PIC16F7X7
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 package types will also
affect the oscillation frequency, especially for low CEXT
values. The user also needs to take into account varia-
tion due to tolerance of external R and C components
used. Figure 4-4 shows how the R/C combination is
connected.
The PIC16F7X7 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 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
• Two-Speed Start-up
• Fail-Safe Clock Monitor
FIGURE 4-4:
RC OSCILLATOR MODE
VDD
These features are discussed in greater detail in
Section 15.0 “Special Features of the CPU”.
REXT
Internal
OSC1
The clock source frequency (INTOSC direct, INTRC
direct or INTOSC postscaler) is selected by configuring
the IRCF bits of the OSCCON register (page 38).
Clock
CEXT
VSS
PIC16F7X7
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).
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).
FIGURE 4-5:
RCIO OSCILLATOR MODE
VDD
REXT
Internal
OSC1
Clock
CEXT
PIC16F7X7
VSS
I/O (OSC2)
RA6
Recommended values: 3 kΩ ≤ REXT ≤ 100 kΩ
CEXT > 20 pF
2004 Microchip Technology Inc.
DS30498C-page 35
MICROCHIP [ MICROCHIP ]
