41594B datasheet(54/372 Pages) MICROCHIP

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PIC12LF1840T48A

DS41594B-page 54

Preliminary

 2011-2012 Microchip Technology Inc.

5.2.2.6

32 MHz Internal Oscillator

Frequency Selection

The Internal Oscillator Block can be used with the

4xPLL associated with the External Oscillator Block to

produce a 32 MHz internal system clock source. The

following settings are required to use the 32 MHz inter-

nal clock source:

• The FOSC bits in Configuration Word 1 must be

set to use the INTOSC source as the device sys-

tem clock (FOSC<2:0> = 100).

• The SCS bits in the OSCCON register must be

cleared to use the clock determined by

FOSC<2:0> in Configuration Word 1

(SCS<1:0> = 00).

• The IRCF bits in the OSCCON register must be

set to the 8 MHz HFINTOSC set to use

(IRCF<3:0> = 1110).

• The SPLLEN bit in the OSCCON register must be

set to enable the 4xPLL, or the PLLEN bit of the

Configuration Word 2 must be programmed to a

‘1’.

The 4xPLL is not available for use with the internal

oscillator when the SCS bits of the OSCCON register

are set to ‘1x’. The SCS bits must be set to ‘00’ to use

the 4xPLL with the internal oscillator.

5.2.2.7

Internal Oscillator Clock Switch

Timing

When switching between the HFINTOSC, MFINTOSC

and the LFINTOSC, the new oscillator may already be

shut down to save power (see Figure 5-7). If this is the

case, there is a delay after the IRCF<3:0> bits of the

OSCCON register are modified before the frequency

selection takes place. The OSCSTAT register will

reflect the current active status of the HFINTOSC,

MFINTOSC and LFINTOSC oscillators. The sequence

of a frequency selection is as follows:

IRCF<3:0> bits of the OSCCON register are

modified.

If the new clock is shut down, a clock start-up

delay is started.

Clock switch circuitry waits for a falling edge of

the current clock.

The current clock is held low and the clock

switch circuitry waits for a rising edge in the new

clock.

The new clock is now active.

The OSCSTAT register is updated as required.

Clock switch is complete.

See Figure 5-7 for more details.

If the internal oscillator speed is switched between two

clocks of the same source, there is no start-up delay

before the new frequency is selected. Clock switching

time delays are shown in Table 5-1.

Start-up delay specifications are located in the

oscillator

tables

Section 31.0

“Electrical

Specifications”

Note:

When using the PLLEN bit of the

Configuration Word 2, the 4xPLL cannot

be disabled by software and the 8 MHz

HFINTOSC option will no longer be

available.

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