PIC16F627A Datasheet, PDF (98/168 Pages) Microchip Technology – FLASH-Based 8-Bit CMOS Microcontrollers

PIC16F627A/628A/648A
FIGURE 14-2:
+5V
10K
4.7K
74AS04
EXTERNAL PARALLEL
RESONANT CRYSTAL
OSCILLATOR CIRCUIT
TO OTHER
DEVICES
74AS04
PIC16F627A/628A/648A
CLKIN
10K
XTAL
10K
C1
C2
Figure 14-3 shows a series resonant oscillator circuit.
This circuit is also designed to use the fundamental
frequency of the crystal. The inverter performs a 180°
phase shift in a series resonant oscillator circuit. The
330 kΩ resistors provide the negative feedback to bias
the inverters in their linear region.
FIGURE 14-3:
EXTERNAL SERIES
RESONANT CRYSTAL
OSCILLATOR CIRCUIT
330 KΩ
74AS04
330 KΩ
74AS04
0.1 PF
XTAL
TO OTHER
DEVICES
74AS04
CLKIN
PIC16F627A/
628A/648A
14.2.4 PRECISION INTERNAL 4 MHZ
OSCILLATOR
The internal precision oscillator provides a fixed 4 MHz
(nominal) system clock at VDD = 5 V and 25°C. See
Section 17.0, Electrical Specifications, for information
on variation over voltage and temperature.
14.2.5 EXTERNAL CLOCK IN
For applications where a clock is already available
elsewhere, users may directly drive the PIC16F627A/
628A/648A provided that this external clock source
meets the AC/DC timing requirements listed in
Section 17.6. Figure 14-4 below shows how an exter-
nal clock circuit should be configured.
FIGURE 14-4:
Clock From
ext. system
RA6
EXTERNAL CLOCK INPUT
OPERATION (EC, HS, XT
OR LP OSC
CONFIGURATION)
RA7/OSC1/CLKIN
PIC16F627A/628A/648A
RA6/OSC2/CLKOUT
14.2.6 RC OSCILLATOR
For applications where precise timing is not a require-
ment, the RC oscillator option is available. The opera-
tion and functionality of the RC oscillator is dependent
upon a number of variables. The RC oscillator fre-
quency 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 14-5 shows how the R/C combination is con-
nected.
FIGURE 14-5:
RC OSCILLATOR MODE
VDD
REXT
CEXT
VSS
FOSC/4
PIC16F627A/628A/648A
RA7/OSC1/
CLKIN
Internal
Clock
RA6/OSC2/CLKOUT
The RC Oscillator mode has two options that control
the unused OSC2 pin. The first allows it to be used as
a general purpose I/O port. The other configures the
pin as an output providing the Fosc signal (internal
clock divided by 4) for test or external synchronization
purposes.
14.2.7 CLKOUT
The PIC16F627A/628A/648A can be configured to pro-
vide a clock out signal by programming the configura-
tion word. The oscillator frequency, divided by 4 can be
used for test purposes or to synchronize other logic.
DS40044A-page 96
Preliminary
 2002 Microchip Technology Inc.