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russk2txb
Joined: 30 Mar 2008 Posts: 109 Location: New Jersey
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Crystal frequency for PIC18F4550 |
Posted: Mon Jul 12, 2010 10:06 am |
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I'm looking for some advice on selecting the best crystal frequency. The Microchip data sheets give various configurations but there seems to be no suggestions why one would be used compared to another. For example, it appears that I can use a 48 MHz crystal and no divide for the PLL or MCU clock (in HS mode). Would that be best? Or is it better to use the 20 MHz crystal and use PLL / 5 and CPU / 2 in HSPLL mode? I guess the question boils down to, is it better to use PLL and a low frequency crystal?
Thanks, Russ |
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Ttelmah
Joined: 11 Mar 2010 Posts: 19385
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Posted: Mon Jul 12, 2010 10:15 am |
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You can't use a 48MHz crystal. Highest frequency supported by the oscillator circuitry, is 25MHz. Above 25MHz, you have to use an external _oscillator_, not just a crystal.
For 'fast' USB, you can use any mutliple of 4MHz, up to 24MHz.
The USB PLL, seems _slightly_ more reliable, when fed with a divided source, rather than directly from a crystal. So, 8MHz, 12Mhz, 16MHz, 20MHz, and 24MHz.
Generally, lower frequency crystals, reduce the risk of RF noise problems, however if the board is well designed, this is a minor.
Realistically choosing between these possible crystal values comes down to price/availability, possible duplication of stock if used in other projects, reducing the number of component lines needed, etc. etc..
Best Wishes |
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russk2txb
Joined: 30 Mar 2008 Posts: 109 Location: New Jersey
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Posted: Mon Jul 12, 2010 2:26 pm |
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Thanks Ttelmah. I was leaning towards the 20 MHz crystal anyway as that's what I used in another project. I did not see where it says that 25 MHz is the highest frequency crystal that can be used.
Thanks, again,
Russ |
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ckielstra
Joined: 18 Mar 2004 Posts: 3680 Location: The Netherlands
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Posted: Mon Jul 12, 2010 4:41 pm |
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russk2txb wrote: | I did not see where it says that 25 MHz is the highest frequency crystal that can be used. | It is in Table 28.8 of the datasheet: 'TABLE 28-8: EXTERNAL CLOCK TIMING REQUIREMENTS' |
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KnowMore
Joined: 05 Jul 2010 Posts: 9 Location: Vietnam
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Posted: Wed Jul 14, 2010 12:37 am |
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ckielstra wrote: | russk2txb wrote: | I did not see where it says that 25 MHz is the highest frequency crystal that can be used. | It is in Table 28.8 of the datasheet: 'TABLE 28-8: EXTERNAL CLOCK TIMING REQUIREMENTS' |
I think we have something in wrong. It's no needed divide_by_12 before PLL if primary oscillator cannot run with above 25MHz crystal.
I've ever use both 48MHz and 40MHz crystal in HS mode, and didn't mind about any error .
I think we should use Low-Frequency Crystal and PLL, more power-consumption but less noise. _________________ GOOD TODAY IS ENEMY OF BETTER TOMORROW |
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Ttelmah
Joined: 11 Mar 2010 Posts: 19385
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Posted: Wed Jul 14, 2010 4:07 am |
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Don't confuse the internal oscillator, with the clock input.
If you look first at the table of crystals/oscillator configurations (2-3), you will see that the top three lines, using /10, /12, and no division att all (direct feed), are only listed for EC, ECIO, ECPLL, and ECPIO modes. These modes use an external _oscillator_. All the ones using 'HS', or 'HSPLL' (or XT), and the internal _oscillator_ block, are below 24MHz.
If you look at the table Ckielstra points you to, you will see that the clock is specified to run at 48MHz, but _not_ using a crystal.
If you have used it at this, you have been lucky. It is common for the oscillator to work well past spec, _but_ should not be relied on....
Best Wishes |
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KnowMore
Joined: 05 Jul 2010 Posts: 9 Location: Vietnam
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Posted: Thu Jul 15, 2010 12:11 am |
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Ttelmah wrote: | Don't confuse the internal oscillator, with the clock input.
If you look first at the table of crystals/oscillator configurations (2-3), you will see that the top three lines, using /10, /12, and no division att all (direct feed), are only listed for EC, ECIO, ECPLL, and ECPIO modes. These modes use an external _oscillator_. All the ones using 'HS', or 'HSPLL' (or XT), and the internal _oscillator_ block, are below 24MHz.
If you look at the table Ckielstra points you to, you will see that the clock is specified to run at 48MHz, but _not_ using a crystal.
If you have used it at this, you have been lucky. It is common for the oscillator to work well past spec, _but_ should not be relied on....
Best Wishes |
I think you are right.
Thanks! _________________ GOOD TODAY IS ENEMY OF BETTER TOMORROW |
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