PIC18FXX20 Datasheet, PDF (26/366 Pages) Microchip Technology – 64/80-Pin High Performance 1 Mbit Enhanced FLASH Microcontrollers with A/D

PIC18FXX20
2.6 Oscillator Switching Feature
The PIC18FXX20 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 PIC18FXX20 devices, this alternate clock
source is the Timer1 oscillator. If a low frequency crys-
tal (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 Configuration Register1H to a
‘0’. Clock switching is disabled in an erased device.
See Section 12.0 for further details of the Timer1 oscil-
lator. See Section 23.0 for Configuration Register
details.
FIGURE 2-7:
DEVICE CLOCK SOURCES
OSC2
OSC1
T1OSO
T1OSI
PIC18FXX20
Main Oscillator
SLEEP
Timer1 Oscillator
4 x PLL
TOSC
TOSC/4
TT1P
T1OSCEN
Enable
Oscillator
Clock Source Option
for Other Modules
Clock
Source
TSCLK
DS39609A-page 24
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 2003 Microchip Technology Inc.