Subject:
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Re: digital and analog port on HD
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Newsgroups:
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lugnet.robotics.handyboard
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Date:
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Tue, 19 Jan 1999 13:02:09 GMT
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Original-From:
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FTHOMPSON9@spamlessAOL.COM
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Viewed:
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859 times
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I remember the Basic Stamp "POT" command which is used to read the
position of a pot using a digital port. The Basic Stamps are made from PIC
microcontrollers and the digital ports are bidirectional. That is they may be
used as digital inputs or digital outputs. As outputs there are CMOS FETs
pulling the pin to + supply or Ground. Source or singk is around 20ma. To
work a POT instruction, you have a POT as a variable resistor in series
between the I/O Pin and a capacitor. The other end of the capacitor is
connected to ground. To execute a POT command you would place a logic one on
the port and set the data direction bit for that pin to 0 (output). You leave
it there for a reletively long to insure that the capacitor is fully charged.
Then you change the data direction register to input and switch the I/O Port
bit value to 0. You then enter a loop where you switch the data direction bit
to 0 (output) for a microsecond or two, and check the value of the input port
(0 or 1) each time through the loop you increment (or decrement) a counter.
When the port reads 0, you exit the loop and adjust the counter value through
some correction factors gained through a calibration procedure. As was
mentioned in a previous post, this process is slow, but it will allow some
digital ports to do some analog work.
With the 68HC11 you have 8 analog ports, and going through all this might
not be the best approach. You can get "analog switches" (like the CD4066)
which can multiplex the analog ports if you don't have enough analog channels.
Hope this is interesting
Pherd
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