16F87 datasheet(44/228 Pages) MICROCHIP

44 / 228 page

PIC16F87/88

DS30487C-page 42

 2005 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

various clock switching sequences. It also shows the

delays invoked for POR and wake-up.

TABLE 4-3:

OSCILLATOR DELAY EXAMPLES

Clock Switch

Frequency

Oscillator Delay

Comments

From

Sleep/POR

INTRC

T1OSC

31.25 kHz

32.768 kHz

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.

INTOSC/

INTOSC

Postscaler

125 kHz-8 MHz

4 ms (approx.) and

CPU Start-up(1)

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

125 kHz-8 MHz

4 ms (approx.)

Refer to Section 4.6.4 “Modifying the

IRCF Bits” for further details.

Note 1:

The 5-10

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

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