> We're getting one of the A/D channels run out to a mux on our
> daughterboard, which will make life simpler (as soon as I figure out
> how to read a muxed A/D signal.)
>
> Anyone know how you go about reading a muxed signal? I know in
> principle how a mux works but I've never tried to read one.
Just to clarify; you have multiple analog signals routed
to an analog mux, and then to an ADC channel on the processor.
In principle, you'd write to the GPIO controlling the mux
select line, wait for a board-dependent settling, and ADC
setting time, and then sample the channel.
Assuming the analog mux just looks like a resistor, then
your waiting time will depend on the EP9302 ADC impedance,
and the driving circuit impedance. If you're driving the
mux inputs with op-amps, then you're ok. If your hardware
designer was lazy and used RC filters, then you might have
some 'scope work :)
Setup the software to cycle through two muxed inputs, put
one input at close to max voltage, the other at close to
min, then cycle between the channels. If you get weird
readings, then you can look with a scope, and you'll
probably see little glitches on the input as the ADC
sample-and-hold capacitor is charging between the levels
(assuming thats how the EP9302 ADC works). The solution are
to either fix the driver circuit to include a low output
impedance op-amp (a follower configuration is fine), or
increase the delay between selecting the mux channel, and
reading the ADC (which may or may not work depending on
the ADC).
Cheers
Dave
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