Whats the purpose of this ANDLW? - Electronic Circuits Projects Diagrams Free

Suraj143 · 8th October 2007, 04:22 AM

I have this code. What I want to know is what’s the purpose of this anldw line?

From my view I can say it will never allow to compare D’125’.Bcuz it allows only D’127’.

Help me to solve this problem.

Code:

	movf	COUNT,W		;get the COUNT value
	andlw	b'01111111'	;AND with 7Fh (D’127’)
	xorlw	.125		;b'01111101'
	btfss	STATUS,Z	;compare with zero bit
	goto	Exit
	goto	Main

SPDCHK · 8th October 2007, 04:35 AM

As for the ANDLW, it looks like it's used for BIT stripping. All values from bits 0-6 will be passed through but bit 7 not. (Maybe the original code was used for 7-segment display where bit <7> was not used ??)

blueroomelectronics · 8th October 2007, 04:50 AM

MPASM has a nifty "bz" command, means branch on zero (there is also a bnz) these can make the program slightly easier to read.
so
bz Main
goto Exit

Pommie · 8th October 2007, 06:00 AM

You have to look at the rest of the code in order to work out why you need the andlw 0x7f. Bit 7 is used to keep track of the state of the LED. From your other thread,

Code:

		movlw	.6
		addwf	TMR0,F
		incf	TIME,F
		movf	TIME,W
		andlw	7fh
		xorlw	.125
		btfss	STATUS,Z
		goto	Away
		btfss	TIME,7
		goto	$+4
		clrf	TIME			;Bit 7 = 0
		bcf	LED			;turn OFF led
		goto	Away
		clrf	TIME
		bsf	TIME,7			;Bit 7 = 1
		bsf	LED			;turn ON led

Mike.

Suraj143 · 8th October 2007, 06:28 AM

Thanks for the replies guys.

Now I got it. It has used a family bit of the same Time variable.
To avoid confusion I can remove andlw & use another variable before writing to the LED.

BTW I have another doubt question.

I want to check whether the 7th bit of COUNT variable set.

This will set for sure bcuz it has set only the 7th bit.

Code:

	COUNT = b’10000000’
	Btfss	COUNT, 7

What about this It has some other values in first 5 bits but 7th bit is set.

Code:

COUNT = b’10111111’
	Btfss	COUNT, 7

Will this also be set?

Thanks

**Nigel Goodwin** · 8th October 2007, 09:12 AM

Yes, BTFSS is only checking one specific bit, the rest make no difference.

As for ANDLW, it's the normal logical AND instruction, as used in computers since their first days. With micro-processors (and lesser micro-controllers) your only option form checking (or setting) individual bits is to use combinations of logical AND, OR, XOR instructions.

donniedj · 9th October 2007, 04:37 AM

Quote:

Originally Posted by Suraj143

I have this code. What I want to know is what’s the purpose of this anldw line?

Code:

	movf	COUNT,W		;get the COUNT value
	andlw	b'01111111'	;AND with 7Fh (D’127’)
	xorlw	.125		;b'01111101'
	btfss	STATUS,Z	;compare with zero bit
	goto	Exit
	goto	Main

"Just as 256 is not a true max, 256 is also not a true minimum" (donniedj circa see post time stamp):

As already stated, it and its accomplice is used to compare an unknown count to a known comparator value. But whats important for you to take from this is how you can compare PARTS of a value, in this case bits 6-0, while not caring about the other parts for the moment. A single variable can have multiple segments. Bit 7 of COUNT could be used for some other purpose such as a flag. Imagine a processor where the smallest data was 4 bytes. A resource limited yet smart coder could make use of each PART as their own 4 separate 1 byte variables coexisting inside 1 entity by a parts compare technique as above.

Code:

	
	;--COMPARE SEGMENT OF COUNT TO COMPARATOR--
	MOVF	COUNT, W	;STORING COUNT[6-0] INTO W
	ANDLW	B'01111111'	;
	XORLW	.125		;ASSIGN COMPARATOR = 125
	BTFSS	STATUS, Z	;DOES COUNT[6-0] = COMPARATOR?,
	GOTO	EXIT		;NO, EXIT
	GOTO	MAIN		;YES, GOTO MAIN.
	;-------------------------------

Do yourself a favor, comment on the significance and not on the literate. Stating lines like

Code:

	movf	COUNT,W		;get the COUNT value
	andlw	b'01111111'	;AND with 7Fh (D’127’)
	xorlw	.125		;b'01111101'
	btfss	STATUS,Z	;compare with zero bit

helps as much as an empty comment line.

Suraj143 · 9th October 2007, 04:59 AM

Hi I got cover up all your comments.And donniedj thanks for your excellent comparator idea.worth of thanks for that.

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8th October 2007, 04:22 AM	(permalink)
Suraj143	Whats the purpose of this ANDLW? I have this code. What I want to know is what’s the purpose of this anldw line? From my view I can say it will never allow to compare D’125’.Bcuz it allows only D’127’. Help me to solve this problem. Code: movf COUNT,W ;get the COUNT value andlw b'01111111' ;AND with 7Fh (D’127’) xorlw .125 ;b'01111101' btfss STATUS,Z ;compare with zero bit goto Exit goto Main

8th October 2007, 04:35 AM	(permalink)
SPDCHK	As for the ANDLW, it looks like it's used for BIT stripping. All values from bits 0-6 will be passed through but bit 7 not. (Maybe the original code was used for 7-segment display where bit <7> was not used ??)

8th October 2007, 04:50 AM	(permalink)
blueroomelectronics	MPASM has a nifty "bz" command, means branch on zero (there is also a bnz) these can make the program slightly easier to read. so bz Main goto Exit __________________ Bill Smart Kits build Smart People http://www.blueroomelectronics.com

8th October 2007, 06:00 AM	(permalink)
Pommie	You have to look at the rest of the code in order to work out why you need the andlw 0x7f. Bit 7 is used to keep track of the state of the LED. From your other thread, Code: movlw .6 addwf TMR0,F incf TIME,F movf TIME,W andlw 7fh xorlw .125 btfss STATUS,Z goto Away btfss TIME,7 goto $+4 clrf TIME ;Bit 7 = 0 bcf LED ;turn OFF led goto Away clrf TIME bsf TIME,7 ;Bit 7 = 1 bsf LED ;turn ON led Mike.

8th October 2007, 06:28 AM	(permalink)
Suraj143	Thanks for the replies guys. Now I got it. It has used a family bit of the same Time variable. To avoid confusion I can remove andlw & use another variable before writing to the LED. BTW I have another doubt question. I want to check whether the 7th bit of COUNT variable set. This will set for sure bcuz it has set only the 7th bit. Code: COUNT = b’10000000’ Btfss COUNT, 7 What about this It has some other values in first 5 bits but 7th bit is set. Code: COUNT = b’10111111’ Btfss COUNT, 7 Will this also be set? Thanks

8th October 2007, 09:12 AM	(permalink)
Nigel Goodwin	Yes, BTFSS is only checking one specific bit, the rest make no difference. As for ANDLW, it's the normal logical AND instruction, as used in computers since their first days. With micro-processors (and lesser micro-controllers) your only option form checking (or setting) individual bits is to use combinations of logical AND, OR, XOR instructions. __________________ PIC programmer software, and PIC Tutorials at: http://www.winpicprog.co.uk

9th October 2007, 04:59 AM	(permalink)
Suraj143	Hi I got cover up all your comments.And donniedj thanks for your excellent comparator idea.worth of thanks for that.