PIC16F720-ESLASHML datasheet(68/244 Pages) MICROCHIP

68 / 244 page

PIC16F/LF720/721

DS41430A-page 68

 2010 Microchip Technology Inc.

7.2

Clock Source Modes

Clock source modes can be classified as external or

internal.

• Internal clock source (INTOSC) is contained

within the oscillator module and derived from a

500 kHz high precision oscillator. The oscillator

module has eight selectable output frequencies,

with a maximum internal frequency of 16 MHz.

• The external clock mode (EC) relies on an

external signal for the clock source.

The system clock can be selected between external or

internal clock sources via the FOSC bits of the

Configuration Word 1.

7.3

Internal Clock Modes

The oscillator module has eight output frequencies

derived from a 500 kHz high precision oscillator. The

IRCF bits of the OSCCON register select the

postscaler applied to the clock source dividing the

frequency by 1, 2, 4 or 8. Setting the PLLEN bit of the

Configuration Word 1 locks the internal clock source to

16 MHz before the postscaler is selected by the IRCF

bits. The PLLEN bit must be set or cleared at the time

of programming; therefore, only the upper or low four

clock source frequencies are selectable in software.

Internal clock sources are contained internally within

the oscillator module. The internal oscillator block has

two internal oscillators and a dedicated Phase Locked

Loop that are used to generate three internal system

clock sources: the 16 MHz High-Frequency Internal

Oscillator (HFINTOSC) and the 500 kHz (MFINTOSC).

Both can be user-adjusted via software using the

OSCTUNE register (Register 7-2).

7.3.1

INTOSC AND INTOSCIO MODES

The INTOSC and INTOSCIO modes configure the

internal oscillators as the system clock source when

the device is programmed using the oscillator selection

or the FOSC<2:0> bits in the CONFIG1 register. See

Section 8.0

“Device

Configuration”

for

information.

In INTOSC mode, CLKIN is available for general

purpose I/O. CLKOUT outputs the selected internal

oscillator frequency divided by 4. The CLKOUT signal

may be used to provide a clock for external circuitry,

synchronization, calibration, test or other application

requirements.

In INTOSCIO mode, CLKIN and CLKOUT are available

for general purpose I/O.

7.3.2

FREQUENCY SELECT BITS (IRCF)

The output of the 500 kHz MFINTOSC and 16 MHz

HFINTOSC, with Phase Locked Loop enabled, connect

to a postscaler and multiplexer (see Figure 7-1). The

Internal Oscillator Frequency Select bits (IRCF) of the

OSCCON register select the frequency output of the

internal oscillator. Depending upon the PLLEN bit, one

of four frequencies of two frequency sets can be

selected via software:

If PLLEN = 1, HFINTOSC frequency selection is as

follows:

•16 MHz

• 8 MHz (default after Reset)

•4 MHz

•2 MHz

If PLLEN = 0, MFINTOSC frequency selection is as

follows:

•500 kHz

• 250 kHz (default after Reset)

•125 kHz

•62.5 kHz

There is no start-up delay before a new frequency

selected in the IRCF bits takes effect. This is because

the old and new frequencies are derived from INTOSC

via the postscaler and multiplexer.

Start-up delay specifications are located in the

Table 23-2

Section 23.0

“Electrical

Specifications”.

Note:

Following any Reset, the IRCF<1:0> bits

of the OSCCON register are set to ‘10’ and

the frequency selection is set to 8 MHz or

250 kHz. The user can modify the IRCF

bits to select a different frequency.

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