參數(shù)資料
型號(hào)： MCV14A-I/SL
元件分類： 微控制器/微處理器
英文描述： 8-BIT, FLASH, 20 MHz, RISC MICROCONTROLLER, PDSO14
封裝： 3.90 MM, PLASTIC, SOIC-14
文件頁(yè)數(shù)： 32/82頁(yè)
文件大小： 1053K
代理商： MCV14A-I/SL
MCV14A
DS41338C-page 38
Preliminary
2009 Microchip Technology Inc.
TABLE 7-2:
CAPACITOR SELECTION FOR
CRYSTAL OSCILLATOR(2)
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 7-3 shows implementation of a parallel resonant
oscillator circuit. The circuit is designed to use the fun-
damental frequency of the crystal. The 74AS04 inverter
performs the 180-degree phase shift that a parallel
oscillator requires. The 4.7 k
Ω resistor provides the
negative feedback for stability. The 10 k
Ω potentiome-
ters bias the 74AS04 in the linear region. This circuit
could be used for external oscillator designs.
FIGURE 7-3:
EXTERNAL PARALLEL
RESONANT CRYSTAL
OSCILLATOR CIRCUIT
Figure 7-4 shows a series resonant oscillator circuit.
This circuit is also designed to use the fundamental
frequency of the crystal. The inverter performs a
180-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 7-4:
EXTERNAL SERIES
RESONANT CRYSTAL
OSCILLATOR CIRCUIT
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 resis-
tor (REXT) and capacitor (CEXT) values, and the operat-
ing temperature. In addition to this, the oscillator
frequency will vary from unit-to-unit due to normal pro-
cess 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.
how
the
R/C combination is
connected to the MCV14A device. For REXT values
below 3.0 k
Ω, the oscillator operation may become
unstable, or stop completely. For very high REXT values
(e.g., 1 M
Ω), the oscillator becomes sensitive to noise,
humidity and leakage. Thus, we recommend keeping
REXT between 5.0 k
Ω 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.
frequency variation from part-to-part due to normal
process variation. The variation is larger for larger val-
ues of R (since leakage current variation will affect RC
frequency more for large R) and for smaller values of C
(since variation of input capacitance will affect RC
frequency more).
Osc
Type
Resonator
Freq.
Cap. Range
C1
Cap. Range
C2
LP
32 kHz(1)
15 pF
XT
200 kHz
1 MHz
4 MHz
47-68 pF
15 pF
47-68 pF
15 pF
HS
20 MHz
15-47 pF
Note 1:
For VDD > 4.5V, C1 = C2
≈ 30 pF is
recommended.
2:
These values are for design guidance
only. Rs may be required to avoid over-
driving crystals with low drive level specifi-
cation. Since each crystal has its own
characteristics, the user should consult
the crystal manufacturer for appropriate
values of external components.
20 pF
+5V
20 pF
10k
4.7k
10k
74AS04
XTAL
10k
74AS04
CLKIN
To Other
Devices
MCV14A
330
74AS04
MCV14A
CLKIN
To Other
Devices
XTAL
330
74AS04
0.1 mF
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