PIC16F87-ISLASHML datasheet(40/214 Pages) MICROCHIP

40 / 214 page

PIC16F87/88

DS30487B-page 38

Preliminary

 2003 Microchip Technology Inc.

4.5.1

INTRC MODES

Using the internal oscillator as the clock source can

eliminate the need for up to two external oscillator pins,

after which it can be used for digital I/O. Two distinct

configurations are available:

• In INTIO1 mode, the OSC2 pin outputs FOSC/4,

while OSC1 functions as RA7 for digital input and

output.

• In INTIO2 mode, OSC1 functions as RA7 and

OSC2 functions as RA6, both for digital input and

output.

4.5.2

OSCTUNE REGISTER

The internal oscillator’s output has been calibrated at the

factory but can be adjusted in the application. This is

done by writing to the OSCTUNE register (Register 4-1).

The tuning sensitivity is constant throughout the tuning

range. The OSCTUNE register has a tuning range of

±12.5%.

When the OSCTUNE register is modified, the INTOSC

and INTRC frequencies will begin shifting to the new fre-

quency. The INTRC clock will reach the new frequency

within 8 clock cycles (approximately 8 * 32

µs = 256 µs);

the INTOSC clock will stabilize within 1 ms. Code execu-

tion continues during this shift. There is no indication that

the shift has occurred. Operation of features that depend

on the 31.25 kHz INTRC clock source frequency, such

as the WDT, Fail-Safe Clock Monitor and peripherals,

will also be affected by the change in frequency.

OSCTUNE: OSCILLATOR TUNING REGISTER (ADDRESS 90h)

U-0

R/W-0

—

TUN5

TUN4

TUN3

TUN2

TUN1

TUN0

bit 7

bit 0

bit 7-6

Unimplemented: Read as ‘0’

bit 5-0

TUN<5:0>: Frequency Tuning bits

011111

= Maximum frequency

011110

•

000001

000000

= Center frequency. Oscillator module is running at the calibrated frequency.

111111

•

100000

= Minimum frequency

Legend:

R = Readable bit

W = Writable bit

U = Unimplemented bit, read as ‘0’

-n = Value at POR

‘1’ = Bit is set

‘0’ = Bit is cleared

x = Bit is unknown

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