'*/' operator

[Darrel Taylor]

*/ is the MidWord multiplier.

When you multiply 2 16-bit values together, you'll get up to a 32-bit result (4 bytes). The Midword, is the 2 bytes in the middle.

For instance, let's say you have an A/D value of 600, and you multiply it times 5000, that gives you 3,000,000. And if you express that in Hexadecimal, it looks like ...
002D:C6C0

The MidWord is 2DC6, or 11,718.

That's the same as (600 * 5000) / 256. The /256 is implied, and happens without the extra time required to do a division. So, it can be allot faster than using DIV32.

The rest of the formula is, >> 2, or shiftright 2 places. Which is the same as /4. Since the result has already been divided by 256, /4 makes the total divisor equal to 1024 (256*4). And, the result will be 2,929, which represents 2.929 Volts input to the ADC.

So, the whole formula is really just (600 * 5000) / 1024. But in a way that PBP can handle it with 16-bit numbers.<hr>
OK, so how to go the other direction...

I would like to take 0.01 to 0.250V

You'll need the integer numbers (10 to 250) I'll call it Offset. Then just do the reverse formula.

AD_Cal = (Offset * 1024) / 5000

This one doesn't fit in with the */ operator, so you'll need to use DIV32.

Offset = 250          ; 0.250V - Must be a WORD Var
AD_Cal = Offset * 1024
AD_Cal = DIV32 5000

This gives a result of 51, which can then be added or subtracted from the AD result as necessary.

HTH,

[rhino]

To supplement Darrel's example/description, I'd suggest looking HERE for some good examples.