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#1
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Re: PIC XT oscillator question.
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Last edited by Rickertsen2 : 21-01-2004 at 20:52. |
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#2
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Re: PIC XT oscillator question.
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Again you have to really refer to the data sheet on the PIC. It describes a variety of oscillator circuits. When a crystal refers to capacitance it is referring to the maximum loading (capacitance in parallel with the crystal) that the crystal will oscillate reliably with. When the crystal is referred as series, it is cut to only work with series fed oscillator circuits. The PIC data sheet specifically calls for a parallel cut crystal or resonator. It also lists the crystals that were tested for various frequencies. Unfortunately none of your selection was on the list. As you page through the catalogs and manufacturers websites, you will see crystals marked for overtone operation and the intended frequency range. The PIC data sheet has a lot of info. For other crystal oscillators you need to get a textbook to help you. I use the Radio Amatuer's Handbook (pub. Amateur Radio Relay League or ARRL) a lot for this kind of help and your library should have a recent copy as well as other electronics books on oscillator design and layout. |
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#3
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Re: PIC XT oscillator question.
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#4
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Re: PIC XT oscillator question.
we keep switching words here between crystal, oscillator and (ceramic) resonator.
these are three different things if you really feel overwhelmed by getting the crystal circuit right, you can buy oscillator packages in surface mount or DIP packages. these have a crystal and buffer circuit inside, all you need to do is put power and gnd on two pins, and a squarewave clock comes out the output pin. Some also have an 'enable' pin that allows you to turn it off, if you want to run in standby or lowpower mode. If you are building a one of a kind board, and you dont want to waste time messing with the crystal or ceramic resonator to get it to run, just get an oscillator device. They are not that expensive, esp considering the amount of time they might potentially save you. DigiKey sells these too. |
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#5
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Re: PIC XT oscillator question.
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Really i havn't been confusing Ceramic, and crystal, but i may have used oscillator in place of crystal. I am still unclear whether or not i can use 40mhz clock sources with the PIC or if i must use a 10mhz clock source and the PIC's PLL to acheive 40Mhz. To be safe i went w/a 10mhz crystal and the PLL for my final design. I was looking at the oscillator things you spoke of. They look REALLY simple. I will have to keep those in mind for future projects. Last edited by Rickertsen2 : 22-01-2004 at 17:49. |
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#6
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Re: PIC XT oscillator question.
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Like many microprocs or microcontrollers, this chip has an onboard oscillator that can use either a ceramic resonator, crystal, an RC oscillator or an external clock. The External clock option might be needed if there was a need to run this from a master for some reason, but as a stand alone it may not be worth the extra expense. I made the recommendation on cost, parts count and complexity. I am sorry for any confusion on the use of the terms and hope that everyone understands that given the right circuitry a crystal and/or a ceramic resonator and/or an RC network all work well in an oscillator. The stability of an oscillator is what governs the decision on which to use. Least stable being the RC followed by the ceramic, crystal, external clock, temperature stabilized (oven) external crystal oscillator and finally atomic reference. I would think in this application the crystal at 4X PLL is the way to go. |
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