Datasheet

PIC16F785/HV785
DS41249E-page 26 © 2008 Microchip Technology Inc.
TABLE 3-3: CAPACITOR SELECTION FOR
CRYSTAL OSCILLATOR
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: 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 (R
EXT) 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 dif-
ference in lead frame capacitance between package
types will also affect the oscillation frequency or low
C
EXT values. The user also needs to take into account
variation due to tolerance of external RC components
used.
Osc Type
Crystal
Freq.
Cap. Range
C1
Cap. Range
C2
LP 32 kHz 15-33 pF 15-33 pF
XT 200 kHz 47-68 pF 47-68 pF
1 MHz 15-33 pF 15-33 pF
4 MHz 15-33 pF 15-33 pF
HS 4 MHz 15-33 pF 15-33 pF
8 MHz 15-33 pF 15-33 pF
20 MHz 15-33 pF 15-33 pF
Note: These values are for design guidance
only. See notes following this table.
Note 1: Higher capacitance increases the stability
of the oscillator, but also increases the
start-up time.
2: Since each resonator/crystal has its own
characteristics, the user should consult
the resonator/crystal manufacturer for
appropriate values of external
components.
3: RS may be required to avoid overdriving
crystals with low drive level specification.
OSC2/CLKOUT
CEXT
REXT
OSC1
F
OSC/4
Internal
Clock
VDD
VSS
Recommended values: 3 kΩ ≤ REXT ≤ 100 kΩ (VDD ≥ 3.0V)
10 kΩ ≤ R
EXT ≤ 100 kΩ (VDD < 3.0V)
C
EXT > 20 pF
PIC16F785/HV785
CEXT
REXT
OSC1
Internal
Clock
VDD
VSS
I/O (OSC2)
RA4
PIC16F785/HV785
Recommended values: 3 kΩ ≤ R
EXT ≤ 100 kΩ (VDD ≥ 3.0V)
10 kΩ ≤ R
EXT ≤ 100 kΩ (VDD < 3.0V)
C
EXT > 20 pF