Stamp Math Questions

Archiver

I am having a little problem getting may hands around some math used in the
Stamp. I am using a MAX186 to read data from an AMP100 hall effect current
sensor. (http://www.ampsense.com/AMP100.html )

The sensor puts out an offset voltage so that 0 amps is read as 2.5 volts. Full
range for the 100 amps is 1.9V or 19mV per amp. I am using the following to read
the ADC into the Stamp and then outputting it to debug. Not all of the code is
here but you should get the idea.

Shiftin Din, clk, msbpost, [noparse][[/noparse]ADCin\12]
Debug dec ADCin dig 3,"."dec ADCin dig 2, dec ADCin dig 1

When run I get an output in debug that reads 2.52 which is correct. What I want
to do is offset this output so it reads 0. I just haven't figured out how to
properly do math on ADCin. What the main goal is to feed this info to StampPlot
to plot a graph of the current draw. Anyone have any pointers or tutorials on
performing math on binary numbers such as ADCin? Thanks!


Hank

Archiver

Hi Hank,

It sounds like the resolution of your ADC is 1 mV. When you use the
DIG operator, the rightmost digit is DIG 0, not DIG 1.

But I don't think you need to use DIG.

mA var word
mV var word
ADCin var word
mV=ADCin-2500 ' subtract the 2500 millivolt (2.5V) offset
debug dec mV,cr ' display raw reading, mV above 2500
debug dec mV/1000, ".", dec3 mV,cr ' display volts
mA=mV */ 13474 ' convert to milliamps
debug dec mA,cr ' display milliamps
debug dec mA/1000, ".", dec3 mA,cr ' display amps


Now, you might be asking, why the factor */13474 to convert from
millivolts to milliamps? You said the calibration is 19 millivolts
per amp, or that is the same as 19 millivolts per 1000 milliamps.
Divide 1000/19=52.6316. That is the multiplier you would use on a
calculator to get from millivolts to milliamps.
milliamps = 52.6316 * millivolts

To make it work with the stamp, multiply 52.6316 times 256, round
off, and you get 13474.

The stamp does the problem like this: 13474/256. The division by
256 is implied in the */ operator. (Divide it on your calculator and
show yourself that 13474/256 is very close to 52.6316).
milliamps = 13474 */ millivolts

-- Tracy




>I am having a little problem getting may hands around some math used
>in the Stamp. I am using a MAX186 to read data from an AMP100 hall
>effect current sensor. (http://www.ampsense.com/AMP100.html )
>
>The sensor puts out an offset voltage so that 0 amps is read as 2.5
>volts. Full range for the 100 amps is 1.9V or 19mV per amp. I am
>using the following to read the ADC into the Stamp and then
>outputting it to debug. Not all of the code is here but you should
>get the idea.
>
>Shiftin Din, clk, msbpost, [noparse][[/noparse]ADCin\12]
>Debug dec ADCin dig 3,"."dec ADCin dig 2, dec ADCin dig 1
>
>When run I get an output in debug that reads 2.52 which is correct.
>What I want to do is offset this output so it reads 0. I just
>haven't figured out how to properly do math on ADCin. What the main
>goal is to feed this info to StampPlot to plot a graph of the
>current draw. Anyone have any pointers or tutorials on performing
>math on binary numbers such as ADCin? Thanks!
>
>Hank

Archiver

Thanks for the info Tracy. It worked much better then what I was trying to do.
One day I'll fully understand all of these "special" math functions... [noparse]:)[/noparse]

Hank

---

Original Message

---

From: "Tracy Allen" <tracy@e...>
To: <basicstamps@yahoogroups.com>
Sent: Tuesday, February 25, 2003 9:31 PM
Subject: Re: [noparse][[/noparse]basicstamps] Stamp Math Questions


> Hi Hank,
>
> It sounds like the resolution of your ADC is 1 mV. When you use the
> DIG operator, the rightmost digit is DIG 0, not DIG 1.
>
> But I don't think you need to use DIG.
>
> mA var word
> mV var word
> ADCin var word
> mV=ADCin-2500 ' subtract the 2500 millivolt (2.5V) offset
> debug dec mV,cr ' display raw reading, mV above 2500
> debug dec mV/1000, ".", dec3 mV,cr ' display volts
> mA=mV */ 13474 ' convert to milliamps
> debug dec mA,cr ' display milliamps
> debug dec mA/1000, ".", dec3 mA,cr ' display amps
>
>
> Now, you might be asking, why the factor */13474 to convert from
> millivolts to milliamps? You said the calibration is 19 millivolts
> per amp, or that is the same as 19 millivolts per 1000 milliamps.
> Divide 1000/19=52.6316. That is the multiplier you would use on a
> calculator to get from millivolts to milliamps.
> milliamps = 52.6316 * millivolts
>
> To make it work with the stamp, multiply 52.6316 times 256, round
> off, and you get 13474.
>
> The stamp does the problem like this: 13474/256. The division by
> 256 is implied in the */ operator. (Divide it on your calculator and
> show yourself that 13474/256 is very close to 52.6316).
> milliamps = 13474 */ millivolts
>
> -- Tracy
>
>
>
>
> >I am having a little problem getting may hands around some math used
> >in the Stamp. I am using a MAX186 to read data from an AMP100 hall
> >effect current sensor. (http://www.ampsense.com/AMP100.html )
> >
> >The sensor puts out an offset voltage so that 0 amps is read as 2.5
> >volts. Full range for the 100 amps is 1.9V or 19mV per amp. I am
> >using the following to read the ADC into the Stamp and then
> >outputting it to debug. Not all of the code is here but you should
> >get the idea.
> >
> >Shiftin Din, clk, msbpost, [noparse][[/noparse]ADCin\12]
> >Debug dec ADCin dig 3,"."dec ADCin dig 2, dec ADCin dig 1
> >
> >When run I get an output in debug that reads 2.52 which is correct.
> >What I want to do is offset this output so it reads 0. I just
> >haven't figured out how to properly do math on ADCin. What the main
> >goal is to feed this info to StampPlot to plot a graph of the
> >current draw. Anyone have any pointers or tutorials on performing
> >math on binary numbers such as ADCin? Thanks!
> >
> >Hank
>
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