barcode reader project in asp.net External Oscillator Circuitry System Clock Mux Postscaler Mux LF 31kHz Oscillator in Software

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External Oscillator Circuitry System Clock Mux Postscaler Mux LF 31kHz Oscillator
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HF 8MHz Oscillator
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Figure 9.8 Newer PIC microcontrollers have an internal oscillator circuit that can select operating speeds from 31 kHz to 8 MHz.
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BASIC OPERATING FEATURES
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R /4 OSC 1 C PICMicro Clock
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Figure 9.9 The external RC oscillator circuit, which can be built very inexpensively but is very inaccurate.
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Originally, the advantage of the RC oscillator was its very low cost and its ability to drive an instruction clock whose output can be used by other devices in the circuit. There are many disadvantages to this method compared with the internal oscillator, and they include
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Lower operating speed (normally the RC oscillator can run at a maximum of 1 MHz). Poor accuracy (an error of 30 percent to the target speed is not unusual with poor-
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tolerance capacitors).
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Dif culty in selecting components to be used to produce a speci c operating frequency
(there is no single table de ning the frequency; instead you must use a graph in the datasheet to select the resistor and capacitor values for different frequencies). Generally uses more power than the internal oscillator for the same speed owing to the additional current passing through the resistor. Some devices have a single external resistor option for an inexpensive oscillator, but this has many of the same disadvantages as the full RC oscillator discussed earlier. Before introduction of internal oscillators to PIC microcontrollers, the external RC oscillator was the only option to a very-low-cost system clock. Today, I would discourage you from considering this circuit and instead suggest that you select a PIC microcontroller part number that has an internal oscillator.
External crystals, resonators, and oscillators
Crystal and ceramic resonators use a similar connection scheme for operation. The crystal or ceramic resonator is wired into the circuit as shown in Fig. 9.10. Crystals and ceramic resonators delay the propagation of a signal a set amount of time. This set amount depends on how the crystal is cut. In addition, for best results, a parallel-circuit crystal should be used. The two capacitors shown in Fig. 9.10 are connected to one side of the crystal or ceramic resonator and the other to ground. The values of the two capacitors are speci ed
CONFIGURATION FUSES
OSC 1
OSC 2
PICMicro Clock
Figure 9.10 The external crystal oscillator circuit is used for both parallel-cut crystals and ceramic resonators.
in the PIC microcontroller device s datasheet and usually match one another. You may nd for some speeds that different values are speci ed for the two capacitors. Every PIC microcontroller has three external crystal or ceramic resonator speed ranges requiring different operating parameters within the chip. Microchip refers to these speed ranges as oscillator types, and they are set in the con guration register to enable the different circuitry needed to ensure proper operation. Table 9.4 lists the oscillator types and their operating frequency ranges. Using the crystal or ceramic resonator, the OSC2 pin can be used to drive one CMOS input, as shown in Fig. 9.11. Note that the input capacitance of the CMOS buffer could affect the operation of the PIC microcontrollers oscillator (i.e., causing it to not run at all). Make sure that you understand what is the capacitance of the input circuit that you are using and whether or not it is signi cant enough to affect the operation of the oscillator. You may nd that you have to use an external oscillator and wire its output to the PIC microcontroller and other devices. A ceramic resonator behaves similarly to a crystal resonator, except that it is not built with a quartz device, like the crystal. Ceramic resonators are much more robust (i.e., can withstand more severe physical shocks) than crystals, and in large quantities,
TABLE 9.4 PIC MICROCONTROLLER OSCILLATOR FREQUENCY RANGES OSCILLATOR TYPE OPERATING FREQUENCY RANGE
LP XT HS
0 200 kHz 200 kHz 4 MHz 4 20 MHz (or the device maximum)
BASIC OPERATING FEATURES
OSC 1
Buffered Clock
CMOS Driver OSC 2
PICMicro Clock
Figure 9.11 The OSC2 pin can be used to drive a single CMOS input, which allows the PIC clock to be distributed throughout the application circuit.
they are much cheaper. Many ceramic resonators are available as three-pin devices, which have the external capacitors built into them, meaning that along with just wiring them to the PIC microcontroller s OSC1 and OSC2 pins, just a ground connection is required. The downside of ceramic resonators is that their accuracy is not as good as that of crystal resonators (usually accurate to 0.5 percent versus a crystal accuracy of 0.02 to 0.1 percent), and some devices with built-in capacitors may not be suitable for use with the PIC microcontroller. When designing and laying your application circuit, always remember to keep the parts of the oscillator as close as possible to grounds. When the speci ed parts are used for oscillators and are kept close to the PIC microcontroller, you ll be impressed with the stability and robustness of PIC microcontrollers oscillator and instruction clocking. Finally, you can use an external oscillator driving the OSC1 pin directly, as shown in Fig. 9.12. The OSC2 pin can be used to redrive the clock (although inverted) to other devices. If you look through PICList and other forum archives, you periodically will see postings from people who drive their PIC microcontrollers at much beyond their rated speed. This is possible for most devices because of the design quali cation Microchip does to ensure that PIC microcontrollers run at the rated speeds. It is not recommended that you count on your PIC microcontrollers to run at faster than rated speeds because this capability is not guaranteed, and you will nd devices that will not run at the speeds required by your application.
PIC18 oscillator circuitry As discussed earlier, the typical PIC microcontroller has four oscillator modes that are selected from the con guration fuses when the PIC microcontroller starts up. The PIC18 has one more oscillator option available, along
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