Specifications

PIC16F627A/628A/648A
DS40044B-page 96 Preliminary  2004 Microchip Technology Inc.
FIGURE 14-2: EXTERNAL PARALLEL
RESONANT CRYSTAL
OSCILLATOR CIRCUIT
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
14.2.4 PRECISION INTERNAL 4 MH
Z
OSCILLATOR
The internal precision oscillator provides a fixed 4 MHz
(nominal) system clock at V
DD = 5 V and 25°C. See
Section 17.0 "Electrical Specifications", for informa-
tion 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 "Timing Diagrams and Specifica-
tions". Figure 14-4 below shows how an external clock
circuit should be configured.
FIGURE 14-4: EXTERNAL CLOCK INPUT
OPERATION (EC, HS, XT
OR LP OSC
CONFIGURATION)
14.2.6 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 (R
EXT) 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
connected.
FIGURE 14-5: RC OSCILLATOR MODE
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
provide a clock out signal by programming the configu-
ration word. The oscillator frequency, divided by 4 can
be used for test purposes or to synchronize other logic.
+5V
10K
4.7K
10K
74AS04
XTAL
10K
74AS04
PIC16F627A/628A/648A
CLKIN
TO O THER
DEVICES
C1 C2
330 KΩ
74AS04
74AS04
PIC16F627A
/
CLKIN
TO O THER
DEVICES
XTAL
330 KΩ
74AS04
0.1 PF
628A/648A
Clock From
ext. system
PIC16F627A/628A/648A
RA6
RA7/OSC1/CLKIN
RA6/OSC2/CLKOUT
CEXT
VDD
REXT
VSS
PIC16F627A/628A/648A
F
OSC/4
Internal
Clock
CLKIN
RA7/OSC1/
RA6/OSC2/CLKOUT