PIC17C4X
14.2.4 EXTERNAL CRYSTAL OSCILLATOR
CIRCUIT
14.2.5 RC OSCILLATOR
For timing insensitive applications, the RC device
option offers additional cost savings. RC oscillator fre-
quency is a function of the supply voltage, the resistor
(Rext) and capacitor (Cext) values, and the operating
temperature. In addition to this, 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
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 14-6 shows how the R/C combination is con-
nected to the PIC17CXX. 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 14-5 shows implementation of a parallel reso-
nant 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 par-
allel oscillator requires.The 4.7 kΩ resistor provides the
negative feedback for stability.The 10 kΩ potentiometer
biases the 74AS04 in the linear region. This could be
used for external oscillator designs.
FIGURE 14-5: 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 little
or no external capacitance, oscillation frequency can
vary dramatically due to changes in external capaci-
tances, such as PCB trace capacitance or package
lead frame capacitance.
OSCILLATOR CIRCUIT
+5V
To Other
Devices
10k
74AS04
PIC17CXX
4.7k
OSC1
74AS04
See Section 18.0 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).
10k
XTAL
10k
See Section 18.0 for variation of oscillator frequency
due to VDD for given Rext/Cext values as well as fre-
quency variation due to operating temperature for given
R, C, and VDD values.
20 pF
20 pF
Figure 14-6 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
circuit. 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 14-7: RC OSCILLATOR MODE
FIGURE 14-6: EXTERNAL SERIES
RESONANT CRYSTAL
VDD
OSCILLATOR CIRCUIT
Rext
Internal
clock
OSC1
To Other
Devices
330 kΩ
330 kΩ
PIC17CXX
Cext
VSS
74AS04
74AS04
74AS04
PIC17CXX
OSC2/CLKOUT
OSC1
Fosc/4
0.1 µF
XTAL
DS30412C-page 102
1996 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
