PIC18F25K20T-ISO Datasheet, PDF (37/456 Pages) Microchip Technology – 28/40/44-Pin Flash Microcontrollers with nanoWatt XLP Technology

2.9 Clock Switching
The system clock source can be switched between
external and internal clock sources via software using
the System Clock Select (SCS<1:0>) bits of the
OSCCON register.
PIC18F2XK20/4XK20 devices contain circuitry to pre-
vent clock “glitches” when switching between clock
sources. A short pause in the device clock occurs dur-
ing the clock switch. The length of this pause is the sum
of two cycles of the old clock source and three to four
cycles of the new clock source. This formula assumes
that the new clock source is stable.
Clock transitions are discussed in greater detail in
Section 3.1.2 “Entering Power-Managed Modes”.
2.9.1
SYSTEM CLOCK SELECT
(SCS<1:0>) BITS
The System Clock Select (SCS<1:0>) bits of the
OSCCON register select the system clock source that
is used for the CPU and peripherals.
• When SCS<1:0> = 00, the system clock source is
determined by configuration of the FOSC<2:0>
bits in the CONFIG1H Configuration register.
• When SCS<1:0> = 10, the system clock source is
chosen by the internal oscillator frequency
selected by the INTSRC bit of the OSCTUNE
register and the IRCF<2:0> bits of the OSCCON
register.
• When SCS<1:0> = 01, the system clock source is
the 32.768 kHz secondary oscillator shared with
Timer1.
After a Reset, the SCS<1:0> bits of the OSCCON
register are always cleared.
Note:
Any automatic clock switch, which may
occur from Two-Speed Start-up or Fail-Safe
Clock Monitor, does not update the
SCS<1:0> bits of the OSCCON register.
The user can monitor the T1RUN bit of the
T1CON register and the IOFS and OSTS
bits of the OSCCON register to determine
the current system clock source.
2.9.2
OSCILLATOR START-UP TIME-OUT
STATUS (OSTS) BIT
The Oscillator Start-up Time-out Status (OSTS) bit of
the OSCCON register indicates whether the system
clock is running from the external clock source, as
defined by the FOSC<3:0> bits in the CONFIG1H
Configuration register, or from the internal clock
source. In particular, when the primary oscillator is the
source of the primary clock, OSTS indicates that the
Oscillator Start-up Timer (OST) has timed out for LP,
XT or HS modes.
PIC18F2XK20/4XK20
2.9.3 CLOCK SWITCH TIMING
When switching between one oscillator and another,
the new oscillator may not be operating which saves
power (see Figure 2-7). If this is the case, there is a
delay after the SCS<1:0> bits of the OSCCON register
are modified before the frequency change takes place.
The OSTS and IOFS bits of the OSCCON register will
reflect the current active status of the external and
HFINTOSC oscillators. The timing of a frequency
selection is as follows:
1. SCS<1:0> bits of the OSCCON register are mod-
ified.
2. The old clock continues to operate until the new
clock is ready.
3. Clock switch circuitry waits for two consecutive
rising edges of the old clock after the new clock
ready signal goes true.
4. The system clock is held low starting at the next
falling edge of the old clock.
5. Clock switch circuitry waits for an additional two
rising edges of the new clock.
6. On the next falling edge of the new clock the low
hold on the system clock is released and new
clock is switched in as the system clock.
7. Clock switch is complete.
See Figure 2-1 for more details.
If the HFINTOSC is the source of both the old and new
frequency, there is no start-up delay before the new
frequency is active. This is because the old and new
frequencies are derived from the HFINTOSC via the
postscaler and multiplexer.
Start-up delay specifications are located in
Section 26.0 “Electrical Characteristics”, under AC
Specifications (Oscillator Module).
 2010 Microchip Technology Inc.
DS41303G-page 37