PIC instruction: SUBLW

[eblc1388]

Hi,

On PIC programming.

Can anyone gives an explanation of what is actually meant by "to reverse a sequence of numbers" from the following:

Using the PIC Subtract Instructions

In general, any time it is necessary to reverse
a sequence of numbers from A to B (where A and B
are both in the range of 0 to 255), you can use

SUBLW	A+B

which will work even if the sum A+B is greater than 255.

Thanks

 

[Pommie]

If you have two numbers that you want to switch between, such as 15 and 47 then a simple way to do it is to calculate 62 (47+15) - N.

EG

62 - 47 = 15
62 - 15 = 47

It's a quirk of how the SUBLW works that makes this possible. The instruction subtracts W from a literal and so in the above example, W will flip between 47 and 15 every time the instruction SUBLW 62 is executed.

I don't think I explained that very well.

Mike.

 

[eblc1388]

Thanks, Pommie.

Just wonder as mentioned in the above code, whether the compiler would complaint if the value of the literal is larger than 255, says 199 & 201:

SUBLW 400 ;A+B larger than 255

 

[Pommie]

You would just use the 8 bit value, the Pic will treat it as 9 bit any way - the 9th bit being the carry.

To switch between 200 and 100 do

SUBLW 44

Because

44-200 = - 156 = 100

and

44-100 = -56 = 200

HTH

Mike.

 

[eblc1388]

Got it.

If sum > 256, subtract 256 from it.

 

[blues.sagar@gmail.com]

Query

Hi Pommie.

can you plz tell me

MOVF Valid_Speed,W
SUBLW Speed_Warning - Speed_Warn_Hysteresis

what excatly this piece of code will do ??

 

[Pommie]

It will do,

W=(Speed_Warning - Speed_Warn_Hysteresis) - Valid_Speed

Mike.