Subject:
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Re: digital ins
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Newsgroups:
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lugnet.robotics.handyboard
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Date:
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Tue, 26 Aug 1997 22:47:08 GMT
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Original-From:
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root <root@snotnose.wizard.orgSPAMCAKE>
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Viewed:
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1220 times
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> > > I want to feed an amplified signal to the digital ins on my HB to
> > > exaggerate > > the response of a photosensor [...] signal
> > > will be near 10V. Will I fry my HB doing this?
> >
> > Yes.
>
> ...
>
> Simply hook a large resistor (like around 100K) in series with your signal.
> Then, if your signal goes to 10v, the HC input truncates it to 5v, and
> around (10v-5v)/100K = .05ma flows through the HC input protection
> circuitry. The main problem with this approach is that you must cut the
> trace to the 47k pullup resistor on the handyboard, since the 100K in
> series will make the "digital" signal too wimpy to overpower the 47k. If
> you read the specs of the 6811 closely and find out it can sink more
> current, perhaps you could reduce the resistor to 10k or so (the protection
> circuitry sinks .5ma now) and mostly forget about the 47k influencing
> things (will just change the transition voltage a bit).
>
> Does someone out there remember the current specs for the input protection
> circuitry on the 6811?
>
> -- Randy
From the pink book (rev 3): pp. 2-24,25
"2.4.3 Internal Circuitry - Digital Input-Only Pin
...When the pin current is increased to very high levels (typically more than
100mA, specified limit is 25mA), physical damage can result. [...]
As voltage is driven above Vdd, the protection device will begin to conduct
and tend to clamp the input voltage to protect [the] input buffer. The voltage
at which this condition will occur varies significantly from lot to lot and
over the operating temperature range. At room temperature, the pin typically
does not draw any current until approximately 20V; at 125-C, the pin may start
conducting at a slightly lower level. Up to this point, the pin appears to
function normally and will return a logic one if read. As the pin voltage
increases, the thick-film protection device begins to conduct more current to
the die substrate, which is Vss. There should be some external series
impedance between the pin and the input voltage source if the MCU will be used
in a detrimental environment. If the input voltage is increased even further,
the protection device will avalanche, and the pin voltage will eventually fold
back (typically to about 7 to 12 V). Under these conditions, a parasitic
bipolar transistor... is turned on and is holding the pin at the 7 V level.
This avalanche is still normally not destructive to the pin. Since the foldback
clamp level is relatively low impedance, the pin voltage cannot be raised
further without supplying a large current. If the offending voltage source
is increased to increase the pin current, the pin circuitry will be damaged
(specified limit is 25mA, typically takes more than 100mA). Gate oxides in
these inputs are not intended to be exposed to voltages above 7 V for any
significant amount of time. With the HCMOS processing used in the MC68HC11A8,
a latchup failure is unlikely unless legal drive limits are grossly exceeded."
The writeup is similar (same current specs) for the Digital I/O pin.
John
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