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PICList Thread
'[PIC] Inputs higher than VDD'
2006\05\23@154520 by Phillip

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Hi all

I'm using a 16F676 to control DC through a GPS RF power divider/splitter.

First a little background.

GPS receivers need an antenna preamplifier to over come the line losses at
1575 GHZ(L1) and 1227 MHz(L2) there soon will be other freqs but that are
all in 1-2 GHz band.

DC is sent on the center coaxial center conductor up to the preamplifier in
the antenna and RF is sent back the down the coaxial cable to the GPS
receiver.



The RF input of the splitter is connected to a GPS antenna mounted on a
tower with a GPS receiver connected to each of the RF outputs of the
splitter.

This way you only have to pay a monthly fee for one GPS antenna on the tower
instead of many and the GPS antenna is a low point of failure.

My pick and choose circuit looks for DC on the inputs and selects one and
only one to provide power to the antenna's preamplifier through the
splitter.

If you connect two power supplies together they will fight with one "winner"
burning the others out so there can only be one DC path through the splitter
at any give time to the antenna preamplifier.

The unused DC inputs have a DC load to ground so that the receivers think
they are powering a preamplifier of an antenna and consequently don't throw
antenna faults.

If one of the GPS receivers keels over and stops providing DC voltage to the
antenna then the next receiver's DC load is switched out and now its power
is routed through the device to the antenna preamplifier while the first
input's path is disconnected form the antenna.

It there is an antenna fault (over current or under current) then the
antenna DC is switched open/off and all the DC loads are switched out (open
circuit) so now all GPS receivers will throw antenna faults.



The code for this all works fine but I have a couple of issues that are
giving me pause.



There is no external power available so I steal some of the antenna current
to power my the control and gain block circuitry of the splitter.



Most GPS receivers use 5V DC to bias the antenna preamplifier but some can
output as much as 12V DC so the logic inputs of my PIC can be anywhere from
5V to 12V.

I used a low drop out regulator to power the PIC at 3.3v and the RF
amplifier circuitry (the device can be operated as a passive splitter so the
RF amplifier is optional)

So I put a series resistor and a 3V zeiner to ground on each of the DC
inputs to the PIC to keep the inputs below VDD that was fine till I had to
program the thing.

The zeiner kept the programming lines at 3V and this was no good.

So now there is a series resistor a zeiner to ground and another series
resistor on the input pins so the ICD2 can pull the programming lines higher
than 3 volts during programming.

The ICD2 programming connector is connected directly to the device and the
LDO regulator is not hurt by taking VDD i.e. its output pin or VDD of the
PIC to 5 volts.  (whilst the LDO regulator has no input power of course)

The trouble is that according to the data sheet my 3V zeiner can go as high
as 3.5 when in breakdown.

If the PIC's VDD  is at 3.3V and the input pin is at 3.5V due to the 3V
zeiner heating...is this bad?....how bad?

I can't really find anything on the data sheet and I was hoping that someone
out there could tell me about any latch up or other issues.. (I could lower
the zeiner to 2.7 but I'd rather not unless I have to.)



Does anyone out there have any experience (good or bad)  with taking inputs
a "little" higher than VDD?







The other issue is that I'm I am doing twenty units for evaluation i.e.
environmental lighting strikes etc.

Out of the twenty 4 did not verify after programming.

They showed incorrect data at a specific location  (all four showed the same
location?????)

Sometimes I can reprogram and they pass other times they might fail ten
times in a row.

If I replace the PIC everything is fine?

How could I get four devices out of twenty that are bad?

I can't imagine this is a quality issue.

All the parts appear to have the same date/batch codes according to my
technician.

Any guesses to what is going on here?







Phillip

Things should be as simple as possible but no simpler







Phillip Coiner

CTO, GPS Source, Inc.





Your source for quality GNSS Networking Solutions and Design Services, Now!




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2006\05\23@155736 by Phillip

picon face




1575GHz should read 1575MHz





Phillip
Things should be as simple as possible but no simpler



Phillip Coiner
CTO, GPS Source, Inc.


Your source for quality GNSS Networking Solutions and Design Services, Now!

{Original Message removed}

2006\05\23@165139 by Bob Axtell

face picon face
Phillip wrote:
{Quote hidden}

I think it would be better to provide a separate supply.

{Quote hidden}

The ICD2 is debugging or programming? In production, I'd use another
programmer, the ICD2 is much too
cranky to use as a production programmer. If you are debugging only, you
can debug at 3.3V OK.


> The ICD2 programming connector is connected directly to the device and the
> LDO regulator is not hurt by taking VDD i.e. its output pin or VDD of the
> PIC to 5 volts.  (whilst the LDO regulator has no input power of course)
>
> The trouble is that according to the data sheet my 3V zeiner can go as high
> as 3.5 when in breakdown.
>
> If the PIC's VDD  is at 3.3V and the input pin is at 3.5V due to the 3V
> zeiner heating...is this bad?....how bad?
>  
Why not use a 2.5V Zener? It will never go above 3.3, and is high enough
to express the correct logic
level. Better is to install schottky diodes to VDD at the chip input, so
that when the value tried to go
above VDD, the tiny current is routed into the VDD supply.  Careful
here: some types of VDD supplies
may not work correctly this way.

> I can't really find anything on the data sheet and I was hoping that someone
> out there could tell me about any latch up or other issues.. (I could lower
> the zeiner to 2.7 but I'd rather not unless I have to.)
>
>  
Those low voltage zeners are expensive, right?

>  
>
> Does anyone out there have any experience (good or bad)  with taking inputs
> a "little" higher than VDD?
>
>  
Bad, bad ideal. The PIC will either lock up of become unable to be reset
properly.
>  
>
>  
>
>  
>
> The other issue is that I'm I am doing twenty units for evaluation i.e.
> environmental lighting strikes etc.
>  
use those cute ESD suppressors in 0805 by Cooper-Bussman. VERY low
capacitance (.5pF).
> Out of the twenty 4 did not verify after programming.
I'll bet.
{Quote hidden}

They are not bad, they simply are never properly reset.

> I can't imagine this is a quality issue.
>
> All the parts appear to have the same date/batch codes according to my
> technician.
>
> Any guesses to what is going on here?
>
>  
>  
Get rid of the external power sources.

--Bob
{Quote hidden}

2006\05\23@183427 by Phillip

picon face






Phillip
Things should be as simple as possible but no simpler



Phillip Coiner
CTO, GPS Source, Inc.


Your source for quality GNSS Networking Solutions and Design Services, Now!

{Original Message removed}

2006\05\23@205951 by Spehro Pefhany

picon face
At 01:57 PM 5/23/2006 -0600, you wrote:


>The trouble is that according to the data sheet my 3V zeiner can go as high
>as 3.5 when in breakdown.
>
>If the PIC's VDD  is at 3.3V and the input pin is at 3.5V due to the 3V
>zeiner heating...is this bad?....how bad?
>
>I can't really find anything on the data sheet and I was hoping that someone
>out there could tell me about any latch up or other issues.. (I could lower
>the zeiner to 2.7 but I'd rather not unless I have to.)

Zeners <= 5V breakdown have a horribly soft "knee". Make sure that the voltage
is guaranteed to be a logic 1 under all conditions (worst case with low
input voltage, series resistor chosen to be okay for maximum input voltage).
See the linked typical graph (this one for the MMBZ5221BLT1
series). http://server2.hostingplex.com/~zstoretr/zim.pdf

An ideal zener would have a curve that was just a vertical line.

There are also unit-to-unit tolerances and temperature coefficients to contend
with.


>Does anyone out there have any experience (good or bad)  with taking inputs
>a "little" higher than VDD?

A little (meaning something like 100-200mV) won't hurt anything but you have
to be able GUARANTEE this under *all* conditions, which is ummm..
"non-trivial".

A homemade RTL inverter (BJT with input resistors) for an input (with
inverse input diode to prevent Vbe from going too negative) would be very
robust and quite cheap.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
spam_OUTspeffTakeThisOuTspaminterlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
->>Test equipment, parts OLED displys http://search.ebay.com/_W0QQsassZspeff


2006\05\24@190605 by Dwayne Reid

flavicon
face
At 07:11 PM 5/23/2006, Spehro Pefhany wrote:

> >The trouble is that according to the data sheet my 3V zeiner can go as high
> >as 3.5 when in breakdown.
> >
> >If the PIC's VDD  is at 3.3V and the input pin is at 3.5V due to the 3V
> >zeiner heating...is this bad?....how bad?
> >
> >I can't really find anything on the data sheet and I was hoping that someone
> >out there could tell me about any latch up or other issues.. (I could lower
> >the zeiner to 2.7 but I'd rather not unless I have to.)
>
>Zeners <= 5V breakdown have a horribly soft "knee". Make sure that the voltage
>is guaranteed to be a logic 1 under all conditions (worst case with low
>input voltage, series resistor chosen to be okay for maximum input voltage).
>See the linked typical graph (this one for the MMBZ5221BLT1
>series). http://server2.hostingplex.com/~zstoretr/zim.pdf

LM-385-2.5 shunt reference?  I use them in place of zeners at that voltage.

TL431 also works but not as low quiescent current as LM385 or LM285.

LM385 is darned cheap in large enough quantities.

dwayne

--
Dwayne Reid   <.....dwaynerKILLspamspam@spam@planet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax

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2006\05\25@025323 by Russell McMahon

face
flavicon
face
> The trouble is that according to the data sheet my 3V zeiner can go
> as high
> as 3.5 when in breakdown.
>
> If the PIC's VDD  is at 3.3V and the input pin is at 3.5V due to the
> 3V
> zeiner heating...is this bad?....how bad?

Bad.
Very bad.
Totally bad.
Fatally bad.
Sometimes.
YMMV.


Any input voltage which exceeds the Vdd - Vss range on pins with
protection diodes by more than a specified amount can and variably
will cause the IC to malfunction. It can do so erratically or
predictably. It can be very very very occasional. ALL data sheet give
the max limits in the "operating" section. The limits given in the
"Absolute maximum values" section do not guarantee correct operation -
just survival of the IC. . For some its 0.3v outside supply rails and
for some its supply rails. in REALITY its "below the level where the
diodes conduct enough to every cause problems". The limits of 0V or
0.3V or whatever are set to be safe enough.

Many will tell you (none in this current thread that I am aware of as
I largely haven't read it) that it doesn't matter. By all means take
their incorrect advice :-).

The problem is that the protection diode current enters the IC at
points where it has not been designed to go. The results are not
designed and can be very very very variable. YMMV. The designers COULD
provide paths back to supply should they wish, but they don't. (The
diodes are usually formed by forward biasing the input pin to
substrate junction so providing designed current paths would be costly
and annoying.)



       Russell McMahon







2006\05\25@080804 by olin piclist

face picon face
Russell McMahon wrote:
>> If the PIC's VDD  is at 3.3V and the input pin is at 3.5V due to the
>> 3V
>> zeiner heating...is this bad?....how bad?
>
> Bad.
> Very bad.
> Totally bad.
> Fatally bad.
> Sometimes.
> YMMV.

Huh?  200mV accross the protection diodes isn't going to turn them on or
cause problems.  There is actually a specification for how far you can go
below Vss and above Vdd.  If I remember right that is 200 or 300mV.  So the
200mV the OP asked about is within spec (again, if I remember right).


******************************************************************
Embed Inc, Littleton Massachusetts, (978) 742-9014.  #1 PIC
consultant in 2004 program year.  http://www.embedinc.com/products

2006\05\25@090501 by Phillip

picon face








Your source for quality GNSS Networking Solutions and Design Services, Now!
-----Original Message-----
From: piclist-bouncesspamKILLspammit.edu [.....piclist-bouncesKILLspamspam.....mit.edu] On Behalf Of
Olin Lathrop
Sent: Thursday, May 25, 2006 6:10 AM
To: Microcontroller discussion list - Public.
Subject: Re: [PIC] Inputs higher than VDD

Russell McMahon wrote:
{Quote hidden}

Huh?  200mV accross the protection diodes isn't going to turn them on or
cause problems.  There is actually a specification for how far you can go
below Vss and above Vdd.  If I remember right that is 200 or 300mV.  So the
200mV the OP asked about is within spec (again, if I remember right).


[Phillip says] Hi Olin   (thanks for the reply)
You do remember correctly.
I found the spec right after I hit send on the first message.
I replied to Bob as follows.
[Phillip says]
I'm not sure about the cost of the lower voltage zeiner but it looks like
I'm buying no matter what.
I just found the spec in the data sheet it says Voltage on all other pins
with respect to VSS -.3V to (VDD + .3V) now 3.5 is less than 3.3V + .3V =
3.6V But if my 12V supply is a tiny bit over 12V then I reckon it will be
too close to the wire.
[Phillip says] also
If the temp rises enough then I will surely be in trouble.  I found some
2.7V zeiners I think that will provide me a safe margin over temp and for
the power supplies of the GPS receivers + or - 10%.

Others offered up advice about some other devices.  I want to look into
those but I was stomping out IT fires all day yesterday instead of doing
development.

Phillip
Things should be as simple as possible but no simpler



Phillip Coiner
CTO, GPS Source, Inc.


******************************************************************
Embed Inc, Littleton Massachusetts, (978) 742-9014.  #1 PIC
consultant in 2004 program year.  http://www.embedinc.com/products

2006\05\25@094454 by Russell McMahon

face
flavicon
face
{Quote hidden}

I (happily) stand by my answer. I'm surprised to see you advising that
people should violate datasheet specs, or providing answers which may
lead to this when you are uncertain if the answer violates specs (even
though you note your uncertainty), when a less experienced person may
take your advice, based on the knowledge that your answers in other
areas are almost always high quality ones.  The better you are the
more incumbent it is on you to be nearly perfect in all areas :-). (ie
given how good your answers usually are you have a duty to make all
your answers as good as you can so that people who rely on you are not
mislead. **)

My response was meant to be useful in answering the actual intended
question rather than deciding if one could shoehorn the application
into reality if the wind was blowing in the desired direction and all
the dead fish were downwind.

While he specified that " ... the input pin is at 3.5V due to the 3V
zener ehating ..." the actual situation is more complex and the real
voltage may be above that. The fact that the zener in question has a
*negative* temperature coefficient actually improves the situation in
most cases.

The "real" question is:

   "If I use a zener diode which I know has a softish knee, which all
zeners have, but especially low voltage ones, and which has a
production variation on zener voltage of 5% or maybe 10% depending on
what I buy, and which will vary with temperature, and if I want the
circuit to work reliably, is it wise to allow the pin voltage to rise
to a small but relatively unknown level above Vdd?"

The real answer is, as it always has been, No!

It happens that memory does and doesn't serve correctly as some PIC
data sheets specify the absolute range of allowable pin voltages as
lying inside Vdd-Vss and some allow a somewhat wider excursion (maybe
up to 0.3 V beyond AFAIITR).

However, the understood principle is that conduction of the protection
diodes by any extent is potentially bad news. At 0.2V, as originally
asked, there is going to be no problem in the real world. Even at 0.3V
all should be well. At 0.4V one may start to become a wee bit twitchy,
and the processor just may as well, and at 0.5V it may be all go, or
all stop, or some random mix.

A typical 3V zener may have a max spec of 3.2V and *cooling* by 30C
(as zeners below ~5V1 have negative tempcos) will add about another
100 mV. This is at 5mA ish for a small zener (SOT23 or 200 to 500mW
glass) which are typical of what is most likely to be used here. So in
fact the 3V zener is lia=ble to only get up to Vdd in most cases and
not to the 3V5 asked about.

I'd still be wary, although the extra 200 mV headroom and the negative
tempco are liable to make all the difference.

The key point is that one is asking about taking processor pins
outside supply range, and many processors datasheets do not guarantee
operation in that area. If you understand the reasons for the spec,
plus any secondary reasons, and if  you do all the sums and it looks
OK, and if you are happy to live with possible failures, then it may
be OK to violate datasheet specs. But, if you are relatively
inexperienced, and the answer to the question is less than fully
obvious, then steering on the safe side for the next decade or so is
liable to lead to a less exciting and experience filled life.



           Russell McMahon
           "Experience is what you get when you don't get what you
want."



** Yes, this is a duty. While nobody is obliged to take advice from
anyone here, and while advice garnered here may be worth what is paid
for it, there is a duty of care for those who often 'strut their
stuff' with pride, not to thereby mislead those who come to worship,
so that advice won't become worth less than is paid for it.






2006\05\25@112328 by olin piclist

face picon face
Russell McMahon wrote:
> I (happily) stand by my answer. I'm surprised to see you advising that
> people should violate datasheet specs,

I didn't.  The OP asked about 200mV over Vdd and the datasheet specifies up
to 300mV is OK.

> My response was meant to be useful in answering the actual intended
> question

OK, but that doesn't make my answer to the question actually asked wrong.  I
suppose I could have gone into some background about protection diodes,
substrate currents, latchup, and the like.  I do that sort of thing
sometimes, but less so on this list lately and less so the more I have other
pressing things to do.  This morning I read the post and chose to only
respond to the question as written.  For all I remember, I may not have even
read the bottom of the post.  Elaboration and background information are
nice, but a direct answer to a question isn't wrong or irresponsible.

> The "real" question is:
>
>    "If I use a zener diode which I know has a softish knee, which all
> zeners have, but especially low voltage ones, and which has a
> production variation on zener voltage of 5% or maybe 10% depending on
> what I buy, and which will vary with temperature, and if I want the
> circuit to work reliably, is it wise to allow the pin voltage to rise
> to a small but relatively unknown level above Vdd?"

If that's really the question, then the "complete" answer is to ask what the
overall problem is.  Is this to clamp a digital signal to the valid range?
Clamp an analog signal?  Provide a voltage reference?  The best answer is to
describe how to achieve the overall objective.

If clamping a digital signal with sufficiencly low impedence, then a
resistor then schottky diode to Vdd should be fine.

If clamping an analog signal, then it gets more tricky since you probably
can't tolerate much leakage else the analog level will get corrupted.  A
hack that is techinically a violation of the data sheet but should cause no
problems is to use clamp with a silicon diode to Vdd, then a resistor from
there to the PIC pin.  The external diode and the protection diode will have
similar voltage characteristics.  The resistor will limit the current to
very small levels if the external diode happens to have a little higher
voltage than the PIC.

The method I prefer for analog signals into PIC pins when a few extra cents
can be tollerated is to buffer with an opamp.  Put a resistor, like 10K
ohms, in series with the opamp output, then clamp to Vdd and Vss with
schottky diodes, then to the PIC pin.  The trick is to take the feedback
from the PIC pin, not the opamp output.  This way the opamp compensates for
the leakage current of the schottky diodes, and also makes the PIC pin a low
impedence node despite the 10Kohm resistor.  Of course this only works
within the opamp's output voltage swing limit.  I usually create a few volts
negative supply with a charge pump or whatever to make sure the opamp output
can swing below ground.

By the way, if you run your main switching supply with a 10F204 you can use
one pin to run a charge pump.  This gets toggled instead of executing NOPs
during deliberate waits.  You get 2 for the price of 1.


******************************************************************
Embed Inc, Littleton Massachusetts, (978) 742-9014.  #1 PIC
consultant in 2004 program year.  http://www.embedinc.com/products

2006\05\25@124414 by Phillip

picon face


Hi thanks Russell and to everyone else.
I am really grateful for all the replies.
I think technically everyone is correct and both replies were useful if only
to make me think..... Henry Ford said thinking is the hardest work you will
ever do that's why most people do so little of it.

I didn't see/couldn't find the value in the data sheet when I sent the
original message.
I suspected there was a latch up issue that is why I asked.
I'm no design genius but I know that if you design right to the wire the
wire will likely end up around your neck.
So I knew that being only 100mV with in the specified tolerance was not
safe.
I also knew the things varied with temp (as Spehro Pefhany pointed out I
wasn't sure if it was a negative or positive coefficient).
Also the power supplied by the GPS receivers can be all over the place.

For the record I can't re-layout the board and I have to send some units to
through verification in a couple of days so I'm going with the 2.7 volt
zeiner.
This will give me .6V of margin so I think it will be all right.
They are available in the exact same package and drop right in.
So I couldn't really consider the other devices or the shottky diodes at
this juncture with out turning/changing the board.

I'm not sure about using the shottky diode and a resistor.
In my mind it would depend if the VDD power supply's output could
handle/dissipate the current from the diode[s].
I suppose it would just be a matter of increasing the resistor to a large
enough value to limit the current but my input voltage varies
widely..........once again more thinking for my one remaining brain cell.
I am more comfortable with the zeiner approach because it does not require
me to think about the power supply sinking the input voltage current....and
I need to send what I currently have, if I can, becuse my deadline is
looming.


Thanks again to everyone.

(If anyone has any GPS or questions don't hesitate to ask)





Phillip
Things should be as simple as possible but no simpler



Phillip Coiner
CTO, GPS Source, Inc.


Your source for quality GNSS Networking Solutions and Design Services, Now!

{Original Message removed}

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