negative value for a variable

Hello
I am in the process od sending datas to a serial PLL (MB88006)
the register A should have a value between 0 and 63 ..
as soon as this variable is out this range I must Increment or decrement another register (N)
question :
assuming variable A value is 0
if I have the order
A= A-1
and after
IF A < 0 then A = 63 : N = N - 1
(for me A as the value -1 (minus 1) and the IF is working !)

it seems NOT ...

what is wrong on my side ?
thanks
Francois

instead of IF A < 0 then A = 63 : N = N - 1 that will not work, you should use:


IF A.7 = 1 Then A = 63

When A = 0 and you sutract 1 (A = A -1), A will overflow to 255. Since your count is limited to 63, you will never use bit 7 that can be used a flag.

Al.

Hi,
BYTES and WORDS are unsigned in PBP. So if A is a BYTE sized variable and has the value of the 0 then A=A-1 will result in A being 255 which isn't less than 0.....

One way is to simply check the highest bit of A.

Code:

---

If A.7 THEN A=63

---

That will tell you if A is < 0 (or >128) but since the maximum value for A is 63 it should work.

HTH
/Henrik.

EDIT: A little late there but damn, that's two almost identical responses, almost scary....

hello
You are GREAT, and I was STUPID (I do not remember the sign or non signed variables value)
IT WORKS, I use the Bit solution IF A.7 = 1 then xxxxx

by the way , I am not very familar with the forum ..
Last week I raised this question, and as far as I know, I didn't see any feedback
here is my post

Thanks for your message, but what do you mean by " Extended Instruction " ?

bye the way, I have a problem, not the same but ...

I try to use also the SHIFTOUT command to send data to a PLL (via Clock and Data pins)
I use the mode (1 or 5) but the clock is always at HIGH level and there is just a DOWN pulse
From the help explanation (clock idles high or low) my understanding was
clock idles low .... clock signal is at LOW level and a positive pulse occur
clock idles High ... clock signal is at HIGH level and a négative pulse occur
as I said I try mode 1 or 5, and I see NO difference on the oscilloscope !

any idea ?

thanks
Francois



Originally Posted by awmt102
UPDATE:

PROBLEM SOLVED!

In case anyone else has the problem I did - i.e. SHIFTOUT not generating any signals, here is how I solved it:

I should have followed the three standard debugging rules when it comes to odd behaviour in PICs:

1. Check the MCLR pin isn't held low (it wasn't)
2. Check there is a clock signal (there was)
3. Check the CONFIG bits - HERES THE PROBLEM!

I went through all of the CONFIG options and discovered that with Extended Instruction Set enabled SHIFTOUT did not work, but as soon as it was disabled everything worked as expected!

I guess the simulation doesn't differentiate between extended and non-extended and just works in either case.

Hope this helps anyone who has the same problem.