PIC16F676-I Datasheet, PDF (58/132 Pages) Microchip Technology – 14-Pin, Flash-Based 8-Bit CMOS Microcontrollers

PIC16F630/676
9.2.3 EXTERNAL CLOCK IN
For applications where a clock is already available
elsewhere, users may directly drive the PIC16F630/
676 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 RA4 to be used as a general purpose I/O
or to output FOSC/4.
FIGURE 9-3:
RC OSCILLATOR MODE
VDD
REXT
CEXT
VSS
FOSC/4
PIC16F630/676
RA5/OSC1/
CLKIN
RA4/OSC2/CLKOUT
Internal
Clock
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 RA4 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.
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 PIC16F630/676 Program-
ming specification. Microchip Develop-
ment Tools maintain all calibration bits to
factory settings.
EXAMPLE 9-1:
CALIBRATING THE
INTERNAL OSCILLATOR
BSF
CALL
MOVWF
BCF
STATUS, RP0
3FFh
OSCCAL
STATUS, RP0
;Bank 1
;Get the cal value
;Calibrate
;Bank 0
9.2.6 CLKOUT
The PIC16F630/676 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
RA4/OSC2/CLKOUT pin. FOSC/4 can be used for test
purposes or to synchronize other logic.
DS40039F-page 58
 2010 Microchip Technology Inc.