【正文】 f 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 capacity. Closed loop gain.Only for the crystal there are more then ten parameters. For a practical evaluation it is almost impossible to include all of them. A second reason for this is that many parameter values are not known to those engineersapplying the ponent. Does this mean that oscillation will be a matter of luck? Without going in much theoretical detail the impedance approach can be used to give an indication on the expected oscillation. The idea behind it is that the effective transconductance (1/ohm) should at least make up for the load (ohm)Figure 5. shows a characteristic that indicates a relation between the Rx,C0 and oscillation. When the coordinates of Rx and C0 are in the upper area, oscillation conditions are not met. When the coordinates cross at the characteristic or are in the lower area there will be proper oscillation.Now the difficulty arises in putting actual values on the Rx and C0 axis, as well as determining the right shapeand position of the characteristic. Practical experience however indicates that a circuit like in figure 3: with C1=C2 = 30 pF will be in the safe area, when C0 is about 10 pF and Rx is about 100 ohm or smaller.A lower value of Rx allows a higher value of C0 and visa versa.5. The drive level issue.Once we have oscillation is there something else to worry about?Most of the information so far is based on experience with the standard 5 Volt microcontrollers. Their oscillator stage was designed to drive a cryst