The way that developer of a lot of MCU wants to receive a right capacitance each to MCU crystal both sides expresses to pay no attention to solution, because of this capacitance sometimes can take out. The author consulted a lot of books, what in discovering a book however, explain is very few, mention most often is: Right capacitance provides stabilization or be equivalent to laden capacitance waiting, undertake theoretical analysis without very thorough ground. And additional on one hand, a lot of lover are direct oversight these two capacitance on the side of crystal, they think to be done by referenced design went. But in fact, this is the oscillating circuit of MCU, weigh “ again at 3 o’clock ” of oscillating circuit of style electric capacity, if pursue,1 is shown.

 

Graph 1: Of MCU at 3 o’clock oscillating circuit of style electric capacity

Among them, y1 is crystal, be equivalent to at 3 o’clock the inductance inside type; C1 and C2 are capacitance, and 5404 realized a NPN dynatron with R1 (everybody can contrast in high frequency book at 3 o’clock oscillating circuit of style electric capacity) .

 

Will analyse this circuit for everybody next: Above all, 5404 indispensible take a resistance, otherwise it will be in saturation to end area, is not enlarge area, slant because of what R1 is equivalent to dynatron buy action, can let 5404 be in enlarge area and act as an inverter, achieve the effect of NPN dynatron thereby, and when common emitter receives a standard, NPN dynatron also is an inverter.

 

Will explain for everybody with common method next this at 3 o’clock type the working principle of oscillating circuit. Well-known, the oscillation condition of oscillating circuit of a sine is: Systematic enlarge multiple is more than 1, this condition comes true more easily; But on the other hand, still need to make phasic and contented 360° . And the problem depends on this phasic: As a result of 5404 it is an inverter, because this already realized 180° ,move, so need C1, C2 and Y1 to realize 180° to move again only the photograph is OK. Apropos, when C1, C2 and Y1 form resonance, can come true 180 move; The simplest implementation way serves as reference with the ground namely, syntonic when, because C1, C2 is medium the electricity that carry is identical, and the ground is between C1, C2, apropos voltage is so contrary, come true thereby 180 move.

 

Again criterion, when C1 increases, the amplitude that C2 carries increases; When C2 is reduced, amplitude also increases. Sometimes although do not solder,C1, C2 also can rise brace up, but this kind of phenomenon is not by do not solder the practice of C1, C2 causes, the distributinging electric capacity that brings a base by chip however is caused, because of the capacitance of C1, C2 the value does not need originally very big, this are very important.

 

So, what do these two capacitance have to affect after all to oscillation stability? Because the voltage feedback of 5404 relies on C2, hypothesis C2 is too big, feedback voltage is too low, at this moment oscillation is not stable; Hypothesis C2 is too small, feedback voltage is exorbitant, store energy is too little, suffer outside interference easily, return meeting radiate to affect the external world. And of the action of C1 and C2 apropos and contrary. In cloth board when, it is double face plate suppose and thicker, so the influence of distributinging capacitance is not very big; But spend for high density suppose multilayer board when, consider distributinging capacitance with respect to need, especially the oscillating circuit of VCO and so on, should consider distributinging capacitance more.

 

Accordingly, those are used at the item that labour charges, the author suggests to had better not use crystal oscillation, the brilliant that receives an active directly however brace up. Everybody can use a lot of time the clock crystal of 32.768K will do clock, is not to carry sheet piece the crystal scale down of machine will do clock, its Central Plains because a lot of people also do not understand most propbably, go up actually the stability that this is He Jing body concerns: Frequency jumps over tall crystal, q value is done hard commonly tall, frequency stability is poorer also; And the 32.768K crystal property performance in each respect such as stability is good, still formed an industry standard, do more easily tall. What be worth to be carried additionally is, 32.768K is the half of 16 Bit data, obligate is highest mark of 1 Bit carry, use as processing of computation of number of time tally interior is very convenient also.