Branch on Less Than - Electronic Circuits Projects Diagrams Free

MrJammin · 11th June 2007, 03:35 PM

I was looking at the PIC instruction set and could not find the equivalent of a Branch on Less Than or Branch on Greater Than that I have found in other assembly languages. Does this instruction not exist for the PIC microprocessors, if so, how do you recommend I circumvent the problem of not being able to use the BLT instruction.

Pommie · 11th June 2007, 04:03 PM

You use the carry flag instead, but the subtract instructions are a bit weird. Sublw 5 does not subtract 5 from W - it subtracts W from 5.

So, you can do,

Code:

         sublw   5
         btfss    STATUS,C
         goto     W_was_greater_than_5

Mike.

MrJammin · 11th June 2007, 08:37 PM

That's very interesting. I'm a little new to flags, do I have to reset that carry flag if I want to use it in another later similar scenario?

geko · 11th June 2007, 10:48 PM

The subtract instruction will set OR clear the flag depending on the result of the subtract.

Therefore the state of the carry flag doesn't matter before the subtract instruction executes.

kchriste · 12th June 2007, 03:28 AM

Quote:

I was looking at the PIC instruction set and could not find the equivalent of a Branch on Less Than or Branch on Greater Than that I have found in other assembly languages.

The 18F series PICs have these instructions. If you are using the 10,12 or 16F series you'll have to do as noted by Pommie above.

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11th June 2007, 03:35 PM	#1
MrJammin	Branch on Less Than I was looking at the PIC instruction set and could not find the equivalent of a Branch on Less Than or Branch on Greater Than that I have found in other assembly languages. Does this instruction not exist for the PIC microprocessors, if so, how do you recommend I circumvent the problem of not being able to use the BLT instruction.

11th June 2007, 04:03 PM	#2
Pommie	You use the carry flag instead, but the subtract instructions are a bit weird. Sublw 5 does not subtract 5 from W - it subtracts W from 5. So, you can do, Code: sublw 5 btfss STATUS,C goto W_was_greater_than_5 Mike. Last edited by Pommie; 13th June 2007 at 01:02 AM.

11th June 2007, 08:37 PM	#3
MrJammin	That's very interesting. I'm a little new to flags, do I have to reset that carry flag if I want to use it in another later similar scenario?

11th June 2007, 10:48 PM	#4
geko	The subtract instruction will set OR clear the flag depending on the result of the subtract. Therefore the state of the carry flag doesn't matter before the subtract instruction executes. __________________ Pete picprojects.org.uk