PIC16F684 datasheet(25/192 Pages) MICROCHIP

25 / 192 page

DS41202F-page 23

PIC16F684

3.4.4

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.

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 3-5 shows

the external RC mode connections.

FIGURE 3-5:

EXTERNAL RC MODES

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:

• 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.

3.5

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

8 MHz. The frequency of the HFINTOSC can be

user-adjusted via software using the OSCTUNE

The

LFINTOSC

(Low-Frequency

Internal

Oscillator) is uncalibrated and 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) bit of the OSCCON register. See Section 3.6

“Clock Switching” for more information.

3.5.1

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 Configuration Word

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.

3.5.2

HFINTOSC

The High-Frequency Internal Oscillator (HFINTOSC) is

a factory calibrated 8 MHz internal clock source. The

frequency of the HFINTOSC can be altered via

software using the OSCTUNE register (Register 3-2).

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<2:0>

bits

the

OSCCON

See

Section 3.5.4 “Frequency Select Bits (IRCF)” for

more information.

The HFINTOSC is enabled by selecting any frequency

between 8 MHz and 125 kHz by setting the IRCF<2:0>

bits of the OSCCON register

≠ 000. Then, set the

System Clock Source (SCS) bit of the OSCCON

the IESO bit in the Configuration Word register

(CONFIG) to ‘1’.

The HF Internal Oscillator (HTS) bit of the OSCCON

OSC2/CLKOUT(1)

CEXT

REXT

PIC® MCU

OSC1/CLKIN

FOSC/4 or

Internal

Clock

VDD

VSS

Recommended values: 10 k

Ω ≤ REXT ≤ 100 kΩ, <3V

3 k

Ω ≤ REXT ≤ 100 kΩ, 3-5V

CEXT > 20 pF, 2-5V

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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