Datasheet

PIC24FJ256GA110 FAMILY
DS39905E-page 116  2010 Microchip Technology Inc.
8.1 CPU Clocking Scheme
The system clock source can be provided by one of
four sources:
• Primary Oscillator (POSC) on the OSCI and
OSCO 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 4x PLL. The frequency of
the FRC clock source can optionally be reduced by the
programmable clock divider. The selected clock source
generates the processor and peripheral clock sources.
The processor clock source is divided by two to pro-
duce the internal instruction cycle clock, F
CY. In this
document, the instruction cycle clock is also denoted
by F
OSC/2. The internal instruction cycle clock, FOSC/2,
can be provided on the OSCO I/O pin for some
operating modes of the Primary Oscillator.
8.2 Initial Configuration on POR
The oscillator source (and operating mode) that is used
at a device Power-on Reset event is selected using
Configuration bit settings. The oscillator Configuration
bit settings are located in the Configuration registers in
the program memory (refer to Section 25.1 “Configu-
ration Bits” for further details). The Primary
Oscillator Configuration bits, POSCMD<1:0>
(Configuration Word 2<1:0>), and the Initial Oscillator
Select Configuration bits, FNOSC<2:0>
(Configuration Word 2<10:8>), select the oscillator
source that is used at a Power-on Reset. The FRC
Primary Oscillator with Postscaler (FRCDIV) is the
default (unprogrammed) selection. The Secondary
Oscillator, or one of the internal oscillators, may be
chosen by programming these bit locations.
The Configuration bits allow users to choose between
the various clock modes, shown in Table 8-1.
8.2.1 CLOCK SWITCHING MODE
CONFIGURATION BITS
The FCKSM Configuration bits (Configuration
Word 2<7:6>) are used to jointly configure device clock
switching and the Fail-Safe Clock Monitor (FSCM).
Clock switching is enabled only when FCKSM1 is
programmed (‘0’). The FSCM is enabled only when
FCKSM<1:0> are both programmed (‘00’).
TABLE 8-1: CONFIGURATION BIT VALUES FOR CLOCK SELECTION
Oscillator Mode Oscillator Source POSCMD<1:0> FNOSC<2:0> Note
Fast RC Oscillator with Postscaler
(FRCDIV)
Internal 11 111 1, 2
(Reserved) Internal xx 110 1
Low-Power RC Oscillator (LPRC) Internal 11 101 1
Secondary (Timer1) Oscillator
(SOSC)
Secondary 11 100 1
Primary Oscillator (XT) with PLL
Module (XTPLL)
Primary 01 011
Primary Oscillator (EC) with PLL
Module (ECPLL)
Primary 00 011
Primary Oscillator (HS) Primary 10 010
Primary Oscillator (XT) Primary 01 010
Primary Oscillator (EC) Primary 00 010
Fast RC Oscillator with PLL Module
(FRCPLL)
Internal 11 001 1
Fast RC Oscillator (FRC) Internal 11 000 1
Note 1: OSCO pin function is determined by the OSCIOFCN Configuration bit.
2: This is the default oscillator mode for an unprogrammed (erased) device.