PIC12CE519-04I Datasheet, PDF (37/113 Pages) Microchip Technology – 8-Pin, 8-Bit CMOS Microcontrollers

8.2.3 EXTERNAL CRYSTAL OSCILLATOR
CIRCUIT
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.
Figure 8-4 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-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.
FIGURE 8-4: EXTERNAL PARALLEL
RESONANT CRYSTAL
OSCILLATOR CIRCUIT
+5V
10k
4.7k
74AS04
To Other
Devices
74AS04
PIC12C5XX
CLKIN
10k
XTAL
10k
20 pF 20 pF
Figure 8-5 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.
FIGURE 8-5:
330
74AS04
EXTERNAL SERIES
RESONANT CRYSTAL
OSCILLATOR CIRCUIT
330
74AS04
To Other
Devices
PIC12C5XX
74AS04
0.1 µF
CLKIN
XTAL
PIC12C5XX
8.2.4 EXTERNAL 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 8-6 shows how the R/C combination is
connected to the PIC12C5XX. 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.
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.
The Electrical Specifications sections show 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 frequency
more).
Also, see the Electrical Specifications sections 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.
FIGURE 8-6: EXTERNAL RC OSCILLATOR
MODE
VDD
Rext
OSC1
Internal
clock
Cext
VSS
N
PIC12C5XX
© 1999 Microchip Technology Inc.
DS40139E-page 37