PIC32MX440F256H-80I Datasheet, PDF (71/646 Pages) Microchip Technology – 64/100-Pin General Purpose and USB 32-Bit Flash Microcontrollers

4.2 Operation: Clock Generation and
Clock Sources
The PIC32MX3XX/4XX device has two internal clocks:
CPU clock and PB clock. They are derived from the
currently selected clock source. The clock source can
be chosen from the 4 available internal or external
clock sources. Some of these clock sources have
Phase Locked Loops (PLLs), programmable output
dividers, or input dividers to scale the input frequency
to suit the application. The clock source can be
changed on-the-fly by software. The oscillator control
register is locked by hardware, it must be unlocked by
a series of writes before software can perform a clock
switch.
There are three main clocks in the PIC32MX3XX/4XX
device:
• The System Clock (SYSCLK) used by CPU and
some peripherals
• The Peripheral Bus Clock (PBCLK) used by most
peripherals
• The USB Clock (USBCLK) used by USB
peripheral
The PIC32MX3XX/4XX clocks are derived from one of
the following sources:
• Primary Oscillator (POSC) on the OSCI and
OSCO pins
• Secondary Oscillator (SOSC) on the SOSCI and
SOSCO pins
• Internal Fast RC Oscillator (FRC)
• Internal Low-Power RC Oscillator (LPRC)
Each of the clock sources has unique configurable
options, such as a PLL, input divider and/or output
divider, that are detailed in their respective sections.
There are up to four internal clocks, depending on the
specific device. The clocks are derived from the
currently selected oscillator source.
Note:
Clock sources for peripherals that use
external clocks, such as the RTCC and
Timer 1, are covered in their respective
sections.
PIC32MX3XX/4XX
© 2008 Microchip Technology Inc.
Preliminary
DS61143E-page 69