PIC16F688-ESLASHST datasheet(29/174 Pages) MICROCHIP

29 / 174 page

 2004 Microchip Technology Inc.

Preliminary

DS41203B-page 27

PIC16F688

3.5

Clock Switching

The system clock source can be switched between

external and internal clock sources via software using

the System Clock Select (SCS) bit.

3.5.1

SYSTEM CLOCK SELECT (SCS) BIT

The System Clock Select (SCS) bit (OSCCON<0>)

selects the system clock source that is used for the

CPU and peripherals.

• When SCS = 0, the system clock source is

determined by configuration of the FOSC<2:0>

bits in the Configuration Word register (CONFIG).

• When SCS = 1, the system clock source is

chosen by the internal oscillator frequency

selected by the IRCF bits. After a Reset, SCS is

always cleared.

3.5.2

OSCILLATOR START-UP TIME-OUT

STATUS BIT

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

(OSCCON<3>) indicates whether the system clock is

running from the external clock source, as defined by

the FOSC bits, or from internal clock source. In

particular, OSTS indicates that the Oscillator Start-up

Timer (OST) has timed out for LP, XT or HS modes.

3.6

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 PIC16F688 is configured for LP, XT or HS

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

(see

Section 3.3.1

“Oscillator

Start-up

Timer

(OST)”). The OST timer 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 (OSCCON<3>) is set, program

execution switches to the external oscillator.

3.6.1

TWO-SPEED START-UP MODE

CONFIGURATION

Two-Speed Start-up mode is configured by the

following settings:

• IESO = 1 (CONFIG<10>) Internal/External Switch

Over bit.

•SCS = 0.

• FOSC configured for LP, XT or HS mode.

Two-Speed Start-up mode is entered after:

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

PWRT has expired, or

• Wake-up from Sleep.

If the external clock oscillator is configured to be any-

thing 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.

3.6.2

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 bits

(OSCCON<6:4>).

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.

3.6.3

CHECKING EXTERNAL/INTERNAL

CLOCK STATUS

Checking the state of the OSTS bit (OSCCON<3>) will

confirm if the PIC16F688 is running from the external

clock source as defined by the FOSC bits in the

Configuration Word (CONFIG) or the internal oscillator.

Note:

Any automatic clock switch, which may

occur

from

Two-Speed

Start-up

Fail-Safe Clock Monitor, does not update

the SCS bit. The user can monitor the

OSTS (OSCCON<3>) to determine the

current system clock source.

Note:

Executing a SLEEP instruction will abort

the oscillator start-up time and will cause

the OSTS bit (OSCCON<3>) to remain

clear.

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