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'[PIC] Min and max voltage for 887'
2008\06\24@220107 by Tomás Ó hÉilidhe

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I'm trying to pick what battery I want for my project so I've been
taking into account the different voltages that various components require.

I know the 887 will work at 5 V but I wanted to figure out how low I
could go.

So...
I...
checked the datasheet and found the following under "Electrical
Specifications":

Voltage on VDD with respect to VSS:  -0.3V to +6.5V

Now see this is the reason why newbies shy away from datasheets so much.
I haven't a clue what it's trying to tell me that I can put -.3 volts on
Vdd.//

So now that I've tried to "do my homework by myself", could someone
please let me know how low I can go with the voltage supply to the 887?
(Also, if there's somewhere in the datasheet that mentions this, I'd
appreciate if you could point me to it)

2008\06\24@222226 by Adam Field

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face
Those are the absolute maximum ratings you can get away with without
damaging the part. It doesn't mean it will work at those values.

Look at section 17.1, you'll find a chart of Fosc vs min and max Vdd.
It's in the next two pages from where you found the absolute maximum
values.

You can go to 2.0V if you're running at 8MHz. 3.0V @ 10Mhz and 4.5V @ 20Mhz.

On Tue, Jun 24, 2008 at 10:00 PM, Tomás Ó hÉilidhe <spam_OUTtoeTakeThisOuTspamlavabit.com> wrote:
{Quote hidden}

>

2008\06\24@223447 by Adam Field

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face
Also, if you keep reading you'll find the supply current (Idd) values
for the 887 running in different oscillator configurations. Of course
the higher speeds draw more current but they also get things done
quicker. For good battery life you want to sleep for as long as
possible anyways.

On Tue, Jun 24, 2008 at 10:22 PM, Adam Field <.....adamKILLspamspam@spam@badtech.org> wrote:
> Those are the absolute maximum ratings you can get away with without
> damaging the part. It doesn't mean it will work at those values.
>
> Look at section 17.1, you'll find a chart of Fosc vs min and max Vdd.
> It's in the next two pages from where you found the absolute maximum
> values.
>
> You can go to 2.0V if you're running at 8MHz. 3.0V @ 10Mhz and 4.5V @ 20Mhz.
>

2008\06\24@223947 by Tomás Ó hÉilidhe

picon face


Adam Field wrote:
> Those are the absolute maximum ratings you can get away with without
> damaging the part. It doesn't mean it will work at those values.
>
> Look at section 17.1, you'll find a chart of Fosc vs min and max Vdd.
> It's in the next two pages from where you found the absolute maximum
> values.
>
> You can go to 2.0V if you're running at 8MHz. 3.0V @ 10Mhz and 4.5V @ 20Mhz.
>  

Great stuff, thanks.

If I run it at 8 MHz and put something like 3 volts on Vdd (e.g. 3 volts
from two AA batteries in series), then what voltage will I get on an
output pin when it's set to high? Will I still get 5 volts, or will I
only get 3 volts?

2008\06\24@224307 by Tomás Ó hÉilidhe

picon face


Adam Field wrote:
> For good battery life you want to sleep for as long as
> possible anyways.

I don't think I'll be able to put the chip into "sleep mode" at all
because I have to keep a multiplexed display constantly lit.

2008\06\24@224559 by PicDude

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face


And make sure you set the Brown-out reset voltage accordingly depending on
what voltage you power it with.



Adam Field wrote:
{Quote hidden}

>> -

2008\06\24@230911 by Adam Field

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face
Best case is Vdd (your input voltage: 3V), worst case is Vdd - 0.7V.
See 17.5 DC Characteristics

> If I run it at 8 MHz and put something like 3 volts on Vdd (e.g. 3 volts
> from two AA batteries in series), then what voltage will I get on an
> output pin when it's set to high? Will I still get 5 volts, or will I
> only get 3 volts?

2008\06\24@233859 by Robert Young

picon face


> If I run it at 8 MHz and put something like 3 volts on Vdd (e.g. 3 volts
> from two AA batteries in series), then what voltage will I get on an
> output pin when it's set to high? Will I still get 5 volts, or will I
> only get 3 volts?

You are pulling our collective leg now, right?  Or perhaps that should be "collector" leg (bad transistor pun).

Rob

2008\06\25@002358 by Jinx

face picon face
> You are pulling our collective leg now, right?  Or perhaps
> that should be "collector" leg

Read a lot of Ocar Wilde do you Rog ?  ;-)

2008\06\25@030717 by Jan-Erik Soderholm

face picon face
Tomás Ó hÉilidhe wrote:

> If I run it at 8 MHz and put something like 3 volts on Vdd (e.g. 3 volts
> from two AA batteries in series), then what voltage will I get on an
> output pin when it's set to high? Will I still get 5 volts, or will I
> only get 3 volts?
>

And where shuold the PIC get the 5V from ?

Jan-Erik.

2008\06\25@071920 by Tomás Ó hÉilidhe

picon face


Jan-Erik Soderholm wrote:
>> If I run it at 8 MHz and put something like 3 volts on Vdd (e.g. 3 volts
>> from two AA batteries in series), then what voltage will I get on an
>> output pin when it's set to high? Will I still get 5 volts, or will I
>> only get 3 volts?
>>
>>    
>
> And where shuold the PIC get the 5V from ?

Internal step-up voltage regulator. Given that I don't know everything
about the 887, I didn't think it impossible.

2008\06\25@074917 by Jan-Erik Soderholm

face picon face
Tomás Ó hÉilidhe wrote:
>
> Jan-Erik Soderholm wrote:
>>> If I run it at 8 MHz and put something like 3 volts on Vdd (e.g. 3 volts
>>> from two AA batteries in series), then what voltage will I get on an
>>> output pin when it's set to high? Will I still get 5 volts, or will I
>>> only get 3 volts?
>>>
>>>    
>> And where shuold the PIC get the 5V from ?
>
> Internal step-up voltage regulator.

Where did you find anything about that one ?
Never mind...

Jan-Erik.

2008\06\25@083929 by olin piclist

face picon face
Tomás Ó hÉilidhe wrote:
> Voltage on VDD with respect to VSS:  -0.3V to +6.5V

Note that this is in the "Absolute Maximum Ratings" section.  That gives you
the voltage limits that are guaranteed not to destroy the device, not what
voltage range it will operate correctly over.  Read the text in the gray box
right below these specs.

> Now see this is the reason why newbies shy away from datasheets so
> much.

Loose the attitude already.

> I haven't a clue what it's trying to tell me that I can put -.3 volts
> on Vdd.

Yes that section is saying that you can put -.3V on Vdd without killing the
PIC.  It is not saying the PIC will work with -.3V on Vdd.

> So now that I've tried to "do my homework by myself", could someone
> please let me know how low I can go with the voltage supply to the
> 887? (Also, if there's somewhere in the datasheet that mentions this,
> I'd appreciate if you could point me to it)

Keep looking in the data sheet nearby.  For most PICs, the next page will
give you a graph showing the valid combinations of Vdd and clock rate.


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