18.4.1 Default start-up clock
In default out of reset, the CPU is clocked from internal Fast IRC (IRC48M). The clocks,
e.g. core clock and bus clock, are programmed via the SCG module. For the default reset
value of divider, please refer to SCG chapter for details.
18.4.2 VLPR mode clocking
The clock dividers should not be changed while in VLPR mode. They must be
programmed prior to entering VLPR mode to guarantee:
• the core/system and bus clocks are less than or equal to 4 MHz
• the flash memory clock is less than or equal to 1 MHz
18.5 Clock Gating
The clock to most of the modules can be individually gated on and off using the PCC
module. After any reset, PCC disables part of the clock to the corresponding module to
conserve power. Prior to initializing a module, set the corresponding clock gating control
bits in PCC register to enable the clock. Before turning off the clock, make sure to disable
the module.
Any bus access to a peripheral that has its bus interface clock disabled (CGC=0 in PCC
module) will generate a bus fault. While any bus access to a peripheral that has its
functional clock disabled (PCS=0 in PCC module) will not return a fault, but the module
cannot work properly.
NOTE
Changes to clock source should be done when clock is gated by
PCC to avoid glitches to output clock.
18.6 Module clocks
The following table summarizes the clocks associated with each module.
Chapter 18 Clock Distribution
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