PIC12CE67 Microchip Technology, PIC12CE67 Datasheet - Page 47

• Manufacturer Part Number: PIC12CE67
• Description: 8-Pin/ 8-Bit CMOS Microcontroller with A/D Converter and EEPROM Data Memory
• Manufacturer: Microchip Technology

9.2.3
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 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-degree phase shift
that a parallel oscillator requires. The 4.7 k
provides the negative feedback for stability. The 10 k
potentiometers bias the 74AS04 in the linear region.
This circuit could be used for external oscillator
designs.
FIGURE 9-4:
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-
degree phase shift in a series resonant oscillator
circuit. The 330
feedback to bias the inverters in their linear region.
FIGURE 9-5:
1998 Microchip Technology Inc.
74AS04
330
10k
EXTERNAL CRYSTAL OSCILLATOR
CIRCUIT
+5V
20 pF
0.1 F
XTAL
10k
74AS04
XTAL
4.7k
EXTERNAL PARALLEL
RESONANT CRYSTAL
OSCILLATOR CIRCUIT
EXTERNAL SERIES
RESONANT CRYSTAL
OSCILLATOR CIRCUIT
74AS04
330
resistors provide the negative
20 pF
10k
74AS04
74AS04
To Other
Devices
To Other
Devices
PIC12CE67X
CLKIN
PIC12CE67X
CLKIN
resistor
Preliminary
9.2.4
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
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 PIC12CE67X. For Rext values below
2.2 k , the oscillator operation may become unstable,
or stop completely. For very high Rext values
(e.g., 1 M ) the oscillator becomes sensitive to noise,
humidity and leakage. Thus, we recommend keeping
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
Rext/Cext values as well as frequency variation due to
operating temperature for given R, C, and V
FIGURE 9-6:
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 V
Cext
V
Rext
SS
V
DD
FOSC/4
EXTERNAL RC OSCILLATOR
parameter
EXTERNAL RC OSCILLATOR
MODE
PIC12CE67X
variation.
OSC1
OSC2/CLKOUT
N
Furthermore,
DS40181B-page 47
PIC12CE67X
DD
DD
Internal
clock
for given
values.
the