參數(shù)資料
型號(hào)： P12C508
廠商： Microchip Technology Inc.
英文描述： 8-Pin, 8-Bit CMOS Microcontroller
中文描述： 8引腳，8位CMOS微控制器
文件頁(yè)數(shù)： 27/84頁(yè)
文件大?。?/td> 526K
代理商： P12C508
1996 Microchip Technology Inc.
Advance Information
DS40139A-page 27
PIC12C5XX
7.2.3
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 7-4 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
resistor
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 7-4:
EXTERNAL PARALLEL
RESONANT CRYSTAL
OSCILLATOR CIRCUIT
Figure 7-5 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
resistors provide the negative
feedback to bias the inverters in their linear region.
FIGURE 7-5:
EXTERNAL SERIES
RESONANT CRYSTAL
OSCILLATOR CIRCUIT
20 pF
+5V
20 pF
10k
4.7k
10k
74AS04
XTAL
10k
74AS04
PIC12C5XX
CLKIN
To Other
Devices
330
74AS04
74AS04
PIC12C5XX
CLKIN
To Other
Devices
XTAL
330
74AS04
0.1
μ
F
7.2.4
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.
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 7-6 shows how the R/C combination is
connected to the PIC12C5XX. 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
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
DD
for given
Rext/Cext values as well as frequency variation due to
operating temperature for given R, C, and V
DD
values.
Furthermore,
the
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