IF condition w/ math

AGCB

I'm working on a thermostat program and would like to turn on the heat at 1/2 degree below the set point. When I try to add the math (-1/2)to this condition, the heat does not come on when inside temp is below set point

if  IS_Temp < (setTemp-1/2) AND (heatflag==0)  '1/2° below set temp and not already on
      outa[16]~~   'turn on heat relay                           
      heatflag:=1  'set flag

I'd like not to have to use floating point math.

Is my IF condition written correctly?

Sometimes a seemingly simple thing racks my brain.

Thanks
Aaron

Dr_Acula

Just a guess here, but assuming that variables setTemp and IS_Temp are integers, then subtracting half may result in the same number. The half will get rounded to zero and then you subtract zero so setTemp is the same.
If you want to avoid floating point, one trick is to multiply everything by ten. So 70 degrees (F) might be 700. Then half will become 5 and you can do all the maths as integers.

AGCB

Thanks
I'll give that a try

JonnyMac

I'm with Mike: if you're wanting to deal with fractional values in an integer world then you must express all values in the fractional domain. Your code becomes

if (istemp < (settemp - 0_5))
    outa[HT_RELAY] := 1
    heatflag := 1

One of the things I like about the Spin compiler is that it allows the underscore character in numbers; I use this in place of a decimal point in situations like this.

AGCB

Regardless how far the IS_temp is below the setTemp, the heat never comes on when that math is a part of the condition. Only if I remove the math does it come on. I have tried several ways of writing it all to no avail.

Dave Hein

Because of operator precedence "setTemp-1/2" is the same as "setTemp-(1/2)". The (1/2) will be computed as zero. You could do "IS_TEMP*2 < setTemp*2-1", but logically you could just use "IS_TEMP < setTemp" and get the same result.

JonnyMac

Make sure that you've multiplied istemp and settemp by 10 to be in your fractional domain.

AGCB

This works

if  (IS_Temp < setTemp) AND (heatflag==0)

this doesn't

if  (IS_Temp < (setTemp - 0_5)) AND (heatflag==0)

or using decimal point

AGCB

OK let me decipher some of these posts. Time out for me.

Thanks for the replies and I'll be back

tonyp12

>if (IS_Temp < (setTemp - 0_5))

ALL the numbers have to be 10x
if (IS_Temp*10 < (setTemp*10 - 0_5))

But best is to just read/store/write all temperature vars as 10x to start with.
You know that there is a xx.x decimal point in there, but if you don't tell math operation it will be our secret.
If ever needed to display the value you would force a char(".") in before that last digit.

AGCB

Great! It works. It took Tonyp12's simple example for my dumb old noggin to see it, even though others said the same.

Thanks for all the replies. You guys are great!

Aaron

Duane Degn

tonyp12 wrote: »

If ever needed to display the value you would force a char(".") in before that last digit.

I've posted a "DecPoint" method to the forum several times. It will insert the decimal point for you.


There are several other methods floating around which do the same thing.