PIC16F684-ESLASHP datasheet(24/164 Pages) MICROCHIP

24 / 164 page

PIC16F684

DS41202C-page 22

Preliminary

 2004 Microchip Technology Inc.

3.3.4

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.

In RC mode, the RC circuit connects to the OSC1 pin.

The OSC2/CLKOUT pin 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 3-5 shows the RC mode connections.

FIGURE 3-5:

RC MODE

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

pin. The OSC2 pin becomes an additional general

purpose I/O pin. The I/O pin becomes bit 4 of PORTA

(RA4). Figure 3-6 shows the RCIO mode connections.

FIGURE 3-6:

RCIO MODE

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:

• 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.

3.4

Internal Clock Modes

The

PIC16F684

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

8 MHz. The frequency of the HFINTOSC can be

user adjusted ±12% via software using the

OSCTUNE register (Register 3-1).

The

LFINTOSC

(Low-Frequency

Internal

Oscillator) is uncalibrated and operates at

approximately 31 kHz.

The system clock speed can be selected via software

using the Internal Oscillator Frequency Select (IRCF)

bits.

The system clock can be selected between external or

internal clock sources via the System Clock Selection

(SCS) bit (see Section 3.5 “Clock Switching”).

3.4.1

INTRC AND INTRCIO MODES

The INTRC and INTRCIO modes configure the internal

oscillators as the system clock source when the device

is programmed using the Oscillator Selection (FOSC)

bits in the Configuration Word register (Register 12-1).

In INTRC mode, the OSC1 pin is available for general

purpose I/O. The OSC2/CLKOUT pin 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 INTRCIO mode, the OSC1 and OSC2 pins are

available for general purpose I/O.

3.4.2

HFINTOSC

The High-Frequency Internal Oscillator (HFINTOSC) is

a factory calibrated 8 MHz internal clock source. The

frequency

the

HFINTOSC

can

altered

approximately ±12% via software using the OSCTUNE

The output of the HFINTOSC connects to a postscaler

and multiplexer (see Figure 3-1). One of seven

frequencies can be selected via software using the

IRCF bits (see Section 3.4.4 “Frequency Select Bits

(IRCF)”).

The HFINTOSC is enabled by selecting any frequency

between 8 MHz and 125 kHz (IRCF

≠ 000) as the

system clock source (SCS = 1), or when Two-Speed

Start-up is enabled (IESO = 1 and IRCF

≠ 000).

The HF Internal Oscillator (HTS) bit (OSCCON<2>)

indicates whether the HFINTOSC is stable or not.

OSC2/CLKOUT

CEXT

REXT

PIC16F684

OSC1

FOSC/4

Internal

Clock

VDD

VSS

Recommended values: 3 k

Ω ≤ REXT ≤ 100 kΩ

CEXT > 20 pF

CEXT

REXT

PIC16F684

OSC1

Internal

Clock

VDD

VSS

Recommended values: 3 k

Ω ≤ REXT ≤ 100 kΩ

CEXT > 20 pF

I/O (OSC2)

RA4

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