PIC12F635ESLASHSS datasheet(36/196 Pages) MICROCHIP

36 / 196 page

PIC12F635/PIC16F636/639

DS41232B-page 34

Preliminary

3.4.3

LFINTOSC

The Low-Frequency Internal Oscillator (LFINTOSC) is

an uncalibrated (approximate) 31 kHz internal clock

source.

The output of the LFINTOSC connects to a postscaler

and multiplexer (see Figure 3-1). 31 kHz can be

selected via software using the IRCF bits (see

Section 3.4.4 “Frequency Select Bits (IRCF)”). The

LFINTOSC is also the clock source for the Power-up

Timer (PWRT), Watchdog Timer (WDT) and Fail-Safe

Clock Monitor (FSCM).

The LFINTOSC is enabled by selecting 31 kHz

(IRCF = 000) as the system clock source (SCS = 1), or

when any of the following are enabled:

• Two-Speed Start-up (IESO = 1 and IRCF = 000)

• Power-up Timer (PWRT)

• Watchdog Timer (WDT)

• Fail-Safe Clock Monitor (FSCM)

The LF Internal Oscillator (LTS) bit (OSCCON<1>)

indicates whether the LFINTOSC is stable or not.

3.4.4

FREQUENCY SELECT BITS (IRCF)

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> (OSCCON<6:4>), select the

frequency output of the internal oscillators. One of eight

frequencies can be selected via software:

•8 MHz

• 4 MHz (Default after Reset)

•2 MHz

•1 MHz

• 500 kHz

• 250 kHz

• 125 kHz

•31 kHz

3.4.5

HFINTOSC AND LFINTOSC CLOCK

SWITCH TIMING

When switching between the LFINTOSC and the

HFINTOSC, the new oscillator may already be shut

down to save power. If this is the case, there is a 10

μs

delay after the IRCF bits are modified before the

frequency selection takes place. The LTS/HTS bits will

reflect the current active status of the LFINTOSC and

the HFINTOSC oscillators. The timing of a frequency

selection is as follows:

IRCF bits are modified.

If the new clock is shut down, a 10

μs clock start-

up delay is started.

Clock switch circuitry waits for a falling edge of

the current clock.

CLKOUT is held low and the clock switch

circuitry waits for a rising edge in the new clock.

CLKOUT is now connected with the new clock.

HTS/LTS bits are updated as required.

Clock switch is complete.

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 the new frequencies are derived from the

HFINTOSC via the postscaler and multiplexer.

Note:

Following any Reset, the IRCF bits 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.

Note:

Care must be taken to ensure a valid

voltage or frequency selection is chosen.

See

voltage

vs.

frequency

diagrams

(Figure 15-2, Figure 15-3 and Figure 15-4)

for more detail.

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