PIC18F010T-ISLASHP datasheet(16/176 Pages) MICROCHIP

16 / 176 page

PIC18F010/020

DS41142A-page 14

Preliminary

 2001 Microchip Technology Inc.

2.11

Base Frequency Change

There are two methods to change frequency during

normal program operation. One option is to switch fre-

quencies using the internal oscillator only; IRCF<2:0>

in the OSCCON register selects the internal oscillator

frequency. Refer to Register 2-1.

Switching for an external clock to an internal oscillator

and vice versa is also possible. Use the SCS bit in the

OSCCON register to select an external or internal clock

source.

2.12

Oscillator Delay Upon Start-up

and Base Frequency Change

When the INTOSC Oscillator Module starts up, an

8-cycle delay of the base frequency is invoked. During

this delay, the FINTOSC output signal is held at ‘0’.

The INTOSC Oscillator Module also allows user to

change frequency during run time. For example, the

frequency can be changed from 8 MHz to 32 kHz, while

the device is operating. When the application requires

a base frequency change, a delay of 8 cycles of the

new base frequency is invoked.

Writing to the OSCTUNE register will not cause any

delay. In applications where the OSCTUNE register is

used to shift the FINTOSC frequency, the application

should not expect the FINTOSC frequency to stabilize

immediately. In this case, the frequency may shift grad-

ually toward the new value. The time for this frequency

shift is less than 8 cycles of the base frequency.

Table 2-4 below, shows examples of when the oscilla-

tor delay is invoked.

TABLE 2-4:

OSCILLATOR DELAY EXAMPLES

Note:

The OSCEN bit in the CONFIG1H configu-

ration byte must be set to allow clock

switching.

Old Frequency

New Frequency

New Base

Frequency

Oscillator Delay

Comments

8 MHz

4 MHz or 1 MHz

None

The 8 MHz, 4 MHz, and 1 MHz are all

running from the same 8 MHz base

frequency.

500 kHz

32 kHz

250

µS nominal

Base frequency changes from 500 kHz

to 32 kHz.

4 MHz

32 kHz

250

µS nominal

Base frequency changes from 8 MHz to

32 kHz.

500 kHz

8 MHz

µS nominal

Base frequency changes from 500 kHz

to 8 MHz.

Off or SLEEP

mode

1 MHz

8 MHz

µS nominal

Upon power-up and wake-up from

SLEEP, there is always oscillator delay.

Off or SLEEP

mode

500 kHz

µS nominal

Upon power-up and wake-up from

SLEEP, there is always oscillator delay.

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