單片機(jī)畢業(yè)設(shè)計(jì)英文翻譯(存儲(chǔ)版)
【正文】 a circuit diagram is very often indicated as Xtal.) oscillator stage.Most microcontroller devices have an oscillator circuit (Xtal1,Xtal2) that will oscillate with an external crystal and external capacitors. The oscillator stage is basically an inverter type gate consisting of a Nchannel and a P channel transistor. The main difference with a digital inverter stage is an integrated bias resistor (also called:feedback resistor) connected between output and input. This (semiconductor) resistor feeds back the output voltage to the input which will balance (bias) the stage in its analog working area. In the quiescent situation this will generate a DC input and output level of about 1/2 Vdd for CMOS devices (For TTL patible versions it is just a little less). Transconductance.When this oscillator stage is driven with an input voltage variation then this will result in an output current variation through an external load. This relation (dVi/dIo) is defined as the transconductance of the oscillator stage. This tranconductance, indicated with “gm” is defined as the amount of current change as a result of the input voltage change. The unit is A/Volt or S (S=Siemens). The Pierce oscillator.The standard circuit for the oscillator is given in figure 3. A Crystal is connected between the output (Xtal2) and the input (Xtal1). Output and input have a capacitor connected to the ground. This is basically a Pierce oscillator circuit. With a proper dimensioning of the external ponents the circuit should generate an (almost) sine wave shape signal on the Xtal2 output pin. One of the parameters related to the oscillator stage that affects the oscillation is the transconductance. A certain value of gm is needed to assure startup. During poweron a noise signal or a transient should result in an amount of energy fed in to the Xtal to make it start and is one of the basic requirements for any oscillator stage.3. The external ponents.The frequency determining element in the external ponents is the crystal (Xtal). Basically a crystal behaves as an LCcircuit for serial resonance. Figure 4. shows a monly used equivalent circuit. The resonant frequency is determined by the value of L and C, so this is series resonance. C0 represents the total parallel capacitance of the crystal and its value is usually much higher then the one of C. However its influence on the resonating frequency is very small.Some typical values for these equivalent ponents based on a 10 MHz crystal are:L = H, C = pF, Rx = 10 ohms, Co = pF.Note that in an application the total equivalent value of Co is also highly influenced by the two external capacitors in the basic Pierce oscillator circuit figure 3. In fact the two capacitors in series shunt the crystal, meaning the Co is in fact increased.4. Oscillation condition.An oscillator stage and external ponents are supposed the generate the clock signal. Is just connecting the external ponents to the oscillator stage the only condition for oscillation? Again, there are theories on the oscillation condition and the Barkhausen rule covers the oscillation condition basics. In a practical situation however there are many circuit parameters that will determine whether an oscillator circuit will show reliable are just some of them: Vdd, supply voltage. fosc, oscillator frequency. gm, oscillator stage transconductance. Rx, equivalent resistor value. C0, equivalent total parallel capacit
公司管理相關(guān)推薦
鄂ICP備17016276號(hào)-1