Question about bank selection, 16F628A, Nigel's tutorial - Electronic Circuits Projects Diagrams Free

kcn · 12th March 2005, 07:21 AM

I am using a 16F628A to go through Nigel's excellent tutorials;
in particular, I am in his second tutorial.

When I compile his code into hex and program it into the chip,
it works absolutely fine... but I am having trouble understanding
the bank selection.

In Tutorial 2.1, the start of his code is the following

Code:

org	0x0000			;org sets the origin, 0x0000 for the 16F628,
				;this is where the program starts running	
movlw	0x07
movwf	CMCON			;turn comparators off (make it like a 16F84)

bsf 	STATUS,	RP0		;select bank 1
movlw 	b'11110000'		;set PortA 4 inputs, 4 outputs
movwf 	LEDTRIS
bcf	STATUS,	RP0		;select bank 0
clrf	LEDPORT			;set all outputs low

BTW, earlier in the .asm file, Nigel had defined

Code:

LEDPORT	Equ	PORTA			;set constant LEDPORT = 'PORTA'
LEDTRIS	Equ	TRISA			;set constant for TRIS register

When Nigel does the BSF STATUS, RP0, how come he doesn't also do a BCF STATUS, RP1?

I'm confused as to how he knows that he is selecting bank 1... couldn't
you also be selecting bank 3 depending upon the values in the register
STATUS when the chip powers up?

Thank you.

kcn

**Nigel Goodwin** · 12th March 2005, 08:10 AM

Quote:

Originally Posted by kcn

I'm confused as to how he knows that he is selecting bank 1... couldn't
you also be selecting bank 3 depending upon the values in the register
STATUS when the chip powers up?

Because a PIC always powers up in bank 0, if you wanted to you could add the extra lines, it wouldn't do any harm (except for using a little space) - but it's not required.

Mike, K8LH · 12th March 2005, 09:47 AM

kcn,

New and old programmers alike can get into trouble with bank selection... Basically, you need to make sure you're in the correct bank for every instruction that accesses a GPR file register (General Purpose Register or RAM) or SFR file register (Special Function Register)... I've been programming awhile but I still find it helpful to track the current bank at the end of each comment line... It makes it much easier for me to verify correct banking when I'm debugging programs...

Code:

        org     0x0000          ;org sets the origin, 0x0000 for the 16F628,
                                ;this is where the program starts running   
        movlw   0x07            ;                                               |B0
        movwf   CMCON           ;turn comparators off (make it like a 16F84)    |B0

        bsf     STATUS, RP0     ;select bank 1                                  |B1
        movlw   b'11110000'     ;set PortA 4 inputs, 4 outputs                  |B1
        movwf   LEDTRIS         ;                                               |B1
        bcf     STATUS,RP0      ;select bank 0                                  |B0
        clrf    LEDPORT         ;set all outputs low                            |B0

Best wishes... Regards, Mike

ryan_ryan · 12th March 2005, 03:22 PM

as noticed, it is a common practise to clear registers and for this eg ledport too when switching back to bank 0, any reason why so?is this a must or a good prging practise?

**Nigel Goodwin** · 13th March 2005, 06:23 AM

Quote:

Originally Posted by ryan_ryan

as noticed, it is a common practise to clear registers and for this eg ledport too when switching back to bank 0, any reason why so?is this a must or a good prging practise?

It depends on what you are doing, if you are specifically writing a value to the registers and ports than there's no point in clearing them. What possible advantage would the following have?.

Code:

clrf PortB
movlw 0x01
movwf PortB

However, if you're not specifically writing values to them it's probably a good idea to clear them - but in any case, it's not a good idea to assume they may be clear if you don't specifically do it!.

12th March 2005, 07:21 AM	(permalink)
kcn New Member	Question about bank selection, 16F628A, Nigel's tutorial I am using a 16F628A to go through Nigel's excellent tutorials; in particular, I am in his second tutorial. When I compile his code into hex and program it into the chip, it works absolutely fine... but I am having trouble understanding the bank selection. In Tutorial 2.1, the start of his code is the following Code: org 0x0000 ;org sets the origin, 0x0000 for the 16F628, ;this is where the program starts running movlw 0x07 movwf CMCON ;turn comparators off (make it like a 16F84) bsf STATUS, RP0 ;select bank 1 movlw b'11110000' ;set PortA 4 inputs, 4 outputs movwf LEDTRIS bcf STATUS, RP0 ;select bank 0 clrf LEDPORT ;set all outputs low BTW, earlier in the .asm file, Nigel had defined Code: LEDPORT Equ PORTA ;set constant LEDPORT = 'PORTA' LEDTRIS Equ TRISA ;set constant for TRIS register When Nigel does the BSF STATUS, RP0, how come he doesn't also do a BCF STATUS, RP1? I'm confused as to how he knows that he is selecting bank 1... couldn't you also be selecting bank 3 depending upon the values in the register STATUS when the chip powers up? Thank you. kcn

12th March 2005, 09:47 AM	(permalink)
Mike, K8LH Experienced Member	kcn, New and old programmers alike can get into trouble with bank selection... Basically, you need to make sure you're in the correct bank for every instruction that accesses a GPR file register (General Purpose Register or RAM) or SFR file register (Special Function Register)... I've been programming awhile but I still find it helpful to track the current bank at the end of each comment line... It makes it much easier for me to verify correct banking when I'm debugging programs... Code: org 0x0000 ;org sets the origin, 0x0000 for the 16F628, ;this is where the program starts running movlw 0x07 ; \|B0 movwf CMCON ;turn comparators off (make it like a 16F84) \|B0 bsf STATUS, RP0 ;select bank 1 \|B1 movlw b'11110000' ;set PortA 4 inputs, 4 outputs \|B1 movwf LEDTRIS ; \|B1 bcf STATUS,RP0 ;select bank 0 \|B0 clrf LEDPORT ;set all outputs low \|B0 Best wishes... Regards, Mike

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12th March 2005, 03:22 PM	(permalink)
ryan_ryan New Member	as noticed, it is a common practise to clear registers and for this eg ledport too when switching back to bank 0, any reason why so?is this a must or a good prging practise?