PIC18F4321 datasheet(36/402 Pages) MICROCHIP

36 / 402 page

PIC18F2221/2321/4221/4321 FAMILY

DS39689F-page 36

3.7.1

OSCILLATOR CONTROL REGISTER

The OSCCON register (Register 3-2) controls several

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

power operation and in power-managed modes.

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

clock source. The available clock sources are the

primary clock (defined by the FOSC<3:0> Configura-

tion bits), the secondary clock (Timer1 oscillator) and

the internal oscillator block. The clock source changes

immediately after either of the SCS<1:0> bits are

changed, following a brief clock transition interval. The

SCS bits are reset on all forms of Reset.

The

Internal

Oscillator

Frequency

Select

bits

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

oscillator block to drive the device clock. The choices

are the INTRC source (31 kHz), the INTOSC source

(8 MHz) or one of the frequencies derived from the

INTOSC postscaler (31.25 kHz to 4 MHz). If the

internal oscillator block is supplying the device clock,

changing the states of these bits will have an immedi-

ate change on the internal oscillator’s output. On

device Resets, the default output frequency of the

internal oscillator block is set at 1 MHz.

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 in the OSCTUNE register (OSCTUNE<7>).

Setting this bit selects INTOSC as a 31.25 kHz clock

source derived from the INTOSC postscaler. Clearing

INTSRC selects INTRC (nominally 31 kHz) as the

clock source and disables the INTOSC to reduce

current consumption.

This option allows users to select the tunable and more

precise INTOSC as a clock source, while maintaining

power savings with a very low clock speed. Addition-

ally, the INTOSC source will already be stable should a

switch to a higher frequency be needed quickly.

Regardless of the setting of INTSRC, INTRC always

remains the clock source for features such as the

Watchdog Timer and the Fail-Safe Clock Monitor.

The OSTS, IOFS and T1RUN bits indicate which clock

source is currently providing the device clock. The

OSTS bit indicates that the Oscillator Start-up Timer

and PLL Start-up Timer (if enabled) have timed out and

the primary clock is providing the device clock in

primary clock modes. The IOFS bit indicates when the

internal oscillator block has stabilized and is providing

the device clock in RC Clock modes. The T1RUN bit

(T1CON<6>) indicates when the Timer1 oscillator is

providing the device clock in secondary clock modes.

In power-managed modes, only one of these three bits

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

INTRC is providing the clock or the internal oscillator

block has just started and is not yet stable.

The IDLEN bit controls whether 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”.

3.7.2

OSCILLATOR TRANSITIONS

The PIC18F2221/2321/4221/4321 family of devices con-

tains circuitry to prevent clock “glitches” when switching

between clock sources. A short pause in the device clock

occurs during the clock switch. The length of this pause

is the sum of two cycles of the old clock source and three

to four cycles of the new clock source. This formula

assumes that the new clock source is stable.

Clock transitions are discussed in greater detail in

Section 4.1.2 “Entering 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 in the Timer1 Control regis-

ter (T1CON<3>). If the Timer1 oscillator

is not enabled, then any attempt to select

a secondary clock source will be ignored.

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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