Datasheet

 2000 Microchip Technology Inc. Preliminary DS40197B-page 33
PIC16HV540
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 crys-
tal 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
(USING XT, HS OR LP
OSCILLATOR MODE)
Figure 7-4 shows a series resonant oscillator circuit.
This circuit is also designed to use the fundamental fre-
quency of the crystal. The inverter performs a 180-
degree phase shift in a series resonant oscillator cir-
cuit. 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
(USING XT, HS OR LP
OSCILLATOR MODE)
7.2.4 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 compo-
nents used.
Figure 7-5 shows how the R/C combination is con-
nected to the PIC16HV540. For Rext values below
2.2 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 3 kΩ and 100 kΩ.
Although the oscillator will operate with no external
capacitor (Cext = 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 pack-
age lead frame capacitance.
Note: If you change from this device to another
device, please verify oscillator characteris-
tics in your application.
20 pF
+5V
20 pF
10k
4.7k
10k
74AS04
XTAL
10k
74AS04
PIC16HV540
CLKIN
To Other
Devices
OSC2
100k
Note: If you change from this device to another
device, please verify oscillator characteris-
tics in your application.
330
74AS04 74AS04
PIC16HV540
CLKIN
To Other
Devices
XTAL
330
74AS04
0.1 µF
OSC2
100k