Datasheet

© 2009 Microchip Technology Inc. DS41236E-page 45
PIC12F508/509/16F505
TABLE 7-2: CAPACITOR SELECTION FOR
CRYSTAL OSCILLATOR –
PIC12F508/509/16F505
(2)
7.2.3 EXTERNAL CRYSTAL OSCILLATOR
CIRCUIT
Either a prepackaged oscillator or a simple oscillator
circuit with TTL gates can be used as an external
crystal oscillator circuit. Prepackaged oscillators
provide a wide operating range and better stability. A
well-designed crystal oscillator will provide good perfor-
mance with TTL gates. Two types of crystal oscillator
circuits can be used: one with parallel resonance, or
one with series resonance.
Figure 7-3 shows implementation of a parallel resonant
oscillator circuit. The circuit is designed to use the fun-
damental frequency of the crystal. The 74AS04 inverter
performs the 180-degree phase shift that a parallel
oscillator requires. The 4.7 kΩ resistor provides the
negative feedback for stability. The 10 kΩ potentiome-
ters bias the 74AS04 in the linear region. This circuit
could be used for external oscillator designs.
FIGURE 7-3: EXTERNAL PARALLEL
RESONANT CRYSTAL
OSCILLATOR CIRCUIT
Figure 7-4 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 in a series resonant oscillator
circuit. The 330 Ω resistors provide the negative
feedback to bias the inverters in their linear region.
FIGURE 7-4: EXTERNAL SERIES
RESONANT CRYSTAL
OSCILLATOR CIRCUIT
7.2.4 EXTERNAL RC OSCILLATOR
For timing insensitive applications, the RC device
option offers additional cost savings. The RC oscillator
frequency is a function of the supply voltage, the resis-
tor (R
EXT) and capacitor (CEXT) values, and the operat-
ing temperature. In addition to this, the oscillator
frequency will vary from unit-to-unit due to normal pro-
cess parameter variation. Furthermore, the difference
in lead frame capacitance between package types will
also affect the oscillation frequency, especially for low
C
EXT values. The user also needs to take into account
variation due to tolerance of external R and C
components used.
Figure 7-5 shows how the R/C combination is
connected to the PIC12F508/509/16F505 devices. For
R
EXT values below 3.0 kΩ, the oscillator operation may
become unstable, or stop completely. For very high
REXT values (e.g., 1 MΩ), the oscillator becomes
sensitive to noise, humidity and leakage. Thus, we
recommend keeping R
EXT between 5.0 kΩ and
100 kΩ.
Osc
Type
Resonator
Freq.
Cap. Range
C1
Cap. Range
C2
LP 32 kHz
(1)
15 pF 15 pF
XT 200 kHz
1 MHz
4 MHz
47-68 pF
15 pF
15 pF
47-68 pF
15 pF
15 pF
HS
(3)
20 MHz 15-47 pF 15-47 pF
Note 1: For V
DD > 4.5V, C1 = C2 ≈ 30 pF is
recommended.
2: These values are for design guidance
only. Rs may be required to avoid over-
driving crystals with low drive level specifi-
cation. Since each crystal has its own
characteristics, the user should consult
the crystal manufacturer for appropriate
values of external components.
3: PIC16F505 only.
20 pF
+5V
20 pF
10k
4.7k
10k
74AS04
XTAL
10k
74AS04
PIC16F505
CLKIN
To O ther
Devices
PIC12F508
PIC12F509
330
74AS04
74AS04
PIC16F505
CLKIN
To Other
Devices
XTAL
330
74AS04
0.1 mF
PIC12F508
PIC12F509