70592C Datasheet, PDF (128/314 Pages) Microchip Technology – High-Performance, 16-bit Microcontrollers

PIC24HJXXXGPX06A/X08A/X10A
9.1 CPU Clocking System
There are seven system clock options provided by the
PIC24HJXXXGPX06A/X08A/X10A:
• FRC Oscillator
• FRC Oscillator with PLL
• Primary (XT, HS or EC) Oscillator
• Primary Oscillator with PLL
• Secondary (LP) Oscillator
• LPRC Oscillator
• FRC Oscillator with postscaler
9.1.1 SYSTEM CLOCK SOURCES
The FRC (Fast RC) internal oscillator runs at a nominal
frequency of 7.37 MHz. The 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 primary oscillator can use one of the following as
its clock source:
• XT (Crystal): Crystals and ceramic resonators in
the range of 3 MHz to 10 MHz. The crystal is con-
nected to the OSC1 and OSC2 pins.
• HS (High-Speed Crystal): Crystals in the range of
10 MHz to 40 MHz. The crystal is connected to
the OSC1 and OSC2 pins.
• EC (External Clock): External clock signal is
directly applied to the OSC1 pin.
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.
The LPRC (Low-Power RC) 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).
The clock signals generated by the FRC and primary
oscillators can be optionally applied to an on-chip
Phase Locked Loop (PLL) to provide a wide range of
output frequencies for device operation. PLL
configuration is described in Section 9.1.3 “PLL
Configuration”.
The FRC frequency depends on the FRC accuracy
(see Table 24-19) and the value of the FRC Oscillator
Tuning register (see Register 9-4).
9.1.2 SYSTEM CLOCK SELECTION
The oscillator source 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 21.1 “Configuration Bits” for further details.)
The Initial Oscillator Selection Configuration bits,
FNOSC<2:0> (FOSCSEL<2:0>), and the Primary Oscil-
lator 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 between
twelve different clock modes, shown in Table 9-1.
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 PIC24HJXXXGPX06A/
X08A/X10A architecture.
Instruction execution speed or device operating
frequency, FCY, is calculated, as shown in
Equation 9-1:
EQUATION 9-1: DEVICE OPERATING
FREQUENCY
FCY = ^F----^O----^S---^C--
2
9.1.3 PLL CONFIGURATION
The primary oscillator and internal FRC oscillator can
optionally use an on-chip PLL to obtain higher speeds
of operation. The PLL provides a significant amount of
flexibility in selecting the device operating speed. A
block diagram of the PLL is shown in Figure 9-2.
The output of the primary oscillator or FRC, denoted as
‘FIN’, is divided down by a prescale factor (N1) of 2, 3,
... or 33 before being provided to the PLL’s Voltage
Controlled Oscillator (VCO). The input to the VCO must
be selected to be in the range of 0.8 MHz to 8 MHz.
Since the minimum prescale factor is 2, this implies that
FIN must be chosen to be in the range of 1.6 MHz to 16
MHz. The prescale factor ‘N1’ is selected using the
PLLPRE<4:0> bits (CLKDIV<4:0>).
The PLL Feedback Divisor, selected using the
PLLDIV<8:0> bits (PLLFBD<8:0>), provides a factor ‘M’,
by which the input to the VCO is multiplied. This factor
must be selected such that the resulting VCO output
frequency is in the range of 100 MHz to 200 MHz.
The VCO output is further divided by a postscale factor
‘N2’. This factor is selected using the PLLPOST<1:0>
bits (CLKDIV<7:6>). ‘N2’ can be either 2, 4 or 8, and
must be selected such that the PLL output frequency
(FOSC) is in the range of 12.5 MHz to 80 MHz, which
generates device operating speeds of 6.25-40 MIPS.
For a primary oscillator or FRC oscillator, output ‘FIN’,
the PLL output ‘FOSC’ is given by:
EQUATION 9-2: FOSC CALCULATION
FOSC = FIN ⋅
⎛
⎝
N^-----1----M⋅^------N^{----2--⎠⎞}
DS70592C-page 128
© 2011 Microchip Technology Inc.