a crystal versus the user environment and expected lifetime of the system.
Table 6-10 details the switching characteristics of the oscillator and the requirements of the input clock.