Re: reading analog signals on a MCU with only digital I/O

Thanks, that's a good start.  I see there's also an ADC0834, which is
still 8-bit but offers 4 channels, for $9.  That's not too bad.


Sounds like the voltage-to-frequency converter idea I had shortly after
my first post.  I'm currently reading over the data sheet for the LM231,
which is under $5.  The thing I like about this approach is that it
should be dead-simple to use from the ARMexpress, which has a PULSIN
function for measuring an input pulse width.  That should make getting a
value from it a one-liner.

(The same technique would work directly with the TSL230 Light to
Frequency converter too, if one had need for such a thing.)

But I haven't yet worked through whether this would actually be faster
than a serial interface.


Hey thanks, that's very neat!  It's just at my level.  :)

So the BASIC Stamp doesn't have analog inputs either?  That surprises
me.  But if so, it puts the ARMexpress in good company, and means I
should find lots of other people who have already solved this problem.

Thanks,
- Joe

Since the ARMexpress is something of a BASIC Stamp "compatible" device
you may want to read up on the documentation for the chip before
spending much more time on this. I think you'll find that like the BASIC
Stamp it supports an RCTIME statement that does exactly what the BASIC
Stamp does with the resistor/capacitor (hence "RC") measurement.

There's not a lot of accuracy with this approach, but it's not bad for
many tasks. For most anything else there are plenty of ADC chips out
there. This is a well-travelled road.

-- Gordon

Seemed like it must be.  I did indeed see RCTIME in the documentation,
but it didn't sound relevant.  It says "Measure the time which pin
remains at *level*, returning the value to variable."  It sounds the
same as PULSIN, except that it applies when the pin is already at the
specified level (low or high), whereas PULSIN waits for the next
transition to that level before starting its measurement.

But both RCTIME and PULSIN seem to measure the time at which a digital
input remains in a given state.  How does that help me with analog
sensors?

An attempt to answer this very question myself (adding RCTIME to my
search terms) did turn up these, which look helpful:

  <http://www.emesystems.com/BS2index.htm>
  <http://www.emesystems.com/BS2rct.htm>

I think I see now -- this doesn't have its own resistor/capacitor
circuit inside the chip; instead you set up your own R/C circuit in such
a way that you can pull the pin low (discharging the capacitor?), and
then let it charge up again in a way that the charging time depends on
the voltage or resistance you're trying to measure.  The RCTIME function
will then return how long this took to cross a threshold indicating a
digital "high" state.  Clever!

I'll have to play with this a bit, but it sounds like a promising
approach when minimizing the component count is desired.  Thanks again!

Best,
- Joe