PIC XT oscillator question.

[Rickertsen2]

Ok, i'm hoping there is someone on these forums who can help me here. My problem is this: I am trying to clock a PIC18F252 at 40Mhz, but can't seem to find a way to do so. I can't find non SMD 40Mhz ceramic resonators. I can find 40Mhz crystal oscillators, but that is not their fundamental frequency. They are all designed to operate on a third harmonic and i have no idea how to do this. What about a 10Mhz crystal + the PLL? I don't understand that either? Anybody have any advice as to how i can get 40Mhz? It must be FIRST legal. Also, i have never designed anything this high frequency before, will 40Mhz even work with through hole?

[Caleb Fulton]

Originally Posted by Rickertsen2

Ok, i'm hoping there is someone on these forums who can help me here. My problem is this: I am trying to clock a PIC18F252 at 40Mhz, but can't seem to find a way to do so. I can't find non SMD 40Mhz ceramic resonators. I can find 40Mhz crystal oscillators, but that is not their fundamental frequency. They are all designed to operate on a third harmonic and i have no idea how to do this. What about a 10Mhz crystal + the PLL? I don't understand that either? Anybody have any advice as to how i can get 40Mhz? It must be FIRST legal. Also, i have never designed anything this high frequency before, will 40Mhz even work with through hole?

http://www.npcamerica.com/oscillators.htm
Would any of these work?

BTW...You have my watch, and your cat is hilarious.

[KenWittlief]

you said oscillators but do you really mean crystals?

dont worry about that third harmonic stuff - just get a 40Mhz crystal and connect it with the correct caps on each side, and it will work fine.

Crystals are standard - they have been for years - you do have to worry about what harmonic its oscillating at, they mark and spec them according to the resulting frequency. If you want a 40Mhz clock you get a 40Mhz crystal.

Have you checked DigiKey?

[Rickertsen2]

Originally Posted by KenWittlief

you said oscillators but do you really mean crystals?

dont worry about that third harmonic stuff - just get a 40Mhz crystal and connect it with the correct caps on each side, and it will work fine.

Crystals are standard - they have been for years - you do have to worry about what harmonic its oscillating at, they mark and spec them according to the resulting frequency. If you want a 40Mhz clock you get a 40Mhz crystal.

Have you checked DigiKey?

Yesi did mean resonators. Are you absolutely sure that they will oscillate on their third harmonic automatically. This is against everything i have read and common sense, which dicates that they would run at 13.333 Mhz in a normal configuration rather that their third harmonic of 40Mhz.

[Al Skierkiewicz]

OK, Hold it guys!
First, Rick, you need to know your prey. the Pic you refer to does not take a 40 MHz crystal input. It has an internal 4 x multiplier PLL that you can enable which will take a 10MHz resonator to 40 MHz internal to the chip.
See p. 19 of http://www.microchip.com/download/li...xx2/39564b.pdf

As far as crystals are concerned, I think the only ones you will find that are in your price range are third overtone crystals. These crystals are cut to produce harmonic energy at intervals above their fundamental in this case three times the frequency there are cut for. By designing an oscillator that will produce significant harmonic energy, using a crystal cut for overtone use and then tuning the output for the third harmonic, you can get crystal operation at higher frequencies. The layout of the circuit is critical and the crystal manufacturers specs need to be followed closely.
I think you will be better off with a 10MHz resonator and the 4X multiplier selected.

[Rickertsen2]

Originally Posted by Al Skierkiewicz

OK, Hold it guys!
First, Rick, you need to know your prey. the Pic you refer to does not take a 40 MHz crystal input. It has an internal 4 x multiplier PLL that you can enable which will take a 10MHz resonator to 40 MHz internal to the chip.
See p. 19 of http://www.microchip.com/download/li...xx2/39564b.pdf

As far as crystals are concerned, I think the only ones you will find that are in your price range are third overtone crystals. These crystals are cut to produce harmonic energy at intervals above their fundamental in this case three times the frequency there are cut for. By designing an oscillator that will produce significant harmonic energy, using a crystal cut for overtone use and then tuning the output for the third harmonic, you can get crystal operation at higher frequencies. The layout of the circuit is critical and the crystal manufacturers specs need to be followed closely.
I think you will be better off with a 10MHz resonator and the 4X multiplier selected.

Thanks, i will go with a 10Mhz crystal x4 w/the PLL, like you suggested. Digikey sells 10Mhz fundamental mode crystals. Specifically i am loking at Digikey PN:X422-ND, X443-ND and XC953-ND. What is the difference? Which should i use? The X443-ND and XC953-ND specify 18pF load caps. The X422-ND just says "series" under load cap. What does that mean? Does this indicate that the crysta lis series cut rather than parallel cut? Also, if i ever want to use a third overtone crystal, do i use it like normal, or do i have to build a circuit that filters out the fundamental or something like that? Your help is GREATLY appreciated.

[Al Skierkiewicz]

Originally Posted by Rickertsen2

Thanks, i will go with a 10Mhz crystal x4 w/the PLL, like you suggested. Digikey sells 10Mhz fundamental mode crystals. Specifically i am loking at Digikey PN:X422-ND, X443-ND and XC953-ND. What is the difference? Which should i use? The X443-ND and XC953-ND specify 18pF load caps. The X422-ND just says "series" under load cap. What does that mean? Does this indicate that the crysta lis series cut rather than parallel cut? Also, if i ever want to use a third overtone crystal, do i use it like normal, or do i have to build a circuit that filters out the fundamental or something like that? Your help is GREATLY appreciated.

James,
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.

[Rickertsen2]

Originally Posted by Al Skierkiewicz

James,
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.

I looked up crystal oscillators in a few of my books (including the 2003 ARRL handbook) and it was quite insightful.

[KenWittlief]

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.

[Rickertsen2]

Originally Posted by KenWittlief

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.

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.

[Al Skierkiewicz]

Originally Posted by KenWittlief

we keep switching words here between crystal, oscillator and (ceramic) resonator.
these are three different things

Ken,
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.