Datasheet

PIC16F526
DS41326E-page 46  2010 Microchip Technology Inc.
TABLE 8-2: CAPACITOR SELECTION FOR
CRYSTAL OSCILLATOR
(2)
8.2.3 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
performance with TTL gates. Two types of crystal
oscillator circuits can be used: one with parallel
resonance, or one with series resonance.
Figure 8-3 shows implementation of 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 resistor provides
the negative feedback for stability. The 10 k
potentiometers bias the 74AS04 in the linear region.
This circuit could be used for external oscillator
designs.
FIGURE 8-3: EXTERNAL PARALLEL
RESONANT CRYSTAL
OSCILLATOR CIRCUIT
Figure 8-4 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  resistors provide the negative
feedback to bias the inverters in their linear region.
FIGURE 8-4: EXTERNAL SERIES
RESONANT CRYSTAL
OSCILLATOR CIRCUIT
8.2.4 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 8-5 shows how the R/C combination is con-
nected to the PIC16F526 device. For R
EXT values
below 3.0 k, the oscillator operation may become
unstable, or stop completely. For very high REXT values
(e.g., 1 M), the oscillator becomes sensitive to noise,
humidity and leakage. Thus, we recommend keeping
R
EXT between 5.0 k and 100 k.
Although the oscillator will operate with no external
capacitor (C
EXT = 0 pF), we recommend using values
above 20 pF for noise and stability reasons. With no or
small external capacitance, the oscillation frequency
can vary dramatically due to changes in external
capacitances, such as PCB trace capacitance or
package lead frame capacitance.
Section 14.0 “Electrical Characteristics” shows RC
frequency variation from part-to-part due to normal
process variation. The variation is larger for larger val-
ues of R (since leakage current variation will affect RC
frequency more for large R) and for smaller values of C
(since variation of input capacitance will affect RC
frequency more).
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 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.
20 pF
+5V
20 pF
10k
4.7k
10k
74AS04
XTAL
10k
74AS04
CLKIN
To O th e r
Devices
PIC16F526
330
74AS04
74AS04
PIC16F526
CLKIN
To Other
Devices
XTAL
330
74AS04
0.1 mF