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 LFOSC0 clock source - external crystal + internal
oscillator or external oscillator. (use any one
option on the board for optimizing the placement and
routing)
- Selection
of LFOSC0 external crystal frequency
- Selection
of crystal load capacitors and value
- Connection of series and parallel resistors for
crystal-based oscillator circuit implementation
- 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 pin when the LFOSC0 is not used
(XI is grounded, XO Unconnected)
- Connection of bulk and decoupling capacitors for
external oscillator supply pin and connection of
oscillator EN pin
Schematic
Review
Follow the below for the
custom schematic design:
- LFOSC0
clock input frequency supported is 32.768kHz.
- Connections of the clock circuit (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 connected to ground).
- Connection of the XI input when the LFOSC0 is not
used (XI is connected to ground, XO is left
unconnected).
- 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.
- LFOSC0 has
limited use cases, the recommendation is to provide
provision to ground the XI input when the clock option is
not used.