PIC16F882-I datasheet(71/328 Pages) MICROCHIP

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DS41291F-page 69

PIC16F882/883/884/886/887

4.6

Clock Switching

The system clock source can be switched between

external and internal clock sources via software using

the System Clock Select (SCS) bit of the OSCCON

4.6.1

SYSTEM CLOCK SELECT (SCS) BIT

The System Clock Select (SCS) bit of the OSCCON

the CPU and peripherals.

• When the SCS bit of the OSCCON register = 0,

the system clock source is determined by

configuration of the FOSC<2:0> bits in the

Configuration Word Register 1 (CONFIG1).

• When the SCS bit of the OSCCON register = 1,

the system clock source is chosen by the internal

oscillator frequency selected by the IRCF<2:0>

bits of the OSCCON register. After a Reset, the

SCS bit of the OSCCON register is always

cleared.

4.6.2

OSCILLATOR START-UP TIME-OUT

STATUS (OSTS) BIT

The Oscillator Start-up Time-out Status (OSTS) bit of

the OSCCON register indicates whether the system

clock is running from the external clock source, as

defined by the FOSC<2:0> bits in the Configuration

Word Register 1 (CONFIG1), or from the internal clock

source. In particular, OSTS indicates that the Oscillator

Start-up Timer (OST) has timed out for LP, XT or HS

modes.

4.7

Two-Speed Clock Start-up Mode

Two-Speed Start-up mode provides additional power

savings by minimizing the latency between external

oscillator start-up and code execution. In applications

that make heavy use of the Sleep mode, Two-Speed

Start-up will remove the external oscillator start-up

time from the time spent awake and can reduce the

overall power consumption of the device.

This mode allows the application to wake-up from

Sleep, perform a few instructions using the INTOSC

as the clock source and go back to Sleep without

waiting for the primary oscillator to become stable.

When the oscillator module is configured for LP, XT or

HS modes, the Oscillator Start-up Timer (OST) is

enabled (see Section 4.4.1 “Oscillator Start-up Timer

(OST)”). The OST will suspend program execution until

1024 oscillations are counted. Two-Speed Start-up

mode minimizes the delay in code execution by

operating from the internal oscillator as the OST is

counting. When the OST count reaches 1024 and the

OSTS bit of the OSCCON register is set, program

execution switches to the external oscillator.

4.7.1

TWO-SPEED START-UP MODE

CONFIGURATION

Two-Speed Start-up mode is configured by the

following settings:

• IESO (of the Configuration Word Register 1) = 1;

Internal/External Switchover bit (Two-Speed Start-

up mode enabled).

• SCS (of the OSCCON register) = 0.

• FOSC<2:0> bits in the Configuration Word

HS mode.

Two-Speed Start-up mode is entered after:

• Power-on Reset (POR) and, if enabled, after

Power-up Timer (PWRT) has expired, or

• Wake-up from Sleep.

If the external clock oscillator is configured to be

anything other than LP, XT or HS mode, then Two-

speed Start-up is disabled. This is because the external

clock oscillator does not require any stabilization time

after POR or an exit from Sleep.

4.7.2

TWO-SPEED START-UP

SEQUENCE

Wake-up from Power-on Reset or Sleep.

Instructions begin execution by the internal

oscillator at the frequency set in the IRCF<2:0>

bits of the OSCCON register.

OST enabled to count 1024 clock cycles.

OST timed out, wait for falling edge of the

internal oscillator.

OSTS is set.

System clock held low until the next falling edge

of new clock (LP, XT or HS mode).

System clock is switched to external clock

source.

Note:

Any automatic clock switch, which may

occur from Two-Speed Start-up or Fail-Safe

Clock Monitor, does not update the SCS bit

of the OSCCON register. The user can

monitor the OSTS bit of the OSCCON

clock source.

Note:

Executing a SLEEP instruction will abort

the oscillator start-up time and will cause

the OSTS bit of the OSCCON register to

remain clear.

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