PIC18F2450-ESLASHMLQTP datasheet(32/320 Pages) MICROCHIP

32 / 320 page

PIC18F2450/4450

DS39760A-page 30

Advance Information

2.4

Clock Sources and Oscillator

Switching

PIC18

enhanced

devices,

the

PIC18F2450/4450 family includes a feature that allows

the device clock source to be switched from the main

oscillator to an alternate low-frequency clock source.

PIC18F2450/4450 devices offer two alternate clock

sources. When an alternate clock source is enabled,

the various power-managed operating modes are

available.

Essentially, there are three clock sources for these

devices:

• Primary oscillators

• Secondary oscillators

• Internal oscillator

The primary oscillators include the External Crystal

and Resonator modes, the External Clock modes and

the internal oscillator. The particular mode is defined by

the FOSC3:FOSC0 Configuration bits. The details of

these modes are covered earlier in this chapter.

The secondary oscillators are those external sources

not connected to the OSC1 or OSC2 pins. These

sources may continue to operate even after the

controller is placed in a power-managed mode.

PIC18F2450/4450 devices offer the Timer1 oscillator

as a secondary oscillator. This oscillator, in all power-

managed modes, is often the time base for functions

such as a Real-Time Clock. Most often, a 32.768 kHz

watch crystal is connected between the RC0/T1OSO/

T1CKI and RC1/T1OSI/UOE pins. Like the XT and HS

Oscillator mode circuits, loading capacitors are also

connected from each pin to ground. The Timer1

oscillator is discussed in greater detail in Section 11.3

“Timer1 Oscillator”.

In addition to being a primary clock source, the internal

oscillator is available as a power-managed mode

clock source. The INTRC source is also used as the

clock source for several special features, such as the

WDT and Fail-Safe Clock Monitor.

2.4.1

OSCILLATOR CONTROL REGISTER

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.

The System Clock Select bits, SCS1:SCS0, select the

clock source. The available clock sources are the

primary clock (defined by the FOSC3:FOSC0 Configura-

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

internal oscillator. The clock source changes immedi-

ately after one or more of the bits is written to, following

a brief clock transition interval. The SCS bits are cleared

on all forms of Reset.

INTRC always remains the clock source for features

such as the Watchdog Timer and the Fail-Safe Clock

Monitor.

The OSTS and T1RUN bits indicate which clock source

is currently providing the device clock. The OSTS bit

indicates that the Oscillator Start-up Timer has timed out

and the primary clock is providing the device clock in

primary clock modes. The T1RUN bit (T1CON<6>) indi-

cates 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 has just started and is

not yet stable.

The IDLEN bit 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”.

2.4.2

OSCILLATOR TRANSITIONS

PIC18F2450/4450 devices contain 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 3.1.2 “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 prior to

switching to it as the clock source; other-

wise, a very long delay may occur while

the Timer1 oscillator starts.

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