topic Re: Multiplying two eight bit numbers numbers in S12 / MagniV Microcontrollers

Multiplying two eight bit numbers numbers

JohnnyP — Wed, 24 Dec 2008 23:30:41 GMT

I am reading two voltages into the A/D, and I want to multiply them.

For example, 1.5v x 1.5v = 2.25v

2.25 x 255/5 = 115 = $72

1.5 x 255/5 = 77 = $4C

$4C x $4C = $1690 = 5776 = 76 x 76

How to get $72 from $1690?

Thanks

Re: Multiplying two eight bit numbers numbers

JohnnyP — Thu, 25 Dec 2008 06:23:52 GMT

Hooray, I got it. Tom Almay has a section on fractional scaling in his book that gave me the clue.

I'll post my code later. I ignored a couple of remainders, and maybe it could be simplified, but it's pretty close.

For those that want to work it out, convert the A/D reading into thousands of millivolts.

For an input of 1.5v = $4D

($4D x $1388)/$FF = $05D2 = 1490mv

If the other input is 2.75v = $8C

($8C x $1388)/$FF = $0AB9 = 2745mv

Then ($05D2 x $0AB9)/$03E8 = $0FFA

Then ($0FFA x $33)/$03E8 = (tadaa)...$D0...

Note that $33 is $FF/5

Takes a lot of EMUL and EDIV's. Good thing the HC08 I was going to use didn't have enough pins for the job.

Re: Multiplying two eight bit numbers numbers

kef — Thu, 25 Dec 2008 06:30:24 GMT

multiply your product by 5, then divide by 255.

Re: Multiplying two eight bit numbers numbers

JohnnyP — Thu, 25 Dec 2008 06:35:55 GMT

Kef:

Lots of products, which one?

Re: Multiplying two eight bit numbers numbers

kef — Thu, 25 Dec 2008 07:32:48 GMT

I replied before you send your second post. I meant product $4C x $4C = $1690. You asked how to get ~$72 from $1690. Answer is multiply it by 5, then divide by 255.

ADcode of 1.5V, having 5V AD reference is sort of this

1.5V

----- * $FF

Similarily ADcode of 1.5V*1.5V=2.25V, with 5V reference is sort of this

1.5V*1.5V

------------- * $FF

What if we replace 1.5V in this expression with reverse expression

$4C

-----*5V ~ 1.5V

$FF

Full expression becomes

$4C * 5V $4C * 5V 1

----------- * ------------ * ------ * $FF

$FF $FF 5V

$4C * $4C * 5V

--------------------

$FF

Message Edited by kef on 2008-12-25 01:34 AM

Message Edited by kef on 2008-12-25 01:35 AM

Re: Multiplying two eight bit numbers numbers

JohnnyP — Thu, 25 Dec 2008 07:54:38 GMT

Wow, that's great. I only worked on it for eight hours. (arg).