Datasheet

PIC16CE62X
DS40182D-page 52  1998-2013 Microchip Technology Inc.
10.2.3 EXTERNAL CRYSTAL OSCILLATOR
CIRCUIT
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 10-3 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 phase shift that a parallel
oscillator requires. The 4.7 k resistor provides the
negative feedback for stability. The 10 k
potentiometers bias the 74AS04 in the linear region.
This could be used for external oscillator designs.
FIGURE 10-3: EXTERNAL PARALLEL
RESONANT CRYSTAL
OSCILLATOR CIRCUIT
Figure 10-4 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 k resistors provide the negative feedback to bias
the inverters in their linear region.
FIGURE 10-4: EXTERNAL SERIES
RESONANT CRYSTAL
OSCILLATOR CIRCUIT
20 pF
+5V
20 pF
10k
4.7k
10k
74AS04
XTAL
10k
74AS04
PIC16CE62X
CLK
IN
To Other
Devices
330
74AS04
74AS04
PIC16CE62X
CLK
IN
To ot h er
Devices
XTAL
330
74AS04
0.1 F
10.2.4 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 10-5 shows how the
R/C combination is connected to the PIC16CE62X. For
Rext values below 2.2 k, the oscillator operation may
become unstable, or stop completely. For very high
Rext values (i.e., 1 M), the oscillator becomes
sensitive to noise, humidity and leakage. Thus, we
recommend to keep Rext between 3 k and 100 k.
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.
See Section 14.0 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).
See Section 14.0 for variation of oscillator frequency
due to V
DD for given Rext/Cext values, as well as
frequency variation due to operating temperature for
given R, C, and VDD values.
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 (Figure 3-2 for
waveform).
FIGURE 10-5: RC OSCILLATOR MODE
OSC2/CLKOUT
Cext
Rext
V
DD
PIC16CE62X
OSC1
F
OSC/4
Internal Clock
VDD