PIC24FJ16MC101_12 Datasheet, PDF (102/350 Pages) Microchip Technology – 16-bit Microcontrollers (up to 32 KB Flash and 2 KB SRAM)

PIC24FJ16MC101/102 AND PIC24FJ32MC101/102/104
8.1 CPU Clocking System
The PIC24FJ16MC101/102 and PIC24FJ32MC101/
102/104 devices provide seven system clock options:
• Fast RC (FRC) Oscillator
• FRC Oscillator with 4x PLL
• Primary (MS, HS or EC) Oscillator
• Primary Oscillator with 4x PLL
• Secondary (LP) Oscillator
• Low-Power RC (LPRC) Oscillator
• FRC Oscillator with postscaler
8.1.1 SYSTEM CLOCK SOURCES
8.1.1.1 Fast RC
The Fast RC (FRC) internal oscillator runs at a nominal
frequency of 7.37 MHz. User software can tune the
FRC frequency. User software can optionally specify a
factor (ranging from 1:2 to 1:256) by which the FRC
clock frequency is divided. This factor is selected using
the FRCDIV<2:0> (CLKDIV<10:8>) bits.
The FRC frequency depends on the FRC accuracy
(see Table 26-18) and the value of the FRC Oscillator
Tuning register (see Register 8-3).
8.1.1.2 Primary
The primary oscillator can use one of the following as
its clock source:
• MS (Crystal): Crystals and ceramic resonators in
the range of 4 MHz to 10 MHz. The crystal is
connected to the OSC1 and OSC2 pins.
• HS (High-Speed Crystal): Crystals in the range of
10 MHz to 32 MHz. The crystal is connected to
the OSC1 and OSC2 pins.
• EC (External Clock): The external clock signal is
directly applied to the OSC1 pin.
8.1.1.3 Secondary
The secondary (LP) oscillator is designed for low power
and uses a 32.768 kHz crystal or ceramic resonator.
The LP oscillator uses the SOSCI and SOSCO pins.
8.1.1.4 Low-Power RC
The Low-Power RC (LPRC) internal oscIllator runs at a
nominal frequency of 32.768 kHz. It is also used as a
reference clock by the Watchdog Timer (WDT) and
Fail-Safe Clock Monitor (FSCM).
8.1.1.5 PLL
The clock signals generated by the FRC and primary
oscillators can be optionally applied to an on-chip 4x
Phase-Locked Loop (PLL) to provide faster output
frequencies for device operation. PLL configuration is
described in Section 8.1.3 “PLL Configuration”.
8.1.2 SYSTEM CLOCK SELECTION
The oscillator source 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 23.1 “Configuration
Bits” for further details.) The Initial Oscillator
Selection Configuration bits, FNOSC<2:0>
(FOSCSEL<2:0>), and the Primary Oscillator Mode
Select Configuration bits, POSCMD<1:0>
(FOSC<1:0>), select the oscillator source that is used
at a Power-on Reset. The FRC primary oscillator is
the default (unprogrammed) selection.
The Configuration bits allow users to choose among 12
different clock modes, shown in Table 8-1.
The output of the oscillator (or the output of the PLL if
a PLL mode has been selected) FOSC is divided by 2 to
generate the device instruction clock (FCY) and the
peripheral clock time base (FP). FCY defines the
operating speed of the device, and speeds up to 40
MHz are supported by the PIC24FJ16MC101/102 and
PIC24FJ32MC101/102/104 architecture.
Instruction execution speed or device operating
frequency, FCY, is given by:
EQUATION 8-1: DEVICE OPERATING
FREQUENCY
FCY = ^F----^O----^S---^C--
2
DS39997C-page 102
Preliminary
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