12F683 datasheet(26/148 Pages) MICROCHIP

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PIC12F683

DS41211B-page 24

Preliminary

 2004 Microchip Technology Inc.

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 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 = 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

HF AND LF INTOSC 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 fre-

quency 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.

3.5

Clock Switching

The system clock source can be switched between

external and internal clock sources via software using

the System Clock Select (SCS) bit.

3.5.1

SYSTEM CLOCK SELECT (SCS) BIT

The System Clock Select (SCS) bit (OSCCON<0>)

selects the system clock source that is used for the

CPU and peripherals.

• When SCS = 0, the system clock source is

determined by configuration of the FOSC<2:0>

bits in the Configuration Word register (CONFIG).

• When SCS = 1, the system clock source is

chosen by the internal oscillator frequency

selected by the IRCF bits. After a Reset, SCS is

always cleared.

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:

Any automatic clock switch, which may

occur from Two-Speed Start-up or Fail-

Safe Clock Monitor, does not update the

SCS bit. The user can monitor the OSTS

(OSCCON<3>) to determine the current

system clock source.

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