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SteveC
Joined: 09 Jul 2004
Posts: 10
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| PIC 18LFxxxx maximum clock frequency @ 3.3V |
 Posted: Thu Aug 04, 2005 10:16 am |
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Hello,
I need some clarification of Fmax on an 18LF8722 (and 18LF4620). The datasheet for the 8722 (39646b.pdf) in figure 28-3 on page 379 shows a graph with Fmax attainable at 4.2V, and stating that Fmax=40MHz if Vddappmin>4.2V.
I am hoping to run the device at 24MHz/3.3V. In fact the demo board is running fine in this way. However, the formula given:
| Code: | | Fmax=(16.36/V) * (Vddappmin - 2) + 4 |
(by the way, what's the difference between 'V' and 'Vddappmin'?)
yields a figure of 12.57MHz for a 4.2V supply, and a figure of 10.44MHz @ 3.3V.
If the '/V' is removed from the first term the calculated Fmax is 39.99MHz. So, hopefully the '/V' is an error. But, it's in all the 18F/LF datasheets. However, the inclusion of the divisor in the first term implies that as the supply increases, the clock frequency limit decreases, unless the 'V' isn't anything to do with the supply and is some other parameter that I don't understand!
Am I missing something? I do hope so.
Thanks a lot.
Steve. |
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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| Posted: Thu Aug 04, 2005 10:47 am |
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| The "MHz slash V" just means "MegaHertz per Volt". |
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SteveC
Joined: 09 Jul 2004
Posts: 10
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| Posted: Thu Aug 04, 2005 10:49 am |
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PCM programmer,
In that case I am a donkey, and you are a god. 
Many, many thanks for that. My design is safe!
Cheers.
Steve. |
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