12F629 datasheet(56/136 Pages) MICROCHIP

56 / 136 page

PIC12F629/675

DS41190G-page 56

 2010 Microchip Technology Inc.

9.2.3

EXTERNAL CLOCK IN

For applications where a clock is already available

elsewhere, users may directly drive the PIC12F629/

675 provided that this external clock source meets the

AC/DC timing requirements listed in Section 12.0

“Electrical Specifications”. Figure 9-2 shows how

an external clock circuit should be configured.

9.2.4

RC OSCILLATOR

For applications where precise timing is not a

requirement, the RC oscillator option is available. The

operation and functionality of the RC oscillator is

dependent upon a number of variables. The RC

oscillator frequency is a function of:

• Supply voltage

• Resistor (REXT) and capacitor (CEXT) values

• Operating temperature.

The oscillator frequency will vary from unit to unit due

to normal process parameter variation. The difference

in lead frame capacitance between package types will

also affect the oscillation frequency, especially for low

CEXT values. The user also needs to account for the

tolerance of the external R and C components.

Figure 9-3 shows how the R/C combination is

connected.

Two options are available for this Oscillator mode

which allow GP4 to be used as a general purpose I/O

or to output FOSC/4.

FIGURE 9-3:

RC OSCILLATOR MODE

9.2.5

INTERNAL 4 MHZ OSCILLATOR

When calibrated, the internal oscillator provides a fixed

4 MHz (nominal) system clock. See Electrical

Specifications, Section 12.0 “Electrical Specifica-

tions”, for information on variation over voltage and

temperature.

Two options are available for this Oscillator mode

which allow GP4 to be used as a general purpose I/O

or to output FOSC/4.

9.2.5.1

Calibrating the Internal Oscillator

A calibration instruction is programmed into the last

location of program memory. This instruction is a

RETLW XX

, where the literal is the calibration value.

The literal is placed in the OSCCAL register to set the

calibration of the internal oscillator. Example 9-1

demonstrates how to calibrate the internal oscillator.

For best operation, decouple (with capacitance) VDD

and VSS as close to the device as possible.

EXAMPLE 9-1:

CALIBRATING THE

INTERNAL OSCILLATOR

9.2.6

CLKOUT

The PIC12F629/675 devices can be configured to

provide a clock out signal in the INTOSC and RC

oscillator modes. When configured, the oscillator

frequency divided by four (FOSC/4) is output on the

GP4/OSC2/CLKOUT pin. FOSC/4 can be used for test

purposes or to synchronize other logic.

GP4/OSC2/CLKOUT

CEXT

VDD

REXT

VSS

PIC12F629/675

GP5/OSC1/

FOSC/4

Internal

Clock

CLKIN

Note:

Erasing the device will also erase the pre-

programmed internal calibration value for

the internal oscillator. The calibration value

must be saved prior to erasing part as

specified in the PIC12F629/675 Program-

ming specification. Microchip Develop-

ment Tools maintain all Calibration bits to

factory settings.

BSF

STATUS, RP0

;Bank 1

CALL

3FFh

;Get the cal value

MOVWF

OSCCAL

;Calibrate

BCF

STATUS, RP0

;Bank 0

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