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Hi, Does anyone know where i could buy some fast resonators? The fastest i could find are 24MHz from rapid. I was hoping for 40 and possibly 60MHz but none of my usual places have any. I would prefer to buy them from the UK but i dont mind buying overseas if its not going to cost too much. I would also like the 3 pin ones not 2 pin
It looks like Digikey has them up to 50 MHz (40, 48, 50).
Don't know that helps you much in the UK, but you might grab the part numbers off the ones Digikey stocks and see if you can find them closer to home.
steve
You aren't planning to use them on a PIC are you?
25mhz is the highest you can go for a PIC oscillator.
Actually it is 20MHz maximum for PIC16's and 40MHz for PIC18's using an oscillator in EC mode.
For crystals/resonators the maximum is 20MHz for PIC16's and 25MHz (10MHz if PLL is enabled) for PIC18's in HS mode.
Yes i was planning to use them with PICs. Im using 24MHz at the moment with PIC18Fs (i know 16Fs can only do 20). Im pretty sure 18Fs can handle upto 60MHz or atleast the ones im using can (according to the datasheet). Even 40MHz would be a help tho. Im struggling at the moment cramming all my PWM code and extras into the 6M instructions you can get with a 24MHz resonator. I will also be looking at writing the code in slightly different ways but a better resonator would help a lot.
Ill have a look at digikey. Ive already tried my local places but it seems that nobody really uses over 16MHz. Thanks for your help
That's because that is the highest that is useable with any PIC18 to get the fastest clock possible - 64MHz with the newer 3.3V parts.Quote:
Originally Posted by The Master
Buy a 10mhz crystal with 2-22pF capacitors.
Use the 4xPLL to get 40mhz.
You can not run a 40mhz crystal or resonator from a PIC's oscillator.
P.S.
Use a crystal. Resonators have their uses, and you can figure out if it'll work in your application, after you have your application working.
Hmm, i think you might have to explain that one too me. Also, how come my 24MHz resonators work fine with a 18F?
I thought PICs ran instructions at a quarter of the speed of the resonator
The resonators i use dont need external capacitors (they are the 3 pin ones). Do i need to use a 2 pin one with caps?
Edit: My circuits are already working. I just want to speed them up a bit
Which part?Quote:
Originally Posted by The Master
Because 18F's oscillate just fine up to 25mhz.Quote:
Also, how come my 24MHz resonators work fine with a 18F?
They Do.Quote:
I thought PICs ran instructions at a quarter of the speed of the resonator
But you seem to be missing the 4xPLL part.
Are you familiar with the 4xPLL?
That is simply my suggestion.Quote:
Originally Posted by The Master
Do your development with a crystal.
When it's all working perfectly with the crystal, test it with a resonator while varying the ambient temperature to the furthest extremes you can or really need.
If everything still works, you should be OK to uses resonators for a production run.
add: If not, you can make allowances for the deviations from "nominal".
But having timing related problems that you can't find in the program, while you're trying to program it, really sucks.
Use a crystal.
No i dont know what 4xPLL is (thats the bit i need explaining).
I definately read in a datasheet somewhere that the PICs supported 60MHz. Now that you mention it im not completely sure that was for the actual resonator value but it had something to do with it.
So far ive been working it out that a 24MHz resonator divided by 4 gives me 6,000,000 instruction cycles per second. Ive also counted how many my program uses. Apart from variable declarations the whole program is in ASM now so i know exactly how many instruction cycles everything should take. I adjusted the timing so it should have taken under 6M cycles and it worked. Then i adjusted it to go slightly over and i started seeing the kind of behaviour that you get when its using too many commands so it seemed like i had worked everything out correctly.
I take it crystals are more accurate than resonators. I like resonators because they are nice and small and simple. I currently have 2 circuits running outside where its -2C. They worked exactly the same inside at room temperature. The timing in just about ever circuit i do isnt critical. As long as it is timed well enough to use serial then its good enough. I do usually add allowances anyway by varying the timing a little.
I do know exactly where my timing problems are and how to fix them (one way being a fastor resonator). I would rather do that first as my other ideas invovle removing functionality.
Edit: I just opened a datasheet randomly for a PIC18F2420. It says "Two External Clock modes, up to 40 MHz". I thought that was the max resonator speed. Is it something else?
You need to get a better understanding of the oscillator in the PIC or you'll just be beating around the bush. Since you are using a PIC18, I suggest that you download the PicMICRO 18C MCU Family Reference Manual from the MicroCHIP website and read Section 2 carefully - especially the part that talks about PLL (Phase Locked Loop).
Also, as I said in my earlier post, the maximum clock frequency (Fosc) for a PIC18 is 40MHz (64MHz for some of the newer 3.3V only parts such as the PIC18FxxKxx). This, however, is not the same thing as the crystal/resonator/external-oscillator frequency.
Hi all,
This whole thread is making me more confused than what I usually am :cool:. For my USB projects I usually use a PIC18F4550 with a 4MHz resonator. But, I believe the USB module runs at 96 MHz. So, how do you go from 4 to 96 MHz?
Rmteo,
As soon as I get a chance I will read chapter two in the reference manual that you mentioned in your last post.
Robert
The oscillator in the USB equipped PIC's is quite different. See Section 2.3 Oscillator Settings for USB (and table 2.3) of the relevant data sheet for detailed info.