Microchip Technology DM183037 Data Sheet

DS30575A-page 38

 2012 Microchip Technology Inc.

The system clock source can be provided by one of
four sources:
• Primary Oscillator (POSC) on the OSC1 and 

OSC2 pins

• Secondary Oscillator (SOSC) on the SOSCI and 

SOSCO pins

• Fast Internal RC (FRC) Oscillator
• Low-Power Internal RC (LPRC) Oscillator

The Primary Oscillator and FRC sources have the option
of using the internal USB PLL block, which generates
both the USB module clock and a separate system clock
from the 96 MHz PLL. Refer to 

 for additional

information.
The internal FRC provides an 8 MHz clock source. It
can optionally be reduced by the programmable clock
divider to provide a range of system clock frequencies.
The selected clock source generates the processor
and peripheral clock sources. The processor clock
source is divided by four to produce the internal instruc-
tion cycle clock, F

CY

. In this document, the instruction

cycle clock is also denoted by F

OSC

/4. The internal

instruction cycle clock, F

OSC

/4, can be provided on the

OSC2 I/O pin for some operating modes of the Primary
Oscillator. The timing diagram in 

 shows the

relationship between the processor clock source and
instruction execution.

F

OSC

PC

F

CY

PC

PC + 2

PC + 4

Fetch INST (PC)

Execute INST (PC – 2)

Fetch INST (PC + 2)

Execute INST (PC)

Fetch INST (PC + 4)

Execute INST (PC + 2)

T

CY