PIC18F23K20 Datasheet, PDF (38/420 Pages) Microchip Technology – 28/40/44-Pin Flash Microcontrollers with 10-Bit A/D and nanoWatt Technology

PIC18F2XK20/4XK20
2.10.2 TWO-SPEED START-UP
SEQUENCE
1. Wake-up from Power-on Reset or Sleep.
2. Instructions begin executing by the internal
oscillator at the frequency set in the IRCF<2:0>
bits of the OSCCON register.
3. OST enabled to count 1024 external clock
cycles.
4. OST timed out. External clock is ready.
5. OSTS is set.
6. Clock switch finishes according to FIGURE 2-7:
“Clock Switch Timing”
2.10.3 CHECKING TWO-SPEED CLOCK
STATUS
Checking the state of the OSTS bit of the OSCCON
register will confirm if the microcontroller is running
from the external clock source, as defined by the
FOSC<2:0> bits in CONFIG1H Configuration register,
or the internal oscillator. OSTS = 0 when the external
oscillator is not ready, which indicates that the system
is running from the internal oscillator.
2.10.4 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 IRCF<2:0> 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. IRCF<2:0> bits of the OSCCON register are
modified.
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).
DS41303B-page 36
Advance Information
© 2007 Microchip Technology Inc.