Datasheet

© 2007 Microchip Technology Inc. DS41268D-page 59
PIC12F510/16F506
TABLE 10-2: CAPACITOR SELECTION FOR
CRYSTAL OSCILLATOR –
PIC12F510/16F506
(2)
10.2.3 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 10-3 shows implementation of a parallel reso-
nant 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 kΩ resistor provides
the negative feedback for stability. The 10 kΩ potenti-
ometers bias the 74AS04 in the linear region. This
circuit could be used for external oscillator designs.
FIGURE 10-3: EXTERNAL PARALLEL
RESONANT CRYSTAL
OSCILLATOR CIRCUIT
Figure 10-4 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 10-4: EXTERNAL SERIES
RESONANT CRYSTAL
OSCILLATOR CIRCUIT
10.2.4 EXTERNAL RC OSCILLATOR
For timing insensitive applications, the EXTRC device
option offers additional cost savings. The EXTRC oscil-
lator 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 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 10-5 shows how the R/C combination is
connected to the PIC12F510/16F506 devices. For
R
EXT values below 5.0 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 REXT 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 beyond the drive level
specification. Since each crystal has its
own characteristics, the user should con-
sult the crystal manufacturer for appropri-
ate values of external components.
3: PIC16F506 only.
20 pF
+5V
20 pF
10k
4.7k
10k
74AS04
XTAL
10k
74AS04
CLKIN
To O the r
Devices
PIC12F510
PIC16F506
330
74AS04
74AS04
CLKIN
To Other
Devices
XTAL
330
74AS04
0.1 mF
PIC12F510
PIC16F506