PIC18F4550ESOSQTP datasheet(30/438 Pages) MICROCHIP

30 / 438 page

PIC18F2455/2550/4455/4550

DS39632E-page 28

2.2.5.2

OSCTUNE Register

The internal oscillator’s output has been calibrated at

the factory but can be adjusted in the user’s applica-

tion. This is done by writing to the OSCTUNE register

(Register 2-1). The tuning sensitivity is constant

throughout the tuning range.

The INTOSC clock will stabilize within 1 ms. Code exe-

cution continues during this shift. There is no indication

that the shift has occurred.

The OSCTUNE register also contains the INTSRC bit.

The INTSRC bit allows users to select which internal

oscillator provides the clock source when the 31 kHz

frequency option is selected. This is covered in greater

detail in Section 2.4.1 “Oscillator Control Register”.

2.2.5.3

Internal Oscillator Output Frequency

and Drift

The internal oscillator block is calibrated at the factory

to produce an INTOSC output frequency of 8.0 MHz.

However, this frequency may drift as VDD or tempera-

ture changes, which can affect the controller operation

in a variety of ways.

The low-frequency INTRC oscillator operates indepen-

dently of the INTOSC source. Any changes in INTOSC

across voltage and temperature are not necessarily

reflected by changes in INTRC and vice versa.

OSCTUNE: OSCILLATOR TUNING REGISTER

R/W-0

U-0

R/W-0

INTSRC

—

TUN4

TUN3

TUN2

TUN1

TUN0

bit 7

bit 0

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

bit 7

INTSRC: Internal Oscillator Low-Frequency Source Select bit

1 = 31.25 kHz device clock derived from 8 MHz INTOSC source (divide-by-256 enabled)

0 = 31 kHz device clock derived directly from INTRC internal oscillator

bit 6-5

Unimplemented: Read as ‘0’

bit 4-0

TUN4:TUN0: Frequency Tuning bits

01111 = Maximum frequency

•

00001

00000 = Center frequency. Oscillator module is running at the calibrated frequency.

11111

•

10000 = Minimum frequency

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