PIC16F818/819
4.4
RC Oscillator
4.5
Internal Oscillator Block
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 (REXT) and capacitor (CEXT)
values and the operating temperature. In addition to
this, the oscillator frequency will vary from unit to unit
due to normal manufacturing variation. Furthermore,
the difference in lead frame capacitance between pack-
age types will also affect the oscillation frequency,
especially for low CEXT 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.
The PIC16F818/819 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 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 µs 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:
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.
• Power-up Timer
• Watchdog Timer
FIGURE 4-4:
RC OSCILLATOR MODE
These features are discussed in greater detail in
Section 12.0 “Special Features of the CPU”.
VDD
The clock source frequency (INTOSC direct, INTRC
direct or INTOSC postscaler) is selected by configuring
the IRCF bits of the OSCCON register (Register 4-2).
REXT
Internal
OSC1
Clock
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).
CEXT
VSS
PIC16F818/819
OSC2/CLKO
FOSC/4
Recommended values: 3 kΩ ≤ REXT ≤ 100 kΩ
CEXT > 20 pF
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).
4.5.1
INTRC MODES
Using the internal oscillator as the clock source can
eliminate the need for up to two external oscillator pins,
which can then be used for digital I/O. Two distinct
configurations are available:
FIGURE 4-5:
RCIO OSCILLATOR MODE
VDD
• In INTIO1 mode, the OSC2 pin outputs FOSC/4
while OSC1 functions as RA7 for digital input and
output.
REXT
Internal
OSC1
Clock
• In INTIO2 mode, OSC1 functions as RA7 and
OSC2 functions as RA6, both for digital input and
output.
CEXT
PIC16F818/819
VSS
I/O (OSC2)
RA6
Recommended values: 3 kΩ ≤ REXT ≤ 100 kΩ
CEXT > 20 pF
2004 Microchip Technology Inc.
DS39598E-page 35
MICROCHIP [ MICROCHIP ]
