General
Review and verify the following for the custom
schematic design:
- Reviewed above "Common
checklist for all sections" section of the user's guide.
- Selection of WKUP_LFOSC0 clock source - external crystal + internal
oscillator or external oscillator.
- Selection of WKUP_LFOSC0 external crystal frequency.
- Selection of crystal load capacitor.
- Connection of series and parallel resistors to the crystal base oscillator
circuit.
- Selection of external oscillator and connection of capacitor and series
resistor.
- Connection of XO when external oscillator output is connected to XI.
- Connection of XI and XO when WKUP_LFOSC0 is not used.
Schematic Review
Follow the below for the custom
schematic design:
- WKUP_LFOSC0 clock input frequency supported is 32.768kHz.
- Connections of the clock circuit (WKUP_LFOSC0), as per the
processor-specific data sheet recommendations.
- Selection of crystal load and load capacitance value (follow the
processor-specific data sheet), with the load capacitance being x2 of the
crystal load (PCB capacitance is not included).
- When external oscillator is used, the recommendation is to add decoupling
capacitor and a bulk capacitor near to the oscillator supply pin and series
resistor on the clock output pin.
- Connection of XO when external oscillator is used (XO is grounded).
- Connection of the XI input when the WKUP_LFOSC0 is not used (XI is
grounded).
- Connection of series and parallel resistors for initial prototype
(preproduction) and production boards (can be removed) as per
processor-specific data sheet requirements.
Additional
- Crystal load capacitance versus
LFOSC0 registers. The only LFOSC0 register bits custom board designers change
are BP_C, PD_C, and CTRLMMR_WKUP_LFXOSC_TRIM[18:16], where PD_C is reset (0) to
enable the oscillator and the BP_C bit is only set (1) to place the oscillator
in bypass mode when using an LVCMOS clock source. The
CTRLMMR_WKUP_LFXOSC_TRIM[18:16] bits are set based on the actual capacitance
load applied to the crystal, as defined by the Load Capacitance
Equation.
- Refer to the processor-specific
data sheet for the recommended circuit configuration (passives) during
preproduction PCB and the production PCB.
- WKUP_LFOSC0 has limited use
cases, the recommendation is to provide provision to ground the XI input when
the clock option is not used.