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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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3
4363A–80C51–07/04
Typical Oscillator
Operation
The process involved in start-up and locking of oscillator is explained hereafter (see Fig-
ure 3):
• Biasing process. The power-up is applied and the amplifier output follows the
power until it reaches its biasing level where it can amplify the noise signal on its
input.
• Oscillation. The amplified noise on the output (Xtal2) is filtered by the feed-back
loop which has a pass-band frequency corresponding to the nominal oscillator
frequency. The filtered output noise is amplified again and starts to increase. The
oscillation level continue to grow and reaches the non-linear area.
• Lock. In the non-linear area both the gain and the oscillation level starts to reduce.
• Steady State. A stabilization point is found where the closed-loop gain is
maintained with the unity.
Figure 3. Process Needed to Reach a Stable Oscillation
Each element plays a role and their electrical characteristics have to be understood. The
next sections explain this matter.
Crystal Model and
Operation
Crystal and ceramic resonators are piezoelectric devices which transform voltage
energy to mechanical vibrations and vice-versa. At certain vibrational frequencies, there
is a mechanical resonance. Main resonances are called: fundamental, third, fifth, ...
overtones. Overtones are not harmonics but different mechanical vibrational modes.
This crystal is an efficient pass-band filter which exhibits a good frequency stability. The
equivalent model, shown in Figure 4, consists of two resonant circuits:
• C1, L1 and R1 is a series resonant circuit (fs),
• In addition the series circuit, C0 in parallel forms a parallel circuit which has a
parallel resonance frequency (fa) .
Vxtal2
Start Oscillation
Steady State
VDD
t
Lock
Biasing
Bias Level
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