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'PIC16C84--Absolute max voltage on RA4?'
1996\10\09@023600 by John Payson

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> A quick look at my databook shows that RA4 is an open drain output pin.
> This means that the pin must have an external pullup resistor to function
> as an output.  Please try adding a 4700 ohm  resistor between the RA4 pin
> and 5 volts, that should fix it.

On a related note: what is the maximum voltage on RA4 when VDD is at zero
volts?  Is there any "safe" current level [e.g. if I were to connect RA4
to +5 through a 1M resistor while VDD was at zero, would that be safe?
What if it were a 10K resistor?  What if it were a Zerohm(tm) "resistor"?]

1996\10\09@024852 by TONY NIXON 54964

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>On a related note: what is the maximum voltage on RA4 when VDD is at zero
>volts?  Is there any "safe" current level [e.g. if I were to connect RA4
>to +5 through a 1M resistor while VDD was at zero, would that be safe?
>What if it were a 10K resistor?  What if it were a Zerohm(tm) "resistor"?]

The trouble with power being applied to any pin while Vdd is zero, is
that the device can still remain in a powered up state. Current flows
into the IO pin, through the protection diode, and back to the chip's
internal Vdd rail. Therefor a 10K resistor would provide 440uA of
current which is more than enough to power a PIC on LP mode. As the
chip may not power down correctly because of this current flow, it
may not start up reliably as well.

Tony


Just when I thought I knew it all,
I learned that I didn't.

1996\10\09@043449 by Keith Dowsett

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<important stuff snipped>

> What if it were a Zerohm(tm) "resistor"?
>

I think I saw one of those a while ago sitting in a bath of liquid helium.
How do you connect it to the PIC?

Keith.
==========================================================
Keith Dowsett         "Variables won't; constants aren't."

E-mail: spam_OUTkdowsettTakeThisOuTspamrpms.ac.uk
WWW:    http://kd.rpms.ac.uk/index.html

1996\10\09@091927 by Jacob Blichfeldt

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>The trouble with power being applied to any pin while Vdd is zero, is
that the device can still remain in a powered up state. Current flows
into the IO pin, through the protection diode, and back to the chip's
internal Vdd rail. Therefor a 10K resistor would provide 440uA of
current which is more than enough to power a PIC on LP mode. As the
chip may not power down correctly because of this current flow, it
may not start up reliably as well.

Tony


As written, RA4 is an O/C-output and does not have any protection-diodes (only to Gnd). The datasheets 'absolute maximum ratings' states max. 14 V on RA4, also when VCC=GND. A resistor is recommend as currentlimiter if RA4 goes low on power-up.

-Jacob Blichfeldt

1996\10\09@105255 by Martin J. Maney

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On Wed, 9 Oct 1996, Jacob Blichfeldt wrote:

> As written, RA4 is an O/C-output and does not have any protection-diodes =

I think you've overlooked the input buffer.

> (only to Gnd). The datasheets 'absolute maximum ratings' states max. 14 =
> V on RA4, also when VCC=3DGND. A resistor is recommend as currentlimiter =
> if RA4 goes low on power-up.

What edition of the datasheet is that?  This sounds like the description
of the MCLR pin in the 16C84 sheet in hte 95/96 databook (and in every
other sheet I recall looking at).  I think you've been misled.

1996\10\09@113213 by Jacob Blichfeldt

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>> As written, RA4 is an O/C-output and does not have any protection-diodes =

>I think you've overlooked the input buffer.

>> (only to Gnd). The datasheets 'absolute maximum ratings' states max. 14 =
>> V on RA4, also when VCC=GND. A resistor is recommend as currentlimiter =
>> if RA4 goes low on power-up.

>What edition of the datasheet is that?  This sounds like the description
of the MCLR pin in the 16C84 sheet in hte 95/96 databook (and in every
other sheet I recall looking at).  I think you've been misled.


The datasheets is DS30081F page 21: (quote) 'Note: I/O pin has protection diodes
to Vss only.'
I just tried measuring a device, and the datasheet is ofcource correct. Only a
diode to Gnd (Vss) can measured.
One of the advantages of an O/C (or Open Drain) output is that you can switch
between Gnd and a voltage higher than Vcc.

-Jacob Blichfeldt

1996\10\09@113426 by John Payson

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> On Wed, 9 Oct 1996, Jacob Blichfeldt wrote:
>
> > As written, RA4 is an O/C-output and does not have any protection-diodes =
>
> I think you've overlooked the input buffer.

An input buffer is usually a mosfet inverter, with the input tied to the
gates of two mosfets.  As such, the gates have no diode behavior and are
effectively open-circuit for any voltage not high enough to destroy the
insulating oxide layer.

The protection diodes in a chip are normally a result of output drivers: a
sinking N-fet driver will have its substrate (which is P-type) biased to
ground; if the voltage on the P-substrate exceeds that on the N-channel,
current will flow through the P-N junction.

A similar situation exists on many normal device pull-up transistors (there
are some exceptions--it's possible for a device to use N-type transistors
on its outputs which would not have the diodes to VDD, but such a device
would need an internal voltage source of 6 volts or so to drive the gate).

> > (only to Gnd). The datasheets 'absolute maximum ratings' states max. 14 =
> > V on RA4, also when VCC=3DGND. A resistor is recommend as currentlimiter =
> > if RA4 goes low on power-up.
>
> What edition of the datasheet is that?  This sounds like the description
> of the MCLR pin in the 16C84 sheet in hte 95/96 databook (and in every
> other sheet I recall looking at).  I think you've been misled.

I would not be surprised if the behavior of the 16C84 is as described, but
it would be nice to have some useful source that actually SAID so...

1996\10\09@182140 by John Payson

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> The datasheets is DS30081F page 21: (quote) 'Note: I/O pin has protection
diodes
>  to Vss only.'
> I just tried measuring a device, and the datasheet is ofcource correct. Only a
>  diode to Gnd (Vss) can measured.
> One of the advantages of an O/C (or Open Drain) output is that you can switch
>  between Gnd and a voltage higher than Vcc.

The data sheet says the pin does not have the diode, but it says nothing
about the maximum allowable voltage on the pin other than the "any pin"
spec.  For any other "any" pin, the requirement that the pin not exceed
VDD makes sense because of the parasitic diode; for the RA4 pin, however,
the documentation does not say you can exceed VDD without damaging the
device--merely that there's no diode to prevent you from doing so.

I'd be thrilled if I had some confirmation that the pin could be used to
switch 9 or 12 volts; I've not seen such a thing documented, however, and
would not wish to use it in a design without assurance that it was safe
to do so.  Does anyone at Microchip know the scoop?

1996\10\10@040719 by Kalle Pihlajasaari

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Hi Jacob,

> The datasheets is DS30081F page 21: (quote) 'Note: I/O pin has protection
diodes
>  to Vss only.'
> I just tried measuring a device, and the datasheet is ofcource correct. Only a
>  diode to Gnd (Vss) can measured.
> One of the advantages of an O/C (or Open Drain) output is that you can switch
>  between Gnd and a voltage higher than Vcc.

Woah boy.  Just remember that you are not talking about a ULN2003 relay
driver here.  The PIC16C/F84(A) could be inadvertently be put into
Probramming mode if you pull the _MCLR pin too high (especially
when Vcc is low)  If you must have a 6V open collector output
from say a set of batteries and you know that you chip will
reset correctly (open collector reset IC connected to _MCLR then
you can probably get away with it even if you put a low drop regulator
onto the rest of the IC and run it at 5V.

If you want open collector at above supply voltage then put in the
transistor and resistor and feel safe.

Cheers
--
Kalle Pihlajasaari     .....kalleKILLspamspam@spam@data.co.za
Interface Products     Box 15775, Doornfontein, 2028, South Africa
+27 (11) 402-7750      Fax: +27 (11) 402-7751

1996\10\10@064051 by Jacob Blichfeldt

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part 0 745 bytes
>Woah boy.  Just remember that you are not talking about a ULN2003 relay
driver here.  The PIC16C/F84(A) could be inadvertently be put into
Probramming mode if you pull the _MCLR pin too high (especially
when Vcc is low)  If you must have a 6V open collector output
from say a set of batteries and you know that you chip will
reset correctly (open collector reset IC connected to _MCLR then
you can probably get away with it even if you put a low drop regulator
onto the rest of the IC and run it at 5V.

>If you want open collector at above supply voltage then put in the
transistor and resistor and feel safe.

>Cheers
>Kalle Pihlajasaari     kallespamKILLspamdata.co.za

What are you talking about? We are discussing RA4 here, not _MCLR...

1996\10\10@113046 by Martin J. Maney

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On Wed, 9 Oct 1996, Jacob Blichfeldt wrote:

> The datasheets is DS30081F page 21: (quote) 'Note: I/O pin has protection
diodes
>  to Vss only.'
> I just tried measuring a device, and the datasheet is ofcource correct. Only a
>  diode to Gnd (Vss) can measured.

Okay as far as it goes, but there IS an input buffer connected to that
pin as well.  You'd need to measure leakage current with 14V applied to
test this empirically, not just ohm it out.

> One of the advantages of an O/C (or Open Drain) output is that you can switch
>  between Gnd and a voltage higher than Vcc.

Can be, if the output is specified to permit it.  Does the F revision of
the datasheet spec that pin for higher voltage tham most of the rest?
The E version does not - all other [than Vdd or MCLR] pins are spec'd
only to 0.6V above Vdd.  Lacking an (input) protection diode is not
enough to permit reliable operation above Vdd -there were LOTS of OC TTL
gates that would fry if you went much over 5V, and those outputs really
were bare open collectors (aside from parasitic junctions).

1996\10\10@211624 by Eric Smith

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Someone said:
> What if it were a Zerohm(tm) "resistor"?

and Keith Dowsett replied:
> I think I saw one of those a while ago sitting in a bath of liquid helium.
> How do you connect it to the PIC?

I acquired a box of zero ohm resistors from an engineering lab clearance at
my previous job.  I'd seen some before with just a single black band, but
these ones are marked black-black-black-silver.  Unfortunately they seem to
be defective; they don't come anywhere close to meeting the specified
tolerance.

Eric

1996\10\11@014449 by Dave Mullenix

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>Someone said:
>> What if it were a Zerohm(tm) "resistor"?
>
>Someone else replied:
>> I think I saw one of those a while ago sitting in a bath of liquid helium.
>> How do you connect it to the PIC?
>
>A third person raised the stakes
>I acquired a box of zero ohm resistors from an engineering lab clearance at
>my previous job.  I'd seen some before with just a single black band, but
>these ones are marked black-black-black-silver.  Unfortunately they seem to
>be defective; they don't come anywhere close to meeting the specified
>tolerance.

And I chipped in:
We used to use them as current limiting resisters for our Darkness Emitting
Arsenide Diodes.  D.E.A.D. diodes are like LEDs, except that they emit
darkness when you put a voltage across them.

1996\10\11@034147 by Keith Dowsett

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>>Someone said:
>>> What if it were a Zerohm(tm) "resistor"?
>>
>>Someone else replied:
>>> I think I saw one of those a while ago sitting in a bath of liquid helium.
>>> How do you connect it to the PIC?
>>
>>A third person raised the stakes
>>I acquired a box of zero ohm resistors from an engineering lab clearance at
>>my previous job.  I'd seen some before with just a single black band, but
>>these ones are marked black-black-black-silver.  Unfortunately they seem to
>>be defective; they don't come anywhere close to meeting the specified
>>tolerance.
>
>And I chipped in:
>We used to use them as current limiting resisters for our Darkness Emitting
>Arsenide Diodes.  D.E.A.D. diodes are like LEDs, except that they emit
>darkness when you put a voltage across them.
>
>
And the story continued...

I believe you can get great big panels of silicon DED's They even use them
to power the parking meters in germany.

Keith.
==========================================================
Keith Dowsett         "Variables won't; constants aren't."

E-mail: .....kdowsettKILLspamspam.....rpms.ac.uk
WWW:    http://kd.rpms.ac.uk/index.html

1996\10\11@041956 by Kalle Pihlajasaari

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Hi Jacob,

> >> One of the advantages of an O/C (or Open Drain) output is that you can
switch
{Quote hidden}

My mistake, not at my most lucid when I posted the message.

There should be no direct side effects from using RA4 above supply
withing the limits of the spec (which is woefully unclear on this
point).

Cheers
--
Kalle Pihlajasaari     kallespamspam_OUTdata.co.za
Interface Products     Box 15775, Doornfontein, 2028, South Africa
+27 (11) 402-7750      Fax: +27 (11) 402-7751

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