單片機(jī)畢業(yè)設(shè)計(jì)英文翻譯(完整版)
【正文】 g. When this is due to the PCBtracks and/or oscillator circuit external ponents it could be eliminated by using an extra capacitor with a non critical value when it is much larger then the value C1 and C2 (see figure 7).Another possibility of reducing the sensitivity for environmental conditions like DCoffsets is shunting the oscillator stage with a resistor rather than shunting the Xtal. This way the input sensitivity is reduced but also the loop gain. It can also be used for amplitude reduction. It may be necessary to modify the value of the C1. See figure 8:All these circuit examples were tested with a 559 device. They all start oscillating already at about 3 Volts Vdd. Driving other circuitryAnother frequently asked question rises when more microcontrollers are used in the same application and/or microcontrollers are used with other IC’s that require a clock signal. In many of these cases it is possible to use one crystal only.Figure 9 shows this situation when using the standard oscillator circuit. The Xtal2 output (micro 1) is directly connected to the Xtal1 input of the second microcontroller. The Xtal2 output from the second microcontroller remains not circuit for the microcontroller 1 may be modified as described earlier in this report. In these cases always connect the input of the microcontroller 2 to the Xtal2 output of microcontroller 1 rather than direct on the is to minimize the effects of the extra load on the oscillator circuit. To avoid any DCoffset for microcontroller2 (a DC offset may result in more asymmetric duty cycle of the signal on Xtal2) the connection between uC1 and 2 may be DCdecoupled by using a capacitor. Figure 10 shows the circuits are applicable for most microcontrollers. Some controllers however have two onboard oscillators one for a direct clock signal and one for a PLL clock. In this last case the Xtal frequency is normally 32 kHz. This oscillator stage has pull down devices on the input and means that transistors are added (onchip)which are switched on under certain conditions, . when the oscillator is transistor then connect Xtal1 and Xtal2 to the ground. To avoid the flow of (too) much current into the pull down transistor it is remended to insert a resistor in the interconnection to the Xtal4input. Figure 11 shows this situation.7. Emulation and the Xtal oscillator.Developing microcontroller applications are usually carried out with the use of an emulator device. In a carefully configured oscillator circuit around a microcontroller the microcontroller is removed and replaced by a connector with a cable connection to emulating device which hopefully has identical oscillator stage when it has, inserting a cable can dramatically influence the oscillator behaviour especially at higher frequencies. Two lowcost emulators each have different ways to handle with this issue: approach.In the PDS51 system this problem is approached by not using the target oscillator hardware. The system has a separate oscillator circuit and the target Xtal has to be removed from the application and then inserted on the PDS51 daughter the application requires the signal Xtal2 to be present on the target (for driving other circuitry) there is a jumper to be set. Note that the signal from the PDS51 is then usually a better square wave shape signal then the original Xtal2 output the target microcontroller is not using its own oscillator but gets a signal on the Xtal1 input from an other sour
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