Testing for transition 255 to 0 - Electronic Circuits Projects Diagrams Free

atferrari · 28th January 2007, 11:45 AM

Micro: 18F Microchip family.

Given a common register, to detect the transition from 0 to 255 (DECF), what can I test? Is it any trick other than looking for the value 255 in the byte?

**Nigel Goodwin** · 28th January 2007, 12:06 PM

Try looking at the flags register!.

atferrari · 28th January 2007, 01:04 PM

Going from 255 to 0 sets C but the opposite has not a single flag set, only for the case. At least, simulation shows that.

Pommie · 28th January 2007, 01:23 PM

Doesn't it set the overflow flag?

Mike.

picasm · 28th January 2007, 01:56 PM

The DECF instruction on a reg with a value of zero to 255 sets the Negative flag (Status reg bit 4)

Mike, K8LH · 28th January 2007, 02:37 PM

Wouldn't it also clear the Zero flag bit? Yep, the DECF instructions affects a whole bunch of STATUS bits. Check out the instruction set summary.

Mike

**Nigel Goodwin** · 28th January 2007, 02:41 PM

Quote:

Originally Posted by Mike, K8LH

Wouldn't it also clear the Zero flag bit?

Yes, but you would have to check that it was set BEFORE you did the DECF, because it would be clear in all cases except one.

Mike, K8LH · 28th January 2007, 02:46 PM

I wonder if the OP is trying to avoid decrementing a register if its value is 0? If that's the case, most of us recommend testing for that condition before performing the decrement operation. Isn't that right guys?

Mike

**Nigel Goodwin** · 28th January 2007, 02:53 PM

Quote:

Originally Posted by Mike, K8LH

I wonder if the OP is trying to avoid decrementing a register if its value is 0? If that's the case, most of us recommend testing for that condition before performing the decrement operation. Isn't that right guys?

If that was the case, then yes!.

picasm · 28th January 2007, 02:57 PM

Quote:

Originally Posted by Mike, K8LH

I wonder if the OP is trying to avoid decrementing a register if its value is 0? If that's the case, most of us recommend testing for that condition before performing the decrement operation. Isn't that right guys?

Mike

Yes, For that situation I have used:
tstfsz reg ; skip if the reg has already reached zero
decf reg,f

If he wants to test if a reg has reached 255, then he could use:
DECF reg,f
BN destination ; Branch if negative

Pommie · 28th January 2007, 03:42 PM

How about
decf file,F
incfsnz file,W ; check if file was zero before the decrement

goto WasZero

Or OV=1 and N=1 gives the same thing.

Checking Z=0 will give all numbers except zero.
Checking N=1 will give all numbers from 0x80 to 0xff.
Checking just OV will give 255 and 127.

Someone with more knowledge about 18 code, does the Carry flag act as a borrow and is therefore reset only if there is a transition from 0 to 255? This would seem likely to me.

Mike.

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28th January 2007, 11:45 AM	(permalink)
atferrari	Testing for transition 255 to 0 Micro: 18F Microchip family. Given a common register, to detect the transition from 0 to 255 (DECF), what can I test? Is it any trick other than looking for the value 255 in the byte? __________________ Agustín Tomás In theory, there is no difference between theory and practice. In practice, however, there is.

28th January 2007, 12:06 PM	(permalink)
Nigel Goodwin	Try looking at the flags register!. __________________ PIC programmer software, and PIC Tutorials at: http://www.winpicprog.co.uk

28th January 2007, 01:04 PM	(permalink)
atferrari	Going from 255 to 0 sets C but the opposite has not a single flag set, only for the case. At least, simulation shows that. __________________ Agustín Tomás In theory, there is no difference between theory and practice. In practice, however, there is.

28th January 2007, 01:23 PM	(permalink)
Pommie	Doesn't it set the overflow flag? Mike.

28th January 2007, 01:56 PM	(permalink)
picasm	The DECF instruction on a reg with a value of zero to 255 sets the Negative flag (Status reg bit 4)

28th January 2007, 02:37 PM	(permalink)
Mike, K8LH	Wouldn't it also clear the Zero flag bit? Yep, the DECF instructions affects a whole bunch of STATUS bits. Check out the instruction set summary. Mike Last edited by Mike, K8LH; 28th January 2007 at 02:44 PM.

28th January 2007, 02:46 PM	(permalink)
Mike, K8LH	I wonder if the OP is trying to avoid decrementing a register if its value is 0? If that's the case, most of us recommend testing for that condition before performing the decrement operation. Isn't that right guys? Mike

28th January 2007, 03:42 PM	(permalink)
Pommie	How about decf file,F incfsnz file,W ; check if file was zero before the decrement goto WasZero Or OV=1 and N=1 gives the same thing. Checking Z=0 will give all numbers except zero. Checking N=1 will give all numbers from 0x80 to 0xff. Checking just OV will give 255 and 127. Someone with more knowledge about 18 code, does the Carry flag act as a borrow and is therefore reset only if there is a transition from 0 to 255? This would seem likely to me. Mike.