Datasheet 搜索 > 微控制器 > ATMEL（爱特美尔） > AT89C51AC3-SLSUM 数据手册 > AT89C51AC3-SLSUM 其他数据使用手册 6/16 页
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AT89C51AC3-SLSUM 其他数据使用手册 - ATMEL（爱特美尔）
制造商:
ATMEL（爱特美尔）
分类:
微控制器
封装:
PLCC-44
描述:
AT89C51 系列 60 MHz 64 KB 闪存 2.25 KB SRAM 8 位 微控制器 - PLCC-44
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AT89C51AC3-SLSUM数据手册
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6
4363A–80C51–07/04
Overtone or
Fundamental Mode
Vibrational mode is used to reduce the crystal cost. Above 20MHz it is costly to produce
such crystals tuned on the fundamental mode. To avoid that, an overtone mode is used
to tune the oscillation frequency. To work properly, this vibrational mode needs a spe-
cific schematic where a frequency trap is installed on the oscillator output to short-circuit
the fundamental mode and force the overtone mode. The trap is an LC filter installed
between the Xtal2 and the ground. The frequency on this filter is calculated on the fun-
damental mode using the Thomson equation (see Figure 6).
Figure 6. A LC trap is Used for an Overtone Oscillator
Drive Level The characteristics of quartz crystals are influenced by the drive level. In particular,
when the drive level increases, the frequency and the resistance change through non-
linear effects. In extreme cases an inharmonic mode may replace the main mode as the
selective element and cause the frequency of the oscillator jump to a different fre-
quency. With an overdrive level, the crystal substrate itself may be damaged. Typical
characteristic of frequency vs. drive levels is shown in Figure 7.
Figure 7. Frequency Shift vs. Drive Level
Drive level is a measurement of the total power dissipated through the crystal operating
in the circuit. Typical drive levels are between 50 uW and 1000 uW (1 mW). Drive levels
should be kept at the minimum level that will initiate and maintain oscillation. It should be
less than half of the maximum drive level. Excessive drive may cause correlation difficul-
ties, frequency drift, spurious emissions, "ringing" wave forms, excessive ageing, and/or
fatal structural damage to the crystal.
Ftrap
1
2 π×
Ltrap Ctrap
××
-----------------------------------------------------------=
0
X1
C1 C2
Ctrap
Ltrap
Xtal1
Xtal2
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