Datasheet

PIC16F8X
DS30430B-page 40  1996 Microchip Technology Inc.
TABLE 8-2: PIC16F83/CR83/F84/CR84
CAPACITOR SELECTION
FOR CRYSTAL OSCILLATOR
8.2.3 EXTERNAL CRYSTAL OSCILLATOR
CIRCUIT
Either a prepackaged oscillator can be used or a simple
oscillator circuit with TTL gates can be built.
Prepackaged oscillators provide a wide operating
range and better stability. A well-designed crystal
oscillator will provide good performance with TTL
gates. Two types of crystal oscillator circuits are
available; one with series resonance, and one with
parallel resonance.
Figure 8-5 shows a parallel resonant oscillator circuit.
The circuit is designed to use the fundamental
frequency of the crystal. The 74AS04 inverter performs
the 180-degree phase shift that a parallel oscillator
requires. The 4.7 k Ω resistor provides negative
feedback for stability. The 10 k Ω potentiometer biases
the 74AS04 in the linear region. This could be used for
external oscillator designs.
Mode Freq OSC1/C1 OSC2/C2
LP 32 kHz
200 kHz
68 - 100 pF
15 - 33 pF
68 - 100 pF
15 - 33 pF
XT 100 kHz
2 MHz
4 MHz
100 - 150 pF
15 - 33 pF
15 - 33 pF
100 - 150 pF
15 - 33 pF
15 - 33 pF
HS 4 MHz
10 MHz
15 - 33 pF
15 - 33 pF
15 - 33 pF
15 - 33 pF
Note: Higher capacitance increases the stability of
oscillator but also increases the start-up time.
These values are for design guidance only. Rs may
be required in HS mode as well as XT mode to
avoid overdriving crystals with low drive level spec-
ification. Since each crystal has its own characteris-
tics, the user should consult the crystal
manufacturer for appropriate values of external
components.
For V DD > 4.5V, C1 = C2 ≈ 30 pF is recommended.
Crystals Tested:
32.768 kHz Epson C-001R32.768K-A ± 20 PPM
100 kHz Epson C-2 100.00 KC-P ± 20 PPM
200 kHz STD XTL 200.000 KHz ± 20 PPM
1.0 MHz ECS ECS-10-13-2 ± 50 PPM
2.0 MHz ECS ECS-20-S-2 ± 50 PPM
4.0 MHz ECS ECS-40-S-4 ± 50 PPM
10.0 MHz ECS ECS-100-S-4 ± 50 PPM
FIGURE 8-5: EXTERNAL PARALLEL
RESONANT CRYSTAL
OSCILLATOR CIRCUIT
Figure 8-6 shows a series resonant oscillator circuit.
This circuit is also designed to use the fundamental
frequency of the crystal. The inverter performs a
180-degree phase shift. The 330 k Ω resistors provide
the negative feedback to bias the inverters in their
linear region.
FIGURE 8-6: EXTERNAL SERIES
RESONANT CRYSTAL
OSCILLATOR CIRCUIT
20 pF
+5V
20 pF
10k
4.7k
10k
74AS04
XTAL
10k
74AS04
PIC16FXX
CLKIN
To Other
Devices
330 kΩ
74AS04
74AS04
PIC16FXX
CLKIN
To Other
Devices
XTAL
330 kΩ
74AS04
0.1 µF