PIC16C62X
9.2.3
EXTERNAL CRYSTAL OSCILLATOR
CIRCUIT
9.2.4
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. Figure 9-6 shows how the
R/C combination is connected to the PIC16C62X. 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 to keep Rext between 3 kΩ and 100 kΩ.
Either a prepackaged oscillator can be used or a simple
oscillator circuit with TTL gates can be built.
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 series resonance, or one with parallel
resonance.
Figure 9-4 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° 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 could be used for external oscillator designs.
FIGURE 9-4: EXTERNAL PARALLEL
RESONANT CRYSTAL
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.
OSCILLATOR CIRCUIT
+5V
To other
Devices
10k
74AS04
4.7k
PIC16C62X
See Section 13.0 for RC frequency variation from part
to part due to normal process variation.The variation is
larger for larger R (since leakage current variation will
affect RC frequency more for large R) and for smaller C
(since variation of input capacitance will affect RC fre-
quency more).
CLKIN
74AS04
10k
XTAL
10k
See Section 13.0 for variation of oscillator frequency
due to VDD for given Rext/Cext values as well as
frequency variation due to operating temperature for
given R, C, and VDD values.
20 pF
20 pF
Figure 9-5 shows a series resonant oscillator circuit.
This circuit is also designed to use the fundamental
frequency of the crystal. The inverter performs a 180°
phase shift in a series resonant oscillator circuit. The
330 kΩ resistors provide the negative feedback to bias
the inverters in their linear region.
The oscillator frequency, divided by 4, is available on
the OSC2/CLKOUT pin, and can be used for test
purposes or to synchronize other logic (Figure 3-2 for
waveform).
FIGURE 9-6: RC OSCILLATOR MODE
FIGURE 9-5: EXTERNAL SERIES
RESONANT CRYSTAL
VDD
PIC16C62X
Rext
OSCILLATOR CIRCUIT
OSC1
To other
Internal Clock
Devices
330 kΩ
330 kΩ
Cext
PIC16C62X
74AS04
74AS04
74AS04
VDD
CLKIN
OSC2/CLKOUT
Fosc/4
0.1 µF
XTAL
DS30235G-page 48
Preliminary
1998 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
