PIC16F87 datasheet(44/214 Pages) MICROCHIP

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PIC16F87/88

DS30487B-page 42

Preliminary

 2003 Microchip Technology Inc.

• Clock before switch: One of INTOSC/INTOSC

postscaler (IRCF<2:0>

≠ 000)

IRCF bits are modified to a different INTOSC/

INTOSC postscaler frequency.

The clock switching circuitry waits for a falling

edge of the current clock, at which point CLKO

is held low.

The clock switching circuitry then waits for eight

falling edges of requested clock, after which it

switches CLKO to this new clock source.

The IOFS bit is set.

Oscillator switchover is complete.

4.6.6

OSCILLATOR DELAY UPON

POWER-UP, WAKE-UP AND

CLOCK SWITCHING

Table 4-3 shows the different delays invoked for vari-

ous clock switching sequences. It also shows the

delays invoked for POR and wake-up.

TABLE 4-3:

OSCILLATOR DELAY EXAMPLES

Switch From Switch To

Frequency

Oscillator Delay

Comments

SLEEP/POR

INTRC

T1OSC

INTOSC/

INTOSC

Postscaler

31.25 kHz

32.768 kHz

125kHz - 8MHz

µs - 10 µs (approx.)

CPU Start-up(1)

Following a wake-up from SLEEP mode or

POR, CPU start-up is invoked to allow the

CPU to become ready for code execution.

INTRC/SLEEP

EC, RC

DC - 20 MHz

INTRC

(31.25 kHz)

EC, RC

DC - 20 MHz

SLEEP

LP, XT, HS 32.768 kHz - 20 MHz

1024 Clock Cycles

(OST)

Following a change from INTRC, an OST

of 1024 cycles must occur.

INTRC

(31.25 kHz)

INTOSC/

INTOSC

Postscaler

125kHz - 8MHz

4ms

Refer to Section 4.6.4 “Modifying the

IRCF bits” for further details.

Note 1:

The 5

µs-10 µs start-up delay is based on a 1 MHz system clock.

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