Datasheet

PIC18F6525/6621/8525/8621
DS39612C-page 24  2003-2013 Microchip Technology Inc.
2.6 Oscillator Switching Feature
The PIC18F6525/6621/8525/8621 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 PIC18F6525/6621/
8525/8621 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 execution 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 the CONFIG1H Configuration register
to a ‘0’. Clock switching is disabled in an erased device.
See Section 12.0 “Timer1 Module” for further details
of the Timer1 oscillator. See Section 24.0 “Special
Features of the CPU” for Configuration register
details.
FIGURE 2-7: DEVICE CLOCK SOURCES
PIC18F6X2X/8X2X
TOSC
4 x PLL
TT1P
TSCLK
Clock
Source
MUX
TOSC/4
T1OSCEN
Enable
Oscillator
T1OSO
T1OSI
Clock Source Option
for Other Modules
OSC1
OSC2
Sleep
Timer1 Oscillator
Main Oscillator