
PIC12F635/PIC16F636/639
DS41232B-page 32
Preliminary
2005 Microchip Technology Inc.
3.3.4
EXTERNAL RC MODES
The External Resistor-Capacitor (RC) modes support
the use of an external RC circuit. This allows the
designer maximum flexibility in frequency choice while
keeping costs to a minimum when clock accuracy is not
required. There are two modes, RC and RCIO.
In RC mode, the RC circuit connects to the OSC1 pin.
The OSC2/CLKOUT pin outputs the RC oscillator
frequency divided by 4. This signal may be used to
provide a clock for external circuitry, synchronization,
calibration, test or other application requirements.
FIGURE 3-5:
RC MODE
In RCIO mode, the RC circuit is connected to the OSC1
pin. The OSC2 pin becomes an additional general
purpose I/O pin. The I/O pin becomes bit 4 of PORTA
(RA4).
Figure 3-6 shows the RCIO mode connections.
FIGURE 3-6:
RCIO MODE
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 threshold voltage. Furthermore, the
difference in lead frame capacitance between package
types will also affect the oscillation frequency or for low
CEXT values. The user also needs to take into account
variation due to tolerance of external RC components
used.
3.4
Internal Clock Modes
The PIC12F635/PIC16F636/639 has two independent,
internal oscillators that can be configured or selected
as the system clock source.
1.
The
HFINTOSC
(High-Frequency
Internal
Oscillator) is factory calibrated and operates at
8 MHz. The frequency of the HFINTOSC can be
user adjusted ±12% via software using the
2.
The
LFINTOSC
(Low-Frequency
Internal
Oscillator) is uncalibrated and operates at
approximately 31 kHz.
The system clock speed can be selected via software
using the Internal Oscillator Frequency Select (IRCF)
bits.
The system clock can be selected between external or
internal clock sources via the System Clock Selection
3.4.1
LFINTOSC AND LFINTOSCIO
MODES
The LFINTOSC and LFINTOSCIO modes configure
the internal oscillators as the system clock source
when the device is programmed using the oscillator
selection (FOSC) bits in the Configuration Word
In LFINTOSC mode, the OSC1 pin is available for
general purpose I/O. The OSC2/CLKOUT pin outputs
the selected internal oscillator frequency divided by 4.
The CLKOUT signal may be used to provide a clock for
external circuitry, synchronization, calibration, test or
other application requirements.
In LFINTOSCIO mode, the OSC1 and OSC2 pins are
available for general purpose I/O.
3.4.2
HFINTOSC
The High-Frequency Internal Oscillator (HFINTOSC) is
a factory calibrated 8 MHz internal clock source. The
frequency
of
the
HFINTOSC
can
be
altered
approximately ±12% via software using the OSCTUNE
The output of the HFINTOSC connects to a postscaler
frequencies can be selected via software using the
The HFINTOSC is enabled by selecting any frequency
between 8 MHz and 125 kHz (IRCF
≠ 000) as the
system clock source (SCS = 1), or when Two-Speed
Start-up is enabled (IESO = 1 and IRCF
≠ 000).
The HF Internal Oscillator (HTS) bit (OSCCON<2>)
indicates whether the HFINTOSC is stable or not.
OSC2/CLKOUT
CEXT
REXT
PIC12F635/PIC16F636/639
OSC1
FOSC/4
Internal
Clock
VDD
VSS
Recommended values: 3 k
Ω ≤ REXT ≤ 100 kΩ
CEXT > 20 pF
CEXT
REXT
PIC12F635/PIC16F636/639
OSC1
Internal
Clock
VDD
VSS
Recommended values: 3 k
Ω ≤ REXT ≤ 100 kΩ
CEXT > 20 pF
I/O (OSC2)
RA4