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'PICs: Good Design More on (ab)use of PIC protectio'
1999\10\10@194311 by Dennis Plunkett

flavicon
face
At 22:57 9/10/99 +1300, you wrote:
{Quote hidden}

Wow Russ,
Yes you are correct in that the protection diodes are not guarrenteed to
save the device, an any way, they are nominaly included for ESD only! Yes
William is correct with the AC APP note, but this is a side issue! But side
issuee or not this does rasie the question of DO MICROCHIP CONDONE the use
of protection diodes for clamping? It would seem that they do, as the APP
NOTE suggests (I think that you could wave this one at them if a device
fails).
But!
Russ is correct in that this is not good design practice (But try telling
that to the people whem desing for the Automotive market) Getting back to
the original post I have a few questions that undoubtably others have also
considered. The 10uA in whhich direction and to where does this go? into
the core or out the pin? The resets and configuration alterations indicate
that the current spikes are in exccess of the diode nominal capacity, or
that they are causing some EMI into the PIC. I think that this circuit
needs to be investagated and some more answers / questions need to be raised.
But in general I agree with Russ, protection diodes are a bonus only
(typically for ESD not nominal high current rates). Also, need to find out
how fast these diodes are (This is a hint at the EMI problem (Been down
this road with MIL stuff many a time))


Dennis

1999\10\11@080637 by Morgan Olsson

picon face
Tracks on PCB:s ar also transmission lines.
i.e if you have a PIC that drives one line leading to another PIC 20cm away on the other end oc a PCB, then that signal will overshoot, and it wil do it with what the transmitting PIC will drive into the line, transiently much more then the continuously allowed 20/25mA.

So the recieving PIC will recieve a transient of more than +-20mA overshoot current.

The theoretical model uses is driving Vdd/Vcc into a transmission line with recieving end Hi Z, but clamped to Vdd & Vcc.  The clamping vill recieve as much ctrrent as the driver some pioseconds earlier was driving into the line, for the PIC this is transiently much more tna 20/25 mA)

So, basically, PICs are incompetent of recieving signals from each other without additional protection?  (such as at least line driving series resistors)

/Morgan
Morgans Reglerteknik, HŠllekŒs, 277 35 KIVIK, SWEDEN
  tel +46(0)414-446620, fax -70331,   .....mrtKILLspamspam@spam@iname.com

1999\10\11@102316 by Jim Hartmann

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face
My experiment that caused spontaneous internal register changes was quite
simple.  Connect a negative voltage through a series resistor to an input
pin (my experiment was with PORTB, RB3 on a 16C622 series).  Raise the
current by increasing the voltage magnitude, the chip begins malfunctioning
at around 2ma.  This current reverse biases the lower "protection diode".

1999\10\11@105607 by Alan Pearce

face picon face
Connecting a negative voltage to an input pin, even through a resistor quite
clearly violates the "operating" portion of the data sheet. The "maximum"
portion of the data sheet is conditions that the chip can withstand without
damage, but are definitely not part of the operating conditions. hence the
protection diode being able to take 20mA is a maximum rating that is safe for
the chip, but for this to happen the operating condition that Vss<Vin<Vdd is
violated. hence you are causing yourself a whole lot of grief by not observing
the operating conditions clauses. It is like saying that a 100V transistor can
take 10A, but to have 1000W dissapation is violating its 150W rating!!!!!!!

1999\10\11@110644 by Jim Hartmann

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Would you kindly point me to a page and line where it says that a negative
voltage WRT Vss on a pin violates the operating conditions?

Jim Hartmann





Alan Pearce <A.B.PearcespamKILLspamRL.AC.UK>spamKILLspamMITVMA.MIT.EDU> on 10/11/1999 09:55:08 AM

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Subject:  Re: PICs: Good Design More on (ab)use of PIC protection diodes &
     bad design


Connecting a negative voltage to an input pin, even through a resistor
quite
clearly violates the "operating" portion of the data sheet. The "maximum"
portion of the data sheet is conditions that the chip can withstand without
damage, but are definitely not part of the operating conditions. hence the
protection diode being able to take 20mA is a maximum rating that is safe
for
the chip, but for this to happen the operating condition that Vss<Vin<Vdd
is
violated. hence you are causing yourself a whole lot of grief by not
observing
the operating conditions clauses. It is like saying that a 100V transistor
can
take 10A, but to have 1000W dissapation is violating its 150W rating!!!!!!!

1999\10\11@113929 by Alan Pearce

face picon face
>Would you kindly point me to a page and line where it says that a negative
>voltage WRT Vss on a pin violates the operating conditions?

I do not have the data sheet for the 16C622 which I believe you are using, but I
looked up the sheet for the 16C6x (DS30234D) and page 183 lists "Maximum ratings
any pin to Vss as -0.3V to Vdd+0.3V, i.e less than a diode voltage drop outside
the supply rails. This is listed the same through subsequent pages for the
various models of this PIC.

Page 186 gives "Standard Operating Conditions" i.e. the conditions it would
expect to operate under as Parameter D030 - Input Low Voltage Min Vss - not
(Vss-0.3V)
The next parameter is "Input High Voltage" Max Vdd - not Vdd+0.3V. the following
pages list the same for the other models of this family.

I also checked the data sheet I have for 16F87x (DS30292A) and this gives the
same set of voltages for the same parameters. I repeat that "Maximum ratings"
are the limits that will not destroy the device, but are not "standard operating
conditions". All micros, digital and analogue ICs, transistors diodes and
practically any component I can think of is specified this way.

An application note which shows a resistor being used off the mains to drive a
pin does not necessarily give licence to use a chip like that. A collegue I
worked with always said "It is not what is in the circuit on the application
note, but what they left out that is important", and he is right. The device
used in the app note may have happened to work because an adjacent pin wasn't
used, or the chip was a preproduction one that had a different mask pattern, and
the circuit designer was lucky and got away with it. It may even be that the
circuit he used had the same problem you do, but the circuit that got printed
did not show the mods used to get around the problem.

1999\10\11@121813 by Jim Hartmann

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Thanks,

The 16C621 Absolute Maximum Ratings says "Voltage on any pin WRT Vss :
-0.6V to VDD+0.6V".

I think there is some confusion as to what the DC Characteristics "Input
Low Voltage" and "Input High Voltage" mean.  I believe that these
specifications do not dictate limits to what can be applied to the pins.
These specifications dictate what Microchip guarantees will be the
thresholds of the input logic circuit.  An input voltage will produce a
zero input guaranteed if the input voltage is VSS.  But the threshold could
be as high as 0.8V maximum (ttl buffer).

Anyone care to differ?

Jim Hartmann





Alan Pearce <@spam@A.B.PearceKILLspamspamRL.AC.UK>KILLspamspamMITVMA.MIT.EDU> on 10/11/1999 10:38:22 AM

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Subject:  Re: PICs: Good Design More on (ab)use of PIC protection diodes &
     bad design


>Would you kindly point me to a page and line where it says that a negative
>voltage WRT Vss on a pin violates the operating conditions?

I do not have the data sheet for the 16C622 which I believe you are using,
but I
looked up the sheet for the 16C6x (DS30234D) and page 183 lists "Maximum
ratings
any pin to Vss as -0.3V to Vdd+0.3V, i.e less than a diode voltage drop
outside
the supply rails. This is listed the same through subsequent pages for the
various models of this PIC.

Page 186 gives "Standard Operating Conditions" i.e. the conditions it would
expect to operate under as Parameter D030 - Input Low Voltage Min Vss - not
(Vss-0.3V)
The next parameter is "Input High Voltage" Max Vdd - not Vdd+0.3V. the
following
pages list the same for the other models of this family.

I also checked the data sheet I have for 16F87x (DS30292A) and this gives
the
same set of voltages for the same parameters. I repeat that "Maximum
ratings"
are the limits that will not destroy the device, but are not "standard
operating
conditions". All micros, digital and analogue ICs, transistors diodes and
practically any component I can think of is specified this way.

An application note which shows a resistor being used off the mains to
drive a
pin does not necessarily give licence to use a chip like that. A collegue I
worked with always said "It is not what is in the circuit on the
application
note, but what they left out that is important", and he is right. The
device
used in the app note may have happened to work because an adjacent pin
wasn't
used, or the chip was a preproduction one that had a different mask
pattern, and
the circuit designer was lucky and got away with it. It may even be that
the
circuit he used had the same problem you do, but the circuit that got
printed
did not show the mods used to get around the problem.

1999\10\11@181820 by Dennis Plunkett

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face
At 11:15 11/10/99 -0500, you wrote:
>Thanks,
>
>The 16C621 Absolute Maximum Ratings says "Voltage on any pin WRT Vss :
>-0.6V to VDD+0.6V".
>
>I think there is some confusion as to what the DC Characteristics "Input
>Low Voltage" and "Input High Voltage" mean.  I believe that these
>specifications do not dictate limits to what can be applied to the pins.
>These specifications dictate what Microchip guarantees will be the
>thresholds of the input logic circuit.  An input voltage will produce a
>zero input guaranteed if the input voltage is VSS.  But the threshold could
>be as high as 0.8V maximum (ttl buffer).
>
>Anyone care to differ?
>
>Jim Hartmann
>


YES I DO!
If you look you will see that Microchip just like all other manufacturers
will have a little note that says
"Stresses abaove and beyond the maximum limits may cause dammage".
The maximum limits are that that can be applied to any pin on the device.

I think that you are getting it wrong
"The 16C621 Absolute Maximum Ratings says "Voltage on any pin WRT Vss :0.6V
to VDD+0.6V""
This is it, go anywhere over these limits and BANG!
In your case you are lookingat why and what current can be sustained. Now
you need to do two things to get this one rapped up. In an earlyer mail,
you indicated that 2mA of negative current casued the PIC to go nar nar.
OK, so was this current constant? Did you monitor the current with a CRO to
see if at any time during that application that the current was in excess
of 20mA? Did you monitor the pin votlage and the supply votage to see if
the supply exteneded beyond 6V (Max limit?) and that the pin voltage was
not more than -0.6V? If you performed this and it was within these limits,
then contact Microchip, as it would seem that you have found an error.


Dennis

1999\10\11@184501 by William Chops Westfield

face picon face
Of course, if you connect up your 100V supply through your 1M resistor
to a PIC input pin, and then measure the voltage at the pin, you WILL
read Vcc+0.6V or so, assuming that the internal protection diodes are
functioning...

BillW

1999\10\11@194522 by Dennis Plunkett

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face
At 15:44 11/10/99 PDT, you wrote:
>Of course, if you connect up your 100V supply through your 1M resistor
>to a PIC input pin, and then measure the voltage at the pin, you WILL
>read Vcc+0.6V or so, assuming that the internal protection diodes are
>functioning...
>
>BillW
>
>

Not quite so,
This will be so only after the diode (Transistor) has begun conduction.
Until this point the pin may be way way over voltage.


Dennis

1999\10\12@111953 by Jim Hartmann

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face
I'm not sure I understand what you disagree with.  I am saying that the
Abs. Max. Ratings and the Input Low and Input High specs are two distinct
issues.  One is a limitation that you must not exceed without introducing
"nar nar" as you say.  The other is a specification of how the chip behaves
under normal conditions, or specifically what the voltage thresholds are
for the input logic.

I injected 2ma DC from a constant DC source, not from field wiring but a
regulated lab power supply.  It never exceeded 2mA and it makes the chip
malfunction.  However, I didn't measure the voltage on the pin.  Its quite
likely that it went below -0.6V and exceeded the voltage limit without
exceeding the current limit.

My point in bringing up this issue was to warn about relying on the
protection diodes when significant current is present, and to present some
details about what happens.

-Jim Hartmann





Dennis Plunkett <TakeThisOuTdennisEraseMEspamspam_OUTRDD.NECA.NEC.COM.AU>EraseMEspamspam_OUTMITVMA.MIT.EDU> on 10/11/1999
05:23:38 PM

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Subject:  Re: PICs: Good Design More on (ab)use of PIC protection diodes &
     bad design


At 11:15 11/10/99 -0500, you wrote:
{Quote hidden}

could
>be as high as 0.8V maximum (ttl buffer).
>
>Anyone care to differ?
>
>Jim Hartmann
>


YES I DO!
If you look you will see that Microchip just like all other manufacturers
will have a little note that says
"Stresses abaove and beyond the maximum limits may cause dammage".
The maximum limits are that that can be applied to any pin on the device.

I think that you are getting it wrong
"The 16C621 Absolute Maximum Ratings says "Voltage on any pin WRT Vss :0.6V
to VDD+0.6V""
This is it, go anywhere over these limits and BANG!
In your case you are lookingat why and what current can be sustained. Now
you need to do two things to get this one rapped up. In an earlyer mail,
you indicated that 2mA of negative current casued the PIC to go nar nar.
OK, so was this current constant? Did you monitor the current with a CRO to
see if at any time during that application that the current was in excess
of 20mA? Did you monitor the pin votlage and the supply votage to see if
the supply exteneded beyond 6V (Max limit?) and that the pin voltage was
not more than -0.6V? If you performed this and it was within these limits,
then contact Microchip, as it would seem that you have found an error.


Dennis

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