Negative back to positive angle

Clint · 2008-07-01 07:34

I am reading an incremental encoder to measure angle. Based on the encoder, I will INC or DEC a word I call "angle" representing the angle in tenths of degrees (3600 = 360 degrees). What I would like to happen is for the angle to switch from -999·to 2600 when decreasing and from 2600 to -999 when increasing.

I thought I could do this, but it only seems to work for increasing. When decreasing, the number just keeps decreasing. In this example, the value of angle remains -1000

angle VAR Word
 
angle=-1000
 
 IF angle=2601 THEN
  angle=-999
 ELSEIF angle=-1000 THEN
  angle=2600
 ENDIF

Am I doing something wrong?

Thanks for any help

Post Edited (Clint) : 7/1/2008 7:44:08 AM GMT

Sens-a-Dat · 2008-07-01 11:32

Clint,

I do not have the answer for you as to why this is not working, but I have learned in using SX SIM on your code is that the compiler is taking your statement of "ELSEIF angle=-1000 THEN" and making it "ELSEIF angle=1000 THEN". Why the minus value is being made a positive value is something others will need to explain. I have checked the help file and see no numeric limits for the IF..THEN..ELSEIF..ENDIF command.

The VIEW LIST of the code for "angle=-1000" is:
182 0067 0C18 MOV angle_LSB,#(-1000 & 255) ; angle=-1000
0068 002D
183 0069 0CFC MOV angle_MSB,#(-1000 >> 8)
006A 002E

The VIEW LIST for the code of "ELSEIF angle=-1000 THEN" is:
196 007B 0CE8 CJNE angle_LSB,#1000 & 255,@__ELSE_2
007C 008D 0743 0010 0A8B
197 0080 0C03 CJNE angle_MSB,#1000 >> 8,@__ELSE_2
0081 008E 0743 0010 0A8B

I know someone will be able to help us better understand this.

Gary

Sens-a-Dat · 2008-07-01 12:17

Clint,

As a workaround, define a CON such as TestLow, giving it a value of -1000. Then use that CON name in the IF..ENDIF area. This makes the program function as desired now, but does not explain my earlier observation.

TestLow CON -1000

angle VAR Word

START: 
angle = -1000

MAIN: 
 IF angle = 2601 THEN
     angle = -999
 ELSEIF angle = TestLow THEN
     angle = 2600
 ENDIF

Gary

Clint · 2008-07-02 01:20

Gary - Thank you very much for giving this your attention. I will give your workaround a try [noparse]:D[/noparse] For a second there, I thought I'd lost my marbles!

Clint · 2008-07-09 03:22

Well that worked okay, but then I realized I need a > or < to take care of situations where the angle skips over the limit (say I go from 2600 to 2610). Now it doesn't work again.

Here's what I'm trying

upperlimit CON 2600
lowerlimit CON -999
angle VAR Word  'angle

 IF absangle>upperlimit THEN
  angle=angle-3600
 ELSEIF angle<lowerlimit THEN
  angle=angle+3600
 ENDIF

Am I doing something wrong?

Bean · 2008-07-09 10:54

Clint,
· WORD variables can only hold positive values. When you do a comparison to a negative value, the compiler takes the "two's complient" of the negative value, and uses that. This works fine for "=" and "<>", but it won't work for ">" or "<".

WORD value      2's Compliement value
-------------------------------------
32766           32766
32767           32767
32768          -32768
32769          -32767
32770          -32766
...            ...
65533          -3
65534          -2
65535          -1

· So when you say "IF var < -999" you are really saying "IF var < 64538" which isn't what you mean.

· One solution is to simply offset the value before doing the comparisons so everything is positive.

upperlimit CON 3600 + 2600
lowerlimit CON 3600 - 999
angle VAR Word  'angle



 absangle = absangle + 3600 ' Make absangle postive
 IF absangle > upperlimit THEN
  angle = angle - 3600
 ELSEIF angle < lowerlimit THEN
  angle = angle + 3600
 ENDIF
 absangle = absangle - 3600 ' Put value back to original (IF NEEDED)

Bean.

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Post Edited (Bean (Hitt Consulting)) : 7/9/2008 11:04:42 AM GMT

Negative back to positive angle

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