Datasheet

PIC18F87J72 FAMILY
DS39979A-page 30 Preliminary  2010 Microchip Technology Inc.
3.4.2 EXTERNAL CLOCK INPUT
(EC MODES)
The EC and ECPLL Oscillator modes require an
external clock source to be connected to the OSC1 pin.
There is no oscillator start-up time required after a
Power-on Reset or after an exit from Sleep mode.
In the EC Oscillator mode, the oscillator frequency
divided by 4 is available on the OSC2 pin. This signal
may be used for test purposes or to synchronize other
logic. Figure 3-3 shows the pin connections for the EC
Oscillator mode.
FIGURE 3-3: EXTERNAL CLOCK
INPUT OPERATION
(EC CONFIGURATION)
An external clock source may also be connected to the
OSC1 pin in the HS mode, as shown in Figure 3-4. In
this configuration, the divide-by-4 output on OSC2 is
not available. Current consumption in this configuration
will be somewhat higher than EC mode, as the internal
oscillator’s feedback circuitry will be enabled (in EC
mode, the feedback circuit is disabled).
FIGURE 3-4: EXTERNAL CLOCK INPUT
OPERATION (HS OSC
CONFIGURATION)
3.4.3 PLL FREQUENCY MULTIPLIER
A Phase Locked Loop (PLL) circuit is provided as an
option for users who want to use a lower frequency
oscillator circuit, or to clock the device up to its highest
rated frequency from a crystal oscillator. This may be
useful for customers who are concerned with EMI due
to high-frequency crystals, or users who require higher
clock speeds from an internal oscillator.
3.4.3.1 HSPLL and ECPLL Modes
The HSPLL and ECPLL modes provide the ability to
selectively run the device at 4 times the external
oscillating source to produce frequencies up to
40 MHz.
The PLL is enabled by programming the FOSC<2:0>
Configuration bits to either ‘111’ (for ECPLL) or ‘101’
(for HSPLL). In addition, the PLLEN bit
(OSCTUNE<6>) must also be set. Clearing PLLEN
disables the PLL, regardless of the chosen oscillator
configuration. It also allows additional flexibility for
controlling the application’s clock speed in software.
FIGURE 3-5: PLL BLOCK DIAGRAM
3.4.3.2 PLL and INTOSC
The PLL is also available to the internal oscillator block
when the internal oscillator block is configured as the
primary clock source. In this configuration, the PLL is
enabled in software and generates a clock output of up
to 32 MHz. The operation of INTOSC with the PLL is
described in Section 3.5.2 “INTPLL Modes”.
OSC1/CLKI
OSC2/CLKO
F
OSC/4
Clock from
Ext. System
PIC18F87J72
OSC1
OSC2
Open
Clock from
Ext. System
(HS Mode)
PIC18F87J72
MUX
VCO
Loop
Filter
OSC2
OSC1
PLL Enable (OSCTUNE)
F
IN
FOUT
SYSCLK
Phase
Comparator
HSPLL or ECPLL (CONFIG2L)
4
HS or EC
Mode