General
Review and verify the following for the custom
schematic design:
- Above sections,
including relevant application notes and FAQ links
- Pin attributes,
signal description, and electrical specifications
- Electrical
characteristics, timing parameters and any additional
available information
- Selection of
processor clock input source, either crystal or
oscillator
- 25MHz is the
clock input frequency currently supported, refer
processor-specific data sheet for supported clock input
frequency
- Selection of
crystal load capacitor versus data sheet
recommendations
- PCB capacitance for WKUP_OSC0 is included in the
calculation of crystal load capacitance value
- When oscillator
is used, add a decoupling capacitor and bulk capacitor near
to the oscillator supply pin
Schematic Review
Follow the below list for the custom schematic
design:
- Connection of 25MHz WKUP_OSC0 clock is
mandatory
- Connections of the crystal circuit (WKUP_OSC0),
as per the data sheet recommendations
- Direct connection
of crystal without series or parallel resistor
- Selection of
crystal load and load capacitance including around 4pF board
capacitance
- Load capacitor is
recommended to be twice the crystal load, including board
capacitance
- Connection of XO
when external oscillator is used, ground XO
Additional
- Refer to the Applications, Implementation, and Layout section of the data
sheet for clock routing guidelines
- Select crystal and load capacitor such that the load capacitor value can be a
standard value
- Connect the 25MHz crystal directly to the processor XI and XO pins, no series or
parallel resistors are recommended. The internal oscillator implements Automatic
Gain Control (AGC) for amplitude control
- The processor is validated only with a 25MHz
(only frequency currently supported) clock source.
- Processor-specific data sheet shows that
WKUP_OSC0 does not start until the core voltage ramps
because there are some cases where the oscillator does not
start until the VDD_CORE ramps. In most cases the oscillator
start when VDDS_OSC0 ramps, although oscillator start when
VDDS_OSC0 ramps is not always the case. The oscillator
start-up diagram in the data sheet shows the maximum
start-up time, which includes the case where the delay is
based on VDD_CORE is valid
- Recommendation is to retain the HFOSC0 registers
in the default state
- Refer processor-specific data
sheet to select the crystal circuit components