LTC1298 Voltmeter Project

Tom P

I am a hardware person needing help with a A/D voltmeter project.
I tried the sample software for interfacing the STAMP 2 and an LTC1298 shown below.
The software example displays the counts from 0 to 4098, rather than voltage or millivolts.
I suck at math -- what do I need to add to the program to display millivolts in the debug window?
HELP -- HELP
*************************************************************************************8
'LTC1298 & STAMP2
CS con 0 ' Chip select; 0 = active
CLK con 1 ' Clock to ADC; out on rising, in on falling edge.
DIO_n con 2 ' Data I/O pin _number_.
config var nib ' Configuration bits for ADC.
AD var word ' Variable to hold 12-bit AD result.
startB var config.bit0 ' Start bit for comm with ADC.
sglDif var config.bit1 ' Single-ended or differential mode.
oddSign var config.bit2 ' Channel selection.
msbf var config.bit3 ' Output 0s after data xfer complete.
high CS ' Deactivate ADC to begin.
high DIO_n ' Set data pin for first start bit.
again: ' Main loop.
for oddSign = 0 to 1 ' Toggle between input channels.
gosub convert ' Get data from ADC.
debug "channel ",DEC oddSign, ": ",DEC AD,cr ' Display data.
pause 500 ' Wait a half second.
next ' Change channels.
goto again ' Endless loop.
convert:
config = config | %1011 ' Set all bits except oddSign.
low CS ' Activate the ADC.
shiftout DIO_n,CLK,lsbfirst,[noparse][[/noparse]config\4] ' Send config bits.
shiftin DIO_n,CLK,msbpost,[noparse][[/noparse]AD\12] ' Get data bits.
high CS ' Deactivate the ADC.
return

PJAllen

What's your "Full Scale" voltage?·

If it's 2V, say, then you're dealing with 2V / 4098 = 0.49mV / step (count.)· So, if it bounces back "1235", then it's 1235 X 0.49 mV = 605 mV.

Tom P

The maximum voltage would be 5 volts input.
So how do I convert the calculation you listed into actual code
Where do I embed the calculations you listed into the actual BS2 program that I listed, so it would display millivolts via debug
I am using a BASIC STAMP II with the LTC1298
thanks

PJAllen

Here's a good read -- http://www.emesystems.com/BS2math1.htm#cookbook

Look for the subject: Cookbook example, how to compute Y=X*1.2207

Here, I'm sure you're asking, "What's the relevance of that, PJ?"·

5V / 4096 = 1.2207 mV (!!)

Tom P

Aha -- I am beginnning to see the light but its a long tunnel!!
Ok, final question here : where in the program (above) can I place the calculation?

thanks
Tom

Mike Green

You put the calculation anywhere you want where the inputs to the calculation are known and you need the result. That statement may seem simplistic, but it's a simple idea. The calculation isn't just a formula that specifies the relationship among several variables. It's a set of steps for doing a calculation and it's applied at a particular time (when it is executed by the Stamp).

Tracy Allen

Tom, to be very specific about it, the most logical places to put that calculations would be either inside the convert subroutine, just before the RETURN, or, in the main routine, just after the GOSUB convert.

AD = AD ** 14464 + AD ' convert counts to millivolts

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Tracy Allen
www.emesystems.com