Bob Blick wrote:
"Marechiare" said:
>> That variations are exactly what I did in real life.
>> For larger variations of the divider's ratio I used
>> Schottky diodes on inputs to limit the input voltage
>> (was within the specs for that parts).
>
> Resistor in series and Schottky diode clamp to the
> 3V supply? Be careful about how much current it
> pumps into the 3V supply so as not to raise it
> or hold the 3V circuit from reset on power up/down.
> Especially if you have several pins to convert levels upon.


Yeah, you are right, that's why I don't advocate the divider's
approach. Your concern about "how much current it pumps into the 3V
supply" you may address to those who advocate the divider's approach.
Was written by one of them:


On Mon, Oct 5, 2009 at 1:07 AM :
> On the other end, the highest high of 3.48V is only
> 250mV above the lowest possible Vdd.  There won't
> be any meaningful current thru any protection
> diode at 250mV.


Bob Blick wrote:
> On another note, you guys seem to have picked ever
> so slightly different ground to defend so the shots don't
> seem to be hitting anything interesting. There also
> doesn't seem to be a lot of [PIC] going on.

No, one point about PIC inputs with Schmitt triggers yet to be mentioned:

> Not really in a typical case.  Let's say we have a nominal 5V +-2%
> (just about any LDO can do that) input and are using 5% resistors,
> 2Kohms followed by 3.9Kohms to ground.  The worst case high
> output is 5.1V into a 1900ohm,4095ohm divider, which comes out
> to 3.48V.  The worst case low is 4.9V into 2100ohm,3705ohm divider,
> which comes out to 3.13V.
>
> Let's again say we're using a 2% LDO on the 3.3V side, so it can
> range from 3.37V to 3.23V.  About the highest logic high threshold
> you'll encounter is 80% of the supply, which is what PIC inputs with
> Schmitt triggers have.  The worst case logic high threshold is therefore
> 3.73V * 0.8 = 2.69V.  That's still significantly less than the lowest
> high of 3.13V computed above.

This phrase (I removed typos) "The worst case logic high threshold is
therefore 3.37V * 0.8 = 2.7V"

The phrase could be misleading to many. My understanding how PIC
Schmitt triggers work is that high threshold 2.7V for Vdd 3.37V is the
BEST case, not the worst case (correct me if I'm wrong).

That is, from the very Schmitt Trigger idea it is guaranteed that the
level below 2.7V won't be considered logic "1". This means that logic
"1" won't be just >=2.7V. It could take much higher level to actually
reach logic "1".

So, the operating range of possible voltage that should be applied as
logic "1" shifts very closely to if not beyond the specs on max
voltage applied to inputs. Especially taking into account that the
system discussed involves RF module that, probably, would:
- surge supply chains destabilizing the LDO;
- cause gradient of potential on the ground lines;
- introduce RF induced voltages;
- etc etc;

Also should be taken into account that OP mentioned not 3.3V, but 3.0V
and the desire to explore the supply voltage down to 1.8V. This would
narrow the gaps even more.


> Care to call a cease-fire until another hill is found to claim?

Any ideas how to make it look like not a fire? I tried hard to stay
within reasonable rules this time. What should I correct in my
approach? Thanks.
-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist