PIC16C71X
8.2.3
EXTERNAL CRYSTAL OSCILLATOR
CIRCUIT
8.2.4
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 8-8 shows how the R/C combina-
tion is connected to the PIC16CXX. 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. Prepack-
aged 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 8-6 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-
ter biases the 74AS04 in the linear region. This could
be used for external oscillator designs.
FIGURE 8-6: 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 pack-
age lead frame capacitance.
OSCILLATOR CIRCUIT
+5V
To Other
Devices
10k
74AS04
PIC16CXXX
4.7k
CLKIN
74AS04
See characterization data for desired device for RC fre-
quency variation from part to part due to normal pro-
cess 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 frequency more).
10k
XTAL
10k
See characterization data for desired device for varia-
tion of oscillator frequency due to VDD for given Rext/
Cext values as well as frequency variation due to oper-
ating temperature for given R, C, and VDD values.
20 pF
20 pF
Figure 8-7 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 kΩ resistors provide the negative feed-
back 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 pur-
poses or to synchronize other logic (see Figure 3-2 for
waveform).
FIGURE 8-8: RC OSCILLATOR MODE
FIGURE 8-7: EXTERNAL SERIES
RESONANT CRYSTAL
VDD
OSCILLATOR CIRCUIT
Rext
Internal
OSC1
To Other
clock
Devices
330 kΩ
330 kΩ
Cext
74AS04
74AS04
74AS04
PIC16CXXX
PIC16CXXX
VSS
CLKIN
0.1 µF
OSC2/CLKOUT
Fosc/4
XTAL
1997 Microchip Technology Inc.
DS30272A-page 51
MICROCHIP [ MICROCHIP ]
