PIC12F635/PIC16F636/639
3.5.3
LFINTOSC
3.5.5
HF AND LF INTOSC CLOCK
SWITCH TIMING
The Low-Frequency Internal Oscillator (LFINTOSC) is
an uncalibrated 31 kHz internal clock source.
When switching between the LFINTOSC and the
HFINTOSC, the new oscillator may already be shut
down to save power (see Figure 3-6). If this is the case,
there is a delay after the IRCF<2:0> bits of the
OSCCON register are modified before the frequency
selection takes place. The LTS and HTS bits of the
OSCCON register will reflect the current active status
of the LFINTOSC and HFINTOSC oscillators. The
timing of a frequency selection is as follows:
The output of the LFINTOSC connects to a postscaler
and multiplexer (see Figure 3-1). Select 31 kHz, via
software, using the IRCF<2:0> bits of the OSCCON
register. See Section 3.5.4 “Frequency Select Bits
(IRCF)” for more information. The LFINTOSC is also the
frequency for the Power-up Timer (PWRT), Watchdog
Timer (WDT) and Fail-Safe Clock Monitor (FSCM).
The LFINTOSC is enabled by selecting 31 kHz
(IRCF<2:0> bits of the OSCCON register = 000)as the
system clock source (SCS bit of the OSCCON
register = 1), or when any of the following are enabled:
1. IRCF<2:0> bits of the OSCCON register are
modified.
2. If the new clock is shut down, a clock start-up
delay is started.
• Two-Speed Start-up IESO bit of the Configuration
Word register = 1and IRCF<2:0> bits of the
OSCCON register = 000
3. Clock switch circuitry waits for a falling edge of
the current clock.
4. CLKOUT is held low and the clock switch
circuitry waits for a rising edge in the new clock.
• Power-up Timer (PWRT)
• Watchdog Timer (WDT)
5. CLKOUT is now connected with the new clock.
LTS and HTS bits of the OSCCON register are
updated as required.
• Fail-Safe Clock Monitor (FSCM)
The LF Internal Oscillator (LTS) bit of the OSCCON
register indicates whether the LFINTOSC is stable or
not.
6. Clock switch is complete.
See Figure 3-1 for more details.
3.5.4
FREQUENCY SELECT BITS (IRCF)
If the internal oscillator speed selected is between
8 MHz and 125 kHz, there is no start-up delay before
the new frequency is selected. This is because the old
and new frequencies are derived from the HFINTOSC
via the postscaler and multiplexer.
The output of the 8 MHz HFINTOSC and 31 kHz
LFINTOSC connects to a postscaler and multiplexer
(see Figure 3-1). The Internal Oscillator Frequency
Select bits IRCF<2:0> of the OSCCON register select
the frequency output of the internal oscillators. One of
eight frequencies can be selected via software:
Start-up delay specifications are located in the A/C
Specifications (Oscillator Module) in Section 15.0
“Electrical Specifications”.
• 8 MHz
• 4 MHz (Default after Reset)
• 2 MHz
• 1 MHz
• 500 kHz
• 250 kHz
• 125 kHz
• 31 kHz (LFINTOSC)
Note:
Following any Reset, the IRCF<2:0> bits of
the OSCCON register are set to ‘110’ and
the frequency selection is set to 4 MHz.
The user can modify the IRCF bits to
select a different frequency.
© 2007 Microchip Technology Inc.
DS41232D-page 41
MICROCHIP [ MICROCHIP ]
