Hi Bob,


----- Original Message -----
From: Bob Ammerman <RAMMERMAN@PRODIGY.NET>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Sunday, August 05, 2001 2:16 PM
Subject: Re: [PIC]: 100 really obscure factoids about PICs


| > | 7. Port pins have a backwards diode to the power supply pin.
Overdriving
| a
| > | port pin can feed energy back to the power supply.  This can be useful
| if
| > | you understand how it works, maddening if you don't.
| > |
| >
| >
| > The clamping diodes are useful when working with large input voltages
| > needing only an external current limiting resistor for this purpose.
| >
|
| This comment is likely to reignite a religious war here on PICLIST. There
| are those that state that you must have more than a simple series
| resistor.
| Others claim that this technique is valid.


:)


| The datasheets give two specifications that seem to apply:
|
| 1: in the absolute maximum ratings section: pin voltage is limited to
| Vss-0.3 to Vdd+0.3


That's for the 'F877. For the 'F84 and other devices, the 'delta' is 0.6 V
and not 0.3 V, so when the diodes come in action, and assuming there is
an external current limitation under the maximum rated [your point (2)],
the voltage in the pin gets clipped to (Vdd + 0.6 V) or (Vss - 0.6 V),
according to the spec.

This point it's not totally clear for me. In some document the Input Clamp
Current is defined as: "The current through the diode to Vss/Vdd IF PIN
VOLTAGE EXCEEDS SPECIFICATION"

Then, under this definition, there's no problem about the input voltage
limitation, and the only issue is the current limitation that can be tackled
with only a series resistor.


| 2: in the dc parameters the maximum current is limited to (for some
| typical chips) 20 ma.


OK.


| The 'series-resistor-is-enough' camp believe that the series resistor will
| handle the current limiting, and the on-chip clamping diodes will limit
| the pin voltage.


My only doubt here is in reliability since, as Wouter has said, these are
"Absolute Maximum Ratings", and the device under these is not running at
normal operating conditions. But assuming the latter definition of Input
Clamp Current is correct, there's no problem if we limit the current
under (and not equal to) the absolute maximum rated.


| The 'something-more-is-needed' group insist that you cannot depend on the
| clamping diodes to do the voltage limiting.
|
| There are several app notes from mChip that seem to endorse the
| 'series-resistor-is-enough' line of thinking.


Yep. So their engineers promote these practices.


| Bob Ammerman
| RAm Systems
| (contract development of high performance, high function, low-level
| software)


Best Regards,

S.-

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