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'[PIC]: 16LF876.'
2002\05\14@014510 by John Dammeyer

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I have a client who desperately wants to use the lower voltage
capability of the L series (3.0V to 3.9V) but also needs to run at
4.9152Mhz. (I think because he has 900 crystals in stock and it would
cost him to have the code modified for a 4MHz crystal).

Does anyone know what would go wrong with the processor at 3.0V and
4.9152MHz?  It's out of spec for speed but not voltage.  Obviously the
oscillator is still programmed for XT mode not HS mode.

Thanks,

John Dammeyer

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2002\05\14@072715 by Olin Lathrop

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> Does anyone know what would go wrong with the processor at 3.0V and
> 4.9152MHz?

Probably.

> It's out of spec for speed but not voltage.

So its out of spec, period.  Some PICs might work like this, or worse *most*
will work like this *most* of the time.


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(978) 742-9014, http://www.embedinc.com

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2002\05\14@080928 by Mark J. Dulcey

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John Dammeyer wrote:
> I have a client who desperately wants to use the lower voltage
> capability of the L series (3.0V to 3.9V) but also needs to run at
> 4.9152Mhz. (I think because he has 900 crystals in stock and it would
> cost him to have the code modified for a 4MHz crystal).
>
> Does anyone know what would go wrong with the processor at 3.0V and
> 4.9152MHz?  It's out of spec for speed but not voltage.  Obviously the
> oscillator is still programmed for XT mode not HS mode.

Some current PICs are rated to work down to 3 volts over the commercial
temperature range (0 to 70 C). (Over the industrial temperature range,
-40C to 85C, the frequency rating is only 4 MHz below 4.5V.) For
example, see the chart on page 132 of the datasheet for the 16F628. The
rated frequency of operation derates linearly from 20 MHz at 4.5V to 10
MHz at 3.0 V.

The picture is slightly different with the 16F876 and 16F877. There, the
standard part is only rated to work down to 4V. The 16LF877, however, is
rated to work at speeds up to 10 MHz at 3V or above. Again, see the
chart on page 150 of the data sheet.

So it looks as though your client's application is not only possible,
but within specs. However, as this frequency is above 4 MHz, I believe
that the oscillator should be programmed for HS mode, not XT.

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2002\05\14@112028 by John Dammeyer

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Thank you .  I missed that chart.


Good point about the HS.  Not sure if it's important at that lower
frequency.

John Dammeyer

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2002\05\14@113734 by Alan B. Pearce

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>Good point about the HS.  Not sure if it's
>important at that lower frequency.

It is more a case of compensating for the gain drop off of the oscillator as
the supply voltage falls.

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