Subject:
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Re: blowing analog inputs [was: Battery Level Indicator]
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Newsgroups:
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lugnet.robotics.handyboard
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Date:
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Thu, 21 Jan 1999 01:10:28 GMT
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Original-From:
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Fred G. Martin <fredm@media.mit.edu/nomorespam/>
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Viewed:
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1019 times
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> When I first got my HC11EVB I ran 12V straight into the E port (one of
> the pins), and was sure I ruined the chip. I called the tech support
> person at motorola, and he said 12V on a A/D port should be no
> problem. Sure enough it all still worked.
this isn't right, so i feel the need to correct you. the hc11's
analog inputs are sensitive; i have seen many of them destroyed (on a
pin-by-pin basis), and voltages above 5v or below 0v are the cause).
more accurately, it's not the voltage that does the damage, it's the
current, but let's take it one step at a time.
here is the relevant section from the Pink Book, section 12.3:
"Since there are no P-channel devices directly connected to the A/D
input or reference pins, voltages above V DD do not pose a latchup
threat. If an A/D input rises above the threshold of the protection
device, an input protection device avalanches, and current into this
device should be limited."
Your tech was probably thinking only of the latchup problem. the
hc11's analog ins won't latch up with overvoltage, but that doesn't
mean they won't be destroyed.
the key phrase is "current into this device should be limited." the
over voltage diodes protect against voltages higher than 5v, but these
only work if the over-voltage is current-limited---e.g., in the case
where you have a 5 to 10K resistor in series between the out-of-spec
high voltage and the hc11 analog in.
also, a guaranteed way to blow the pins is to put in a voltage less
than zero volts:
"Because of an inherent diode to V SS , A/D
inputs must not go below V SS , or the input can be permanently
damaged. A series resistor of 1 kW will prevent damage ..."
Here's more if you're interested. the bottom line is: don't let more
than 25 mA get into the analog input. Use a 1K to 5K resistor in
series with your "dangerous voltage source" (that is: the signal
you're trying to measure) if there's *ANY* chance that voltage will go
over 5v or under 0v.
Fred
more detail from section 12.3:
"The minimum-desirable source impedance for an analog input signal
should ensure the current at the A/D pin never gets high enough to
cause CMOS latchup. The HC-MOS process used on the MC68HC11A8 is much
more robust than older 14xxx-style CMOS, but the current at a pin
should still be limited to 25 mA or less. Although the pins can
withstand much more, 25 mA is considered a good design target. The
source impedance that meets this limitation will depend on the total
system. For example, suppose the worst-case scenario for a particular
system results in an analog source accidentally shorting to P12 V. The
inherent internal diodes to V SS will clamp the volt-age at the pin to
about -0.7 V. This clamped voltage means a maximum current of 25 mA
must cause 11.3 V to be dropped across a series-limiting resistance,
which calcu-lates to 452 ohm. However, some guard band should be allowed
for tolerances on the clamped voltage, the source voltages, the
resistor, etc."
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Message is in Reply To:
 | | Re: Battery Level Indicator
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| (...) When I first got my HC11EVB I ran 12V straight into the E port (one of the pins), and was sure I ruined the chip. I called the tech support person at motorola, and he said 12V on a A/D port should be no problem. Sure enough it all still (...) (26 years ago, 20-Jan-99, to lugnet.robotics.handyboard)
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