Datasheet

PIC24FJ256GB110 FAMILY
DS39897C-page 126  2009 Microchip Technology Inc.
8.4 Clock Switching Operation
With few limitations, applications are free to switch
between any of the four clock sources (POSC, SOSC,
FRC and LPRC) under software control and at any
time. To limit the possible side effects that could result
from this flexibility, PIC24F devices have a safeguard
lock built into the switching process.
8.4.1 ENABLING CLOCK SWITCHING
To enable clock switching, the FCKSM1 Configuration
bit in CW2 must be programmed to ‘0’. (Refer to
Section 26.1 “Configuration Bits” for further details.)
If the FCKSM1 Configuration bit is unprogrammed (‘1’),
the clock switching function and Fail-Safe Clock
Monitor function are disabled. This is the default
setting.
The NOSCx control bits (OSCCON<10:8>) do not
control the clock selection when clock switching is dis-
abled. However, the COSCx bits (OSCCON<14:12>)
will reflect the clock source selected by the FNOSCx
Configuration bits.
The OSWEN control bit (OSCCON<0>) has no effect
when clock switching is disabled; it is held at ‘0’ at all
times.
REGISTER 8-3: OSCTUN: FRC OSCILLATOR TUNE REGISTER
U-0 U-0 U-0 U-0 U-0 U-0 U-0 U-0
— — — — — — — —
bit 15 bit 8
U-0 U-0 R/W-0 R/W-0 R/W-0 R/W-0 R/W-0 R/W-0
— — TUN5
(1)
TUN4
(1)
TUN3
(1)
TUN2
(1)
TUN1
(1)
TUN0
(1)
bit 7 bit 0
Legend:
R = Readable bit W = Writable bit U = Unimplemented bit, read as ‘0’
-n = Value at POR ‘1’ = Bit is set ‘0’ = Bit is cleared x = Bit is unknown
bit 15-6 Unimplemented: Read as ‘0’
bit 5-0 TUN<5:0>: FRC Oscillator Tuning bits
(1)
011111 = Maximum frequency deviation
011110 =



000001 =
000000 = Center frequency, oscillator is running at factory calibrated frequency
111111 =



100001 =
100000 = Minimum frequency deviation
Note 1: Increments or decrements of TUN<5:0> may not change the FRC frequency in equal steps over the FRC
tuning range, and may not be monotonic.
Note: The Primary Oscillator mode has three
different submodes (XT, HS and EC)
which are determined by the POSCMDx
Configuration bits. While an application
can switch to and from Primary Oscillator
mode in software, it cannot switch
between the different primary submodes
without reprogramming the device.