PIC18F23K22 datasheet(42/492 Pages) MICROCHIP

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PIC18(L)F2X/4XK22

DS41412B-page 42

Preliminary

 2010 Microchip Technology Inc.

2.9.3

CLOCK SWITCH TIMING

When switching between one oscillator and another,

the new oscillator may not be operating which saves

power (see Figure 2-9). If this is the case, there is a

delay after the SCS<1:0> bits of the OSCCON register

are modified before the frequency change takes place.

The OSTS and IOFS bits of the OSCCON register will

reflect the current active status of the external and

HFINTOSC oscillators. The timing of a frequency

selection is as follows:

SCS<1:0> bits of the OSCCON register are mod-

ified.

The old clock continues to operate until the new

clock is ready.

Clock switch circuitry waits for two consecutive

rising edges of the old clock after the new clock

ready signal goes true.

The system clock is held low starting at the next

falling edge of the old clock.

Clock switch circuitry waits for an additional two

rising edges of the new clock.

On the next falling edge of the new clock the low

hold on the system clock is released and new

clock is switched in as the system clock.

Clock switch is complete.

See Figure 2-1 for more details.

If the HFINTOSC is the source of both the old and new

frequency, there is no start-up delay before the new

frequency is active. This is because the old and new

frequencies are derived from the HFINTOSC via the

postscaler and multiplexer.

Start-up

delay

specifications

are

located

Section 27.0 “Electrical Characteristics”, under AC

Specifications (Oscillator Module).

2.10

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 HFINTOSC

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 2.4.1 “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.

2.10.1

TWO-SPEED START-UP MODE

CONFIGURATION

Two-Speed Start-up mode is enabled when all of the

following settings are configured as noted:

• Two-Speed Start-up mode is enabled when the

IESO of the CONFIG1H Configuration register is

set.

• SCS<1:0> (of the OSCCON register) = 00.

• FOSC<2:0> bits of the CONFIG1H Configuration

Two-Speed Start-up mode becomes active after:

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

Power-up Timer (PWRT) has expired, or

• Wake-up from Sleep.

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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