Datasheet

PIC18CXX2
DS39026D-page 20  1999-2013 Microchip Technology Inc.
2.6 Oscillator Switching Feature
The PIC18CXX2 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 PIC18CXX2 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 9.0 for further details of the Timer1 oscilla-
tor. See Section 18.0 for Configuration Register details.
FIGURE 2-7: DEVICE CLOCK SOURCES
2.6.1 SYSTEM CLOCK SWITCH BIT
The system clock source switching is performed under
software control. The system clock switch bit, SCS
(OSCCON<0>) controls the clock switching. When the
SCS bit is’0’, the system clock source comes from the
main oscillator that is selected by the FOSC configura-
tion bits in Configuration Register1H. When the SCS bit
is set, the system clock source will come from the
Timer1 oscillator. The SCS bit is cleared on all forms of
RESET.
REGISTER 2-1: OSCCON REGISTER
PIC18CXXX
TOSC
4 x PLL
TT1P
TSCLK
Clock
Source
MUX
TOSC/4
Timer1 Oscillator
T1OSCEN
Enable
Oscillator
T1OSO
T1OSI
Clock Source option
for other modules
OSC1
OSC2
SLEEP
Main Oscillator
Note: The Timer1 oscillator must be enabled and
operating to switch the system clock
source. The Timer1 oscillator is enabled by
setting the T1OSCEN bit in the Timer1
control register (T1CON). If the Timer1
oscillator is not enabled, then any write to
the SCS bit will be ignored (SCS bit forced
cleared) and the main oscillator will con-
tinue to be the system clock source.
U-0 U-0 U-0 U-0 U-0 U-0 U-0 R/W-1
— — — — — — —SCS
bit 7 bit 0
bit 7-1 Unimplemented: Read as '0'
bit 0 SCS: System Clock Switch bit
When
OSCSEN configuration bit = ’0’ and T1OSCEN bit is set:
1 = Switch to Timer1 oscillator/clock pin
0 = Use primary oscillator/clock input pin
When
OSCSEN and T1OSCEN are in other states:
bit is forced clear
Legend:
R = Readable bit W = Writable bit U = Unimplemented bit, read as ‘0’
- n = Value at POR reset ’1’ = Bit is set ’0’ = Bit is cleared x = Bit is unknown