PIC16CR54C
7.2.3
EXTERNAL CRYSTAL OSCILLATOR
CIRCUIT
FIGURE 7-5: EXTERNAL SERIES
RESONANT CRYSTAL
OSCILLATOR CIRCUIT
(USING XT, HS OR LP
OSCILLATOR MODE)
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.
To Other
Devices
330
330
PIC16CR54C
74AS04
74AS04
74AS04
CLKIN
0.1 µF
Figure 7-4 shows implementation of
a
parallel
OSC2
XTAL
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 kΩ resistor
provides the negative feedback for stability. The 10 kΩ
potentiometers bias the 74AS04 in the linear region.
This circuit could be used for external oscillator
designs.
100k
Note:
If you change from this device to
another device, please verify oscillator
characteristics in your application.
FIGURE 7-4: EXTERNAL PARALLEL
RESONANT CRYSTAL
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
resistor (Rext) and capacitor (Cext) values, and the
operating temperature. In addition to this, the oscillator
frequency will vary from unit to unit due to normal
process 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.
OSCILLATOR CIRCUIT
(USING XT, HS OR LP
OSCILLATOR MODE)
+5V
To Other
Devices
10k
74AS04
PIC16CR54C
4.7k
CLKIN
74AS04
OSC2
10k
100k
XTAL
Figure 7-6 shows how the R/C combination is
connected to the PIC16CR54C. 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.
10k
20 pF
20 pF
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
package lead frame capacitance.
Note:
If you change from this device to
another device, please verify oscillator
characteristics in your application.
Figure 7-5 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.
1998 Microchip Technology Inc.
Preliminary
DS40191A-page 27
MICROCHIP [ MICROCHIP ]
