Figure 7-34 shows the recommended crystal circuit. It is recommended that preproduction printed-circuit board (PCB) designs include the two optional resistors Rbias and Rd in case they are required for proper oscillator operation when combined with production crystal circuit components. In most cases, Rbias is not required and Rd is a 0-Ω resistor. These resistors may be removed from production PCB designs after evaluating oscillator performance with production crystal circuit components installed on preproduction PCBs.
Table 7-31 presents LFXOSC modes of operation.
|ACTIVE||0||0||XTAL||XTAL||CLK_OUT||Active oscillator mode providing 32kHz|
|PWRDN||0||1||X||PD||LOW||Output will be pulled down to LOW. PAD to be tri-stated. Active mode disabled|
|BYPASS||1||X||CLK||PD||CLK||XI is driven by external clock source. XO is pulled down to LOW. Due to ESD diode to supply, XI should not be driven unless oscillator supply is present.|
User should set CTRLMMR_WKUP_LFXOSC_TRIM[18:16] i_mult = 3b’001 for CL in the range 6pf to 9.5pf. CTRLMMR_WKUP_LFXOSC_TRIM [18:16] i_mult = 3b’010 for CL in the range 8.5pf to 12pf. Default setting is 3b’010.
The load capacitors, Cf1 and Cf2 in Figure 7-35, 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 WKUP_LFOSC0_XI, WKUP_LFOSC0_XO, and VSS pins.
The crystal must be in the fundamental mode of operation and parallel resonant. Table 7-32 summarizes the required electrical constraints.
|fp||Parallel resonance crystal frequency||32768||Hz|
|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|
|Cshunt||Shunt capacitance||ESRxtal – 40 Ω||4||pF|
|ESRxtal – 60 Ω||3||pF|
|ESRxtal – 80 Ω||2||pF|
|ESRxtal – 100 Ω||1||pF|
|ESR||Crystal effective series resistance||100||Ω|
When selecting a crystal, the system design must consider the temperature and aging characteristics of a based on the worst case environment and expected life expectancy of the system.
Table 7-33 details the switching characteristics of the oscillator and the requirements of the input clock.