PIC16F688 Datasheet, PDF (26/174 Pages) Microchip Technology – 14-Pin Flash-Based, 8-Bit CMOS Microcontrollers with nanoWatt Technology

PIC16F688
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 shows the RC mode connections.
FIGURE 3-5:
VDD
RC MODE
REXT
OSC1
Internal
Clock
CEXT
PIC16F688
VSS
OSC2/CLKOUT
FOSC/4
Recommended values: 3 kΩ ≤ REXT ≤ 100 kΩ
CEXT > 20 pF
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:
VDD
RCIO MODE
REXT
CEXT
VSS
RA4
OSC1
Internal
Clock
PIC16F688
I/O (OSC2)
Recommended values: 3 kΩ ≤ REXT ≤ 100 kΩ
CEXT > 20 pF
The RC oscillator frequency is a function of the supply
voltage, the resistor (REXT) and capacitor (CEXT)
values and the operating temperature. Other factors
affecting the oscillator frequency are:
• threshold voltage variation
• component tolerances
• packaging variations in capacitance
The user also needs to take into account variation due
to tolerance of external RC components used.
3.4 Internal Clock Modes
The PIC16F688 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
OSCTUNE register (Register 3-1).
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
(SCS) bit (see Section 3.5 “Clock Switching”).
3.4.1 INTRC AND INTRCIO MODES
The INTRC and INTRCIO 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 register (Register 11-1).
In INTRC 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 INTRCIO 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
register (Register 3-1).
The output of the HFINTOSC connects to a postscaler
and multiplexer (see Figure 3-1). One of seven
frequencies can be selected via software using the
IRCF bits (see Section 3.4.4 “Frequency Select Bits
(IRCF)”).
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.
DS41203B-page 24
Preliminary
 2004 Microchip Technology Inc.