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Sheet目录3873

157页 158页 159页 160页 [161页]162页 163页 164页 165页

PIC18F2XK20/4XK20

DS41303G-page 32

2010 Microchip Technology Inc.

2.4.4

EXTERNAL RC MODES

The external Resistor-Capacitor (RC) modes support

the use of an external RC circuit. This allows the

designer maximum flexibility in frequency choice while

keeping costs to a minimum when clock accuracy is not

required. There are two modes: RC and RCIO.

2.4.4.1

RC Mode

In RC mode, the RC circuit connects to OSC1. OSC2/

CLKOUT outputs the RC oscillator frequency divided

by 4. This signal may be used to provide a clock for

external circuitry, synchronization, calibration, test or

other application requirements. Figure 2-5 shows the

external RC mode connections.

FIGURE 2-5:

EXTERNAL RC MODES

2.4.4.2

RCIO Mode

In RCIO mode, the RC circuit is connected to OSC1.

OSC2 becomes an additional general purpose I/O pin.

The RC oscillator frequency is a function of the supply

voltage, the resistor (REXT) and capacitor (CEXT) values

and the operating temperature. Other factors affecting

the oscillator frequency are:

input threshold voltage variation

component tolerances

packaging variations in capacitance

The user also needs to take into account variation due

to tolerance of external RC components used.

2.5

Internal Clock Modes

The Oscillator module has two independent, internal

oscillators that can be configured or selected as the

system clock source.

The

HFINTOSC

(High-Frequency

Internal

Oscillator) is factory calibrated and operates at

16 MHz. The frequency of the HFINTOSC can

user-adjusted

via

software

using

the

OSCTUNE register (Register 2-2).

The

LFINTOSC

(Low-Frequency

Internal

Oscillator) operates at 31 kHz.

The system clock speed can be selected via software

using the Internal Oscillator Frequency Select bits

IRCF<2:0> of the OSCCON register.

The system clock can be selected between external or

internal clock sources via the System Clock Selection

(SCS<1:0>) bits of the OSCCON register. See

Section 2.9 “Clock Switching” for more information.

2.5.1

INTOSC AND INTOSCIO MODES

The INTOSC and INTOSCIO modes configure the

internal oscillators as the primary clock source. The

FOSC<3:0> bits in the CONFIG1H Configuration

Section 23.0 “Special Features of the CPU” for more

information.

In INTOSC mode, OSC1/CLKIN is available for general

purpose I/O. OSC2/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, OSC1/CLKIN and OSC2/CLKOUT

are available for general purpose I/O.

2.5.2

HFINTOSC

The output of the HFINTOSC connects to a postscaler

and

multiplexer

(see

Figure 2-1).

One

eight

frequencies can be selected via software using the

IRCF<2:0>

bits

the

OSCCON

See

Section 2.5.4 “Frequency Select Bits (IRCF)” for

more information.

The HFINTOSC is enabled when:

SCS1 = 1 and IRCF<2:0>

000

SCS1 = 1 and IRCF<2:0> = 000 and INTSRC = 1

IESO bit of CONFIG1H = 1 enabling Two-Speed

Start-up.

FCMEM bit of CONFIG1H = 1 enabling Two-

Speed Start-up and Fail-Safe mode.

FOSC<3:0> of CONFIG1H selects the internal

oscillator as the primary clock

The HF Internal Oscillator (IOFS) bit of the OSCCON

OSC2/CLKOUT(1)

CEXT

REXT

PIC MCU

OSC1/CLKIN

FOSC/4 or

Internal

Clock

VDD

VSS

Recommended values: 10 k

REXT 100 k

CEXT > 20 pF

Note

Alternate pin functions are listed in

Section 1.0 “Device Overview”.

Output depends upon RC or RCIO clock mode.

I/O(2)

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