PIC18F452-IPT Datasheet, PDF (22/332 Pages) Microchip Technology – High-Performance, Enhanced Flash Microcontrollers with 10-Bit A/D

PIC18FXX2
2.6 Oscillator Switching Feature
The PIC18FXX2 devices include a feature that allows
the system clock source to be switched from the main
oscillator to an alternate low frequency clock source.
For the PIC18FXX2 devices, this alternate clock source
is the Timer1 oscillator. If a low frequency crystal (32
kHz, for example) has been attached to the Timer1
oscillator pins and the Timer1 oscillator has been
enabled, the device can switch to a Low Power Execu-
tion mode. Figure 2-7 shows a block diagram of the
system clock sources. The clock switching feature is
enabled by programming the Oscillator Switching
Enable (OSCSEN) bit in Configuration Register1H to a
’0’. Clock switching is disabled in an erased device.
See Section 11.0 for further details of the Timer1 oscil-
lator. See Section 19.0 for Configuration Register
details.
FIGURE 2-7:
DEVICE CLOCK SOURCES
OSC2
OSC1
T1OSO
T1OSI
PIC18FXXX
Main Oscillator
SLEEP
Timer1 Oscillator
4 x PLL
TOSC
TOSC/4
TT1P
T1OSCEN
Enable
Oscillator
Clock Source option
for other modules
Clock
Source
TSCLK
DS39564C-page 20
© 2006 Microchip Technology Inc.