Datasheet

 2002-2013 Microchip Technology Inc. DS30487D-page 37
PIC16F87/88
4.4 RC Oscillator
For timing insensitive applications, the “RC” and
“RCIO” device options offer additional cost savings.
The RC oscillator frequency is a function of the supply
voltage, the resistor (R
EXT) and capacitor (CEXT) val-
ues and the operating temperature. In addition to this,
the oscillator frequency will vary from unit to unit due to
normal manufacturing variation. Furthermore, the dif-
ference in lead frame capacitance between package
types will also affect the oscillation frequency,
especially for low C
EXT values. The user also needs to
take into account variation due to tolerance of external
R and C components used. Figure 4-4 shows how the
R/C combination is connected.
In the RC Oscillator mode, the oscillator frequency
divided by 4 is available on the OSC2 pin. This signal may
be used for test purposes or to synchronize other logic.
FIGURE 4-4: RC OSCILLATOR MODE
The RCIO Oscillator mode (Figure 4-5) functions like
the RC mode, except that the OSC2 pin becomes an
additional general purpose I/O pin. The I/O pin
becomes bit 6 of PORTA (RA6).
FIGURE 4-5: RCIO OSCILLATOR MODE
4.5 Internal Oscillator Block
The PIC16F87/88 devices include an internal oscillator
block which generates two different clock signals;
either can be used as the system’s clock source. This
can eliminate the need for external oscillator circuits on
the OSC1 and/or OSC2 pins.
The main output (INTOSC) is an 8 MHz clock source
which can be used to directly drive the system clock. It
also drives the INTOSC postscaler which can provide a
range of six clock frequencies from 125 kHz to 4 MHz.
The other clock source is the internal RC oscillator
(INTRC) which provides a 31.25 kHz (32 s nominal
period) output. The INTRC oscillator is enabled by
selecting the INTRC as the system clock source or
when any of the following are enabled:
• Power-up Timer
• Watchdog Timer
• Two-Speed Start-up
• Fail-Safe Clock Monitor
These features are discussed in greater detail in
Section 15.0 “Special Features of the CPU”.
The clock source frequency (INTOSC direct, INTRC
direct or INTOSC postscaler) is selected by configuring
the IRCF bits of the OSCCON register (page 40).
OSC2/CLKO
CEXT
REXT
PIC16F87/88
OSC1
F
OSC/4
Internal
Clock
VDD
VSS
Recommended values: 3 k  REXT  100 k
C
EXT > 20 pF
CEXT
REXT
PIC16F87/88
OSC1
Internal
Clock
VDD
VSS
Recommended values: 3 k  REXT  100 k
C
EXT > 20 pF
I/O (OSC2)
RA6
Note: Throughout this data sheet, when referring
specifically to a generic clock source, the
term “INTRC” may also be used to refer to
the clock modes using the internal oscillator
block. This is regardless of whether the
actual frequency used is INTOSC (8 MHz),
the INTOSC postscaler or INTRC
(31.25 kHz).