PIC12F508/509/16F505
TABLE 7-2:
CAPACITOR SELECTION FOR
CRYSTAL OSCILLATOR –
PIC12F508/509/16F505(2)
FIGURE 7-3:
EXTERNAL PARALLEL
RESONANT CRYSTAL
OSCILLATOR CIRCUIT
+5V
Osc
Type
Resonator Cap. Range Cap. Range
To Other
Devices
Freq.
C1
C2
10k
32 kHz(1)
15 pF
15 pF
4.7k
74AS04
LP
XT
200 kHz
1 MHz
4 MHz
47-68 pF
15 pF
15 pF
47-68 pF
15 pF
15 pF
CLKIN
74AS04
PIC16F505
PIC12F508
PIC12F509
(3)
10k
HS
20 MHz
15-47 pF
15-47 pF
XTAL
Note 1: For VDD > 4.5V, C1 = C2 ≈ 30 pF is
recommended.
10k
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.
20 pF
20 pF
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.
3: PIC16F505 only.
7.2.3
EXTERNAL CRYSTAL OSCILLATOR
CIRCUIT
FIGURE 7-4:
EXTERNAL SERIES
RESONANT 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.
To Other
Devices
330
330
74AS04
74AS04
74AS04
CLKIN
0.1 mF
XTAL
PIC16F505
PIC12F508
PIC12F509
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.
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 (REXT) 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
CEXT 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 con-
nected to the PIC12F508/509/16F505 devices. For
REXT 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 REXT between 5.0 kΩ and
100 kΩ.
DS41236C-page 42
Preliminary
© 2007 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
