PIC24FJ256GB108-I Datasheet, PDF (122/352 Pages) Microchip Technology – 64/80/100-Pin, 16-Bit Flash Microcontrollers with USB On-The-Go (OTG)

PIC24FJ256GB110 FAMILY
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 USB PLL block, which
generates both the USB module clock and a separate
system clock from the 96 MHZ PLL. Refer to
Section 8.5 “Oscillator Modes and USB Operation”
for additional information.
The Fast Internal FRC provides an 8 MHz clock
source. It can optionally be reduced by the programma-
ble 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 two to produce the internal instruction cycle
clock, FCY. In this document, the instruction cycle clock
is also denoted by FOSC/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 26.1 “Configu-
ration Bits” for further details). The Primary Oscillator
Configuration bits, POSCMD<1:0> (Configuration
Word 2<1:0>), and the Initial Oscillator Select Configu-
ration 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>
Fast RC Oscillator with Postscaler
Internal
11
111
(FRCDIV)
(Reserved)
Internal
xx
110
Low-Power RC Oscillator (LPRC)
Internal
11
101
Secondary (Timer1) Oscillator
Secondary
11
100
(SOSC)
Primary Oscillator (XT) with PLL
Primary
01
011
Module (XTPLL)
Primary Oscillator (EC) with PLL
Primary
00
011
Module (ECPLL)
Primary Oscillator (HS)
Primary
10
010
Primary Oscillator (XT)
Primary
01
010
Primary Oscillator (EC)
Primary
00
010
Fast RC Oscillator with PLL Module
Internal
11
001
(FRCPLL)
Fast RC Oscillator (FRC)
Internal
11
000
Note 1: OSCO pin function is determined by the OSCIOFCN Configuration bit.
2: This is the default oscillator mode for an unprogrammed (erased) device.
Note
1, 2
1
1
1
1
1
DS39897C-page 122
 2009 Microchip Technology Inc.