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 MCU_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
Have you followed the below for the
custom schematic design:
- Connection of 25MHz MCU_OSC0
clock is mandatory
- Connections of the crystal
circuit (MCU_OSC0), as per the data sheet recommendations
- Direct connection of crystal
without series or parallel resistor
- Selection of crystal load and
load capacitance including approximately 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 the 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 with a 25MHz clock source, 25MHz is the only
frequency currently supported
- The data sheet shows that
MCU_OSC0 does not start until the core voltage ramps because there are some
cases where the oscillator may not start until VDD_CORE ramps. In most cases the
oscillator start when VDDS_OSC0 ramps, although oscillator start when VDDS_OSC0
ramps may not always be 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
- Changing the HFOSC0 registers is
not required. The registers remain in a default state
- Refer processor-specific data
sheet to select the crystal circuit components