RFPIC12F675 Datasheet, PDF (58/136 Pages) Microchip Technology – FLASH-Based Microcontroller with ASK/FSK Transmitter

rfPIC12F675
10.2.3 EXTERNAL CLOCK IN
For applications where a clock is already available
elsewhere, users may directly drive the rfPIC12F675
provided that this external clock source meets the AC/
DC timing requirements listed in Section 13.0.
Figure 10-2 shows how an external clock circuit
should be configured.
Note:
The microcontroller oscillator is indepen-
dent of the RF peripheral oscillator. An
accurate time-base is still possible with
only one crystal. Use the RF crystal on
transmitter and tie the REFCLK signal
back into T0CKI or T1CKI to correct the
RC, INTOSC, or EC clocks. Since REF-
CLK is only active when RFEN=1, it is not
a suitable source for CLKIN.
10.2.4 RC OSCILLATOR
For applications where precise timing is not a require-
ment, 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 10-3 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 10-3:
RC OSCILLATOR MODE
VDD
REXT
CEXT
VSS
FOSC/4
PIC12F629/675
GP5/OSC1/
CLKIN
Internal
Clock
GP4/OSC2/CLKOUT
10.2.5 INTERNAL 4 MHZ OSCILLATOR
When calibrated, the internal oscillator provides a fixed
4 MHz (nominal) system clock. See Electrical
Specifications, Section 13.0, 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.
10.2.5.1 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 10-1
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 rfPIC12F675 Programming
specification. Microchip Development
Tools maintain all calibration bits to factory
settings.
EXAMPLE 10-1: CALIBRATING THE
INTERNAL OSCILLATOR
bsf
call
movwf
bcf
STATUS, RP0
3FFh
OSCCAL
STATUS, RP0
;Bank 1
;Get the cal value
;Calibrate
;Bank 0
10.2.6 CLKOUT
The rfPIC12F675 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.
DS70091B-page 58
Preliminary
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