PIC18F23K20 datasheet(28/456 Pages) MICROCHIP

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PIC18F2XK20/4XK20

DS41303G-page 28

 2010 Microchip Technology Inc.

2.2

Oscillator Control

The OSCCON register (Register 2-1) controls several

aspects of the device clock’s operation, both in full

power operation and in power-managed modes.

• Main System Clock Selection (SCS)

• Internal Frequency selection bits (IRCF)

• Clock Status bits (OSTS, IOFS)

• Power management selection (IDLEN)

2.2.1

MAIN SYSTEM CLOCK SELECTION

The System Clock Select bits, SCS<1:0>, select the

main clock source. The available clock sources are

• Primary clock defined by the FOSC<3:0> bits of

CONFIG1H. The primary clock can be the primary

oscillator, an external clock, or the internal oscilla-

tor block.

• Secondary clock (Timer1 oscillator)

• Internal oscillator block (HFINTOSC and

LFINTOSC).

The clock source changes immediately after one or

more of the bits is written to, following a brief clock tran-

sition interval. The SCS bits are cleared to select the

primary clock on all forms of Reset.

2.2.2

INTERNAL FREQUENCY

SELECTION

The

Internal

Oscillator

Frequency

Select

bits

(IRCF<2:0>) select the frequency output of the internal

oscillator block. The choices are the LFINTOSC source

(31 kHz), the HFINTOSC source (16 MHz) or one of

the frequencies derived from the HFINTOSC post-

scaler (31.25 kHz to 8 MHz). If the internal oscillator

block is supplying the main clock, changing the states

of these bits will have an immediate change on the

internal oscillator’s output. On device Resets, the out-

put frequency of the internal oscillator is set to the

default frequency of 1 MHz.

2.2.3

LOW FREQUENCY SELECTION

When a nominal output frequency of 31 kHz is selected

(IRCF<2:0> = 000), users may choose which internal

oscillator acts as the source. This is done with the

INTSRC bit of the OSCTUNE register. Setting this bit

selects the HFINTOSC as a 31.25 kHz clock source by

enabling the divide-by-512 output of the HFINTOSC

postscaler. Clearing INTSRC selects LFINTOSC (nom-

inally 31 kHz) as the clock source.

This option allows users to select the tunable and more

precise HFINTOSC as a clock source, while maintain-

ing power savings with a very low clock speed. Regard-

less of the setting of INTSRC, LFINTOSC always

remains the clock source for features such as the

Watchdog Timer and the Fail-Safe Clock Monitor.

2.2.4

CLOCK STATUS

The OSTS and IOFS bits of the OSCCON register, and

the T1RUN bit of the T1CON register, indicate which

clock source is currently providing the main clock. The

OSTS bit indicates that the Oscillator Start-up Timer

has timed out and the primary clock is providing the

device clock. The IOFS bit indicates when the internal

oscillator block has stabilized and is providing the

device clock in HFINTOSC Clock modes. The IOFS

and OSTS Status bits will both be set when

SCS<1:0> = 00 and HFINTOSC is the primary clock.

The T1RUN bit indicates when the Timer1 oscillator is

providing the device clock in secondary clock modes.

When SCS<1:0>

 00, only one of these three bits will

be set at any time. If none of these bits are set, the

LFINTOSC is providing the clock or the HFINTOSC

has just started and is not yet stable.

2.2.5

POWER MANAGEMENT

The IDLEN bit of the OSCCON register determines if

the device goes into Sleep mode or one of the Idle

modes when the SLEEP instruction is executed.

The use of the flag and control bits in the OSCCON

“Power-Managed Modes”.

Note 1: The Timer1 oscillator must be enabled to

select the secondary clock source. The

Timer1 oscillator is enabled by setting the

T1OSCEN bit of the T1CON register. If

the Timer1 oscillator is not enabled, then

the main oscillator will continue to run

from the previously selected source. The

source will then switch to the secondary

oscillator after the T1OSCEN bit is set.

2: It is recommended that the Timer1

oscillator be operating and stable before

selecting the secondary clock source or a

very long delay may occur while the

Timer1 oscillator starts.

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