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'[EE] Low Voltage PIC'
2009\03\21@034302 by solarwind

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So far, I've only been working with TTL voltage levels (5V) for all my
ICs (PIC, 4000, 74HC, AS1107, etc...).

If I switch to a 3.3 V PIC such as the PIC32 or an 18F26J50 chip,
would my digital signals still work with my other chips (such as the
AS1107)? I'm asking in general... Is 3.3 V considered a logic high? Do
most chips accept 3.3 V a logic high? What about the low voltage PICs
that can run off of 1.8 V? Will those still be compatible?

Also, will the USB module on the 3.3 V chip be able to communicate
directly to the PC which uses 5 V signals or does it require a level
shifter to boost the signal to 5 V?

--
solarwind

2009\03\21@075204 by Rikard Bosnjakovic

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On Sat, Mar 21, 2009 at 08:43, solarwind <spam_OUTx.solarwind.xTakeThisOuTspamgmail.com> wrote:

> Is 3.3 V considered a logic high? Do most chips accept 3.3 V a logic high?

Google "cmos ttl logic levels".

First hit: www.allaboutcircuits.com/vol_4/chpt_3/10.html
Second: http://en.wikipedia.org/wiki/Logic_level


--
- Rikard - http://bos.hack.org/cv/

2009\03\21@083540 by Russell McMahon

face
flavicon
face
> So far, I've only been working with TTL voltage levels (5V) for all my
> ICs (PIC, 4000, 74HC, AS1107, etc...).

> If I switch to a 3.3 V PIC such as the PIC32 or an 18F26J50 chip,
> would my digital signals still work with my other chips (such as the
> AS1107)? I'm asking in general... Is 3.3 V considered a logic high? Do
> most chips accept 3.3 V a logic high? What about the low voltage PICs
> that can run off of 1.8 V? Will those still be compatible?

> Also, will the USB module on the 3.3 V chip be able to communicate
> directly to the PC which uses 5 V signals or does it require a level
> shifter to boost the signal to 5 V?


How long is a piece of answer?

There are many logic families, operating at various Vdd / Vcc levels. With
various high/low thresholds. Some with tolerance for input voltages above
their Vdd level, many not. You will easily be able to find much general and
specific discussion of this on about 3.2 zillion internet sites and doing so
before asking on list is not only good form, it will make you look good, or
at least better than otherwise, when you ask informed intelligent and pithy
questions. And will make it less likely that people who may feel you have
recently slighted their research skills or tendencies (rightly or wrongly as
the case may be) will be able to return the perceived compliment with any
justifcation.


 Russell

A correct linguistic parsing of the above content will probably assist in
the development of neural material and perhaps of vocabulary. E&OE.




2009\03\21@104501 by olin piclist

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solarwind wrote:
> So far, I've only been working with TTL voltage levels (5V) for all my
> ICs (PIC, 4000, 74HC, AS1107, etc...).
>
> If I switch to a 3.3 V PIC such as the PIC32 or an 18F26J50 chip,
> would my digital signals still work with my other chips (such as the
> AS1107)? I'm asking in general... Is 3.3 V considered a logic high? Do
> most chips accept 3.3 V a logic high? What about the low voltage PICs
> that can run off of 1.8 V? Will those still be compatible?
>
> Also, will the USB module on the 3.3 V chip be able to communicate
> directly to the PC which uses 5 V signals or does it require a level
> shifter to boost the signal to 5 V?

Most of this information can be gleaned by careful reading of the datasheets
and maybe a little general electronics research.  I could elaborate, but
won't.  And yes, I'm making the point that deliberately insulting people for
no reason is going to have consequences when you want something from them in
return.  I hope others will join me on principal and not just give you the
answer, whether you personally attacked them too or not.


********************************************************************
Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
(978) 742-9014.  Gold level PIC consultants since 2000.

2009\03\21@113830 by solarwind

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On Sat, Mar 21, 2009 at 6:51 AM, Rikard Bosnjakovic
<.....rikard.bosnjakovicKILLspamspam@spam@gmail.com> wrote:
> On Sat, Mar 21, 2009 at 08:43, solarwind <x.solarwind.xspamKILLspamgmail.com> wrote:
>
>> Is 3.3 V considered a logic high? Do most chips accept 3.3 V a logic high?
>
> Google "cmos ttl logic levels".
>
> First hit: www.allaboutcircuits.com/vol_4/chpt_3/10.html
> Second: http://en.wikipedia.org/wiki/Logic_level

Very nice. I guess I'll just have to find out when my samples arrive :)



--
solarwind

2009\03\21@125728 by Harold Hallikainen

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flavicon
face

> solarwind wrote:
>> So far, I've only been working with TTL voltage levels (5V) for all my
>> ICs (PIC, 4000, 74HC, AS1107, etc...).
>>
>> If I switch to a 3.3 V PIC such as the PIC32 or an 18F26J50 chip,
>> would my digital signals still work with my other chips (such as the
>> AS1107)? I'm asking in general... Is 3.3 V considered a logic high? Do
>> most chips accept 3.3 V a logic high? What about the low voltage PICs
>> that can run off of 1.8 V? Will those still be compatible?
>>
>> Also, will the USB module on the 3.3 V chip be able to communicate
>> directly to the PC which uses 5 V signals or does it require a level
>> shifter to boost the signal to 5 V?
>
> Most of this information can be gleaned by careful reading of the
> datasheets
> and maybe a little general electronics research.


I agree that careful reading of datasheets is in order. Most of the chips
I'm working with now run on 3.3V supply, so the logic levels are
compatible. Some chips require 5V levels. If I only need to send data from
the 3.3V to the 5V chip, I use a "T" chip like the 74HCT244 running at 5V.
The T indicates it has TTL level input thresholds. TTL is not symmetrical.
The threshold is not at 1/2 supply, but lower. CMOS chips generally have
thresholds at 1/2 supply and, if they have schmitt inputs, they have
thresholds at 20% and 80% of supply, typically. Since 80% of 5V is 4V, a
3.3V chip will never get there. But, a CMOS part with a T in the part
number WILL properly receive 3.3V logic levels. If I need to send data in
both directions, I typically use level translator chips designed for that
purpose. Maxim has a whole series. Some can be used on single lines that
send data in both directions (like I2C), and others are both directions,
but on separate lines (like SPI).

Good luck!

Harold



--
FCC Rules Updated Daily at http://www.hallikainen.com - Advertising
opportunities available!

2009\03\21@132750 by solarwind

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On Sat, Mar 21, 2009 at 12:02 PM, Harold Hallikainen
<.....haroldKILLspamspam.....hallikainen.org> wrote:
> I agree that careful reading of datasheets is in order. Most of the chips
> I'm working with now run on 3.3V supply, so the logic levels are
> compatible. Some chips require 5V levels. If I only need to send data from
> the 3.3V to the 5V chip, I use a "T" chip like the 74HCT244 running at 5V.
> The T indicates it has TTL level input thresholds. TTL is not symmetrical.
> The threshold is not at 1/2 supply, but lower. CMOS chips generally have
> thresholds at 1/2 supply and, if they have schmitt inputs, they have
> thresholds at 20% and 80% of supply, typically. Since 80% of 5V is 4V, a
> 3.3V chip will never get there. But, a CMOS part with a T in the part
> number WILL properly receive 3.3V logic levels. If I need to send data in
> both directions, I typically use level translator chips designed for that
> purpose. Maxim has a whole series. Some can be used on single lines that
> send data in both directions (like I2C), and others are both directions,
> but on separate lines (like SPI).
>
> Good luck!
>
> Harold

Thanks for referring the octal line driver and thanks for the other info.

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