On the use of harmonic resonators in MK devices. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Microcontrollers
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The use of harmonic quartz resonators in clock generators of microcontrollers has its own specifics, and often an attempt to use them in a typical switching circuit leads to failure - generation occurs at the first, fundamental harmonic of the resonator.
Connecting the L1 coil and the coupling capacitor C3 (see figure) ensures that the clock generator starts at the third harmonic of the resonator ZQ1, which is facilitated by the oscillatory circuit L1C2, tuned to the frequency of this harmonic.
Similarly, it is possible to excite the generator at the fifth or seventh harmonic, however, in this case, it is necessary to compensate for the static capacitance of the quartz resonator by connecting coil L2 in parallel with it. Together with the static capacitance of the resonator, it forms a parallel oscillatory circuit that prevents generation at the frequency of the first harmonic.
The specific values of the capacitance of the capacitors C1-C3 and the inductance of the coils L1, 12 depend on the frequency and type (cutoff) of the resonator. For example, when using a resonator at a frequency of 40 MHz (third harmonic), the capacitance of the capacitors C1, C2 for the author turned out to be 47 pF, C3 - 0,01 μF, the inductance of the coil L1 - approximately 0,3 μH (8,5 turns of wire PEV-2 0,27 on a frame with a diameter of 5 mm).
Author: E. Grishin, Kursk
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