PIC16F630/676
9.2.3
EXTERNAL CLOCK IN
9.2.5
INTERNAL 4 MHZ OSCILLATOR
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.
Figure 9-2 shows how an external clock circuit should
be configured.
When calibrated, the internal oscillator provides a fixed
4
MHz (nominal) system clock. See Electrical
Specifications, Section 12.0, 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.
9.2.4
RC OSCILLATOR
9.2.5.1
Calibrating the Internal 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:
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.
• 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.
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.
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.
EXAMPLE 9-1:
CALIBRATING THE
INTERNAL OSCILLATOR
bsf
call
movwf OSCCAL
bcf
STATUS, RP0
3FFh
;Bank 1
;Get the cal value
;Calibrate
FIGURE 9-3:
RC OSCILLATOR MODE
STATUS, RP0
;Bank 0
VDD
PIC16F630/676
REXT
RA5/OSC1/
CLKIN
Internal
Clock
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.
CEXT
VSS
FOSC/4
RA4/OSC2/CLKOUT
DS40039E-page 56
© 2007 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
