PIC18F23K22-ESLASHML datasheet(36/496 Pages) MICROCHIP

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PIC18(L)F2X/4XK22

DS41412C-page 36

Preliminary

 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 2-8 shows the

external RC mode connections.

FIGURE 2-8:

EXTERNAL RC MODES

2.4.4.2

RCIO Mode

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

OSC2 becomes a 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 three 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

be user-adjusted via software using the

OSCTUNE register (Register 2-3).

The MFINTOSC (Medium-Frequency Internal

Oscillator) is factory calibrated and operates

at 500 kHz. The frequency of the MFINTOSC

can be user-adjusted via software using the

OSCTUNE register (Register 2-3).

The

LFINTOSC

(Low-Frequency

Internal

Oscillator) is factory calibrated and operates at

31.25 kHz. The LFINTOSC cannot be user-

adjusted, but is designed to be stable over

temperature and voltage.

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 WITH I/O OR CLOCKOUT

Two of the clock modes selectable with the FOSC<3:0>

bits of the CONFIG1H Configuration register configure

the internal oscillator block as the primary oscillator.

Mode selection determines whether OSC2/CLKOUT/

RA7 will be configured as general purpose I/O (RA7) or

FOSC/4 (CLKOUT). In both modes, OSC1/CLKIN/RA7

configured

general

purpose

I/O.

See

Section 24.0 “Special Features of the CPU” for more

information.

The CLKOUT signal may be used to provide a clock for

external circuitry, synchronization, calibration, test or

other application requirements.

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 1:

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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