PIC12F635ESLASHST datasheet(34/196 Pages) MICROCHIP

34 / 196 page

PIC12F635/PIC16F636/639

DS41232B-page 32

Preliminary

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. In addition to

this, the oscillator frequency will vary from unit to unit

due to normal threshold voltage. Furthermore, the

difference in lead frame capacitance between package

types will also affect the oscillation frequency or for low

CEXT values. The user also needs to take into account

variation due to tolerance of external RC components

used.

3.4

Internal Clock Modes

The PIC12F635/PIC16F636/639 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

LFINTOSC AND LFINTOSCIO

MODES

The LFINTOSC and LFINTOSCIO 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

In LFINTOSC 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 LFINTOSCIO 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

PIC12F635/PIC16F636/639

OSC1

FOSC/4

Internal

Clock

VDD

VSS

Recommended values: 3 k

Ω ≤ REXT ≤ 100 kΩ

CEXT > 20 pF

CEXT

REXT

PIC12F635/PIC16F636/639

OSC1

Internal

Clock

VDD

VSS

Recommended values: 3 k

Ω ≤ REXT ≤ 100 kΩ

CEXT > 20 pF

I/O (OSC2)

RA4

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