how would you implement(read) a 256 byte table?

future · Joined: 14 May 2004 Posts: 330

I am using byte const kpa[256] to convert from a/d reading to pressure...

Looking at the simulator log I saw a lot of code just to read this table.

Does someone have a better/faster way?

Maybe a #rom int8.. I did a search but the results that appears to have something useful are from the old forum... unreachable.

Neutone · Joined: 08 Sep 2003 Posts: 839 Location: Houston

future · Joined: 14 May 2004 Posts: 330

Neutone, thank you for the reply.

I use the same way to do the conversion, if you look at the generated code for this table lookup you will see that it is big, and this is what I am complaining.

I am looking for a way to optimize it using tables, program reads, pointers or other method that I dont know yet.

Neutone · Joined: 08 Sep 2003 Posts: 839 Location: Houston

The line that is commented out is a table lookup. The other line uses RAM and runs much faster. I don't think there is a faster method than a table stored in RAM.

Ttelmah · Guest

SherpaDoug · Joined: 07 Sep 2003 Posts: 1640 Location: Cape Cod Mass USA

Of course you can do it smaller with a polynomial, but it will be slower and less accurate (unless you used a polynomial to generate the table).

What is the application? I have done several pressure measurement systems, mostly high accuracy.
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future · Joined: 14 May 2004 Posts: 330

I'm doing an automotive application, I use it to convert the manifold absolute pressure (MAP) to kilopascals.

The map sensors outputs almost 0~5volts for 0~100kpa

SherpaDoug · Joined: 07 Sep 2003 Posts: 1640 Location: Cape Cod Mass USA

It sounds like you could get away with a simple 2 term formula, y=mx+b.
With any decent sensor this will give you better than 1% accuracy with only one multiplication and one addition in fixed point math.
Put your data points into a spreadsheet and find the best fit slope and offset numbers. Those become m and b in the formula.
If you need accuracy better than the linearity of your sensor and A/D you may have to add another term, y=ax^2+mx+b.
_________________
The search for better is endless. Instead simply find very good and get the job done.