Post by thread:Picn up the Pace Q2

Looking at the code on page 20 for 74hc165.asm I understand every line in it except for at the beginning..... bsf status,rp0 ;switch to bank 1 bcf status,rp0 ;switch BACK to bank 1 Huh?? I understand what the statements do (switch memory banks), I'm just completely lost as to WHY they are even there. Please, can anyone help me?

Hi Mike, Apparently, on the original 16C84, the Option, and TriState ( data direction registers ) registers (in Memory Bank 1) were not directly accessable. The OPTION, and TRISx, instructions had to be used to access these registers. On the 16F84s the second bank of registers are accessable by setting the rp0 bit in the status register. This allows the Option, and Trisx registers to be accessed directly. Thus making the TRIS, and OPTION instructions obsolete. These are still supported by the 'F84 but, presummably can become unsupported in the future. BTW, since you've got PIC'n Up the Pace, Easy PIC'n covers these subtleties very nicely, in the same format. Regards, Heinz At 11:31 AM 10/26/98, Mike Ringenberger wrote: {Quote hidden}>Looking at the code on page 20 for 74hc165.asm I understand every line >in it except for at the beginning..... > >bsf status,rp0 ;switch to bank 1 >bcf status,rp0 ;switch BACK to bank 1 > >Huh?? > >I understand what the statements do (switch memory banks), I'm just >completely lost as to WHY they are even there. > >Please, can anyone help me?

Heinz Czychun wrote: > On the 16F84s the second bank of registers are accessable by setting > the rp0 bit in the status register. This allows the Option, and > Trisx registers to be accessed directly. Thus making the TRIS, and > OPTION instructions obsolete. Hey! Whoa there! I wouldn't say that's true *at all*!!! Having TRIS and OPTION registers accessible as such is only useful in terms of performing reads on them, and by implication, Read-Modify-Write or so- called "bit" set/ clear functions which are by common assent, just a bit dicky anyway. As I see it, the combination "movlw value; tris portb" is an awful lot better than "movlw value; bsf status, rp0; movwf portb; bcf status, rp0" but of course, maybe others see it differently. > These are still supported by the 'F84 but, presumably can become > unsupported in the future. Presumably, but obviously can *not* become unsupported in the 16F84. It's all really a rather amusing pretension to code-transportability in an environment where code transportability isn't particularly meaningful. -- Cheers, Paul B.

Whoops! Missed the *most* obvious blunder! Heinz Czychun wrote: > Apparently, on the original 16C84, the Option, and TriState ( data > direction registers ) registers (in Memory Bank 1) were not directly > accessable. The OPTION, and TRISx, instructions had to be used to > access these registers. Should read "on the original 16C54". -- Cheers, Paul B.

Hi Paul, At 2:46 PM 10/27/98, Paul B. Webster VK2BZC wrote: >Whoops! Missed the *most* obvious blunder! > >Heinz Czychun wrote: > >> Apparently, on the original 16C84, the Option, and TriState ( data >> direction registers ) registers (in Memory Bank 1) were not directly >> accessable. The OPTION, and TRISx, instructions had to be used to >> access these registers. > > Should read "on the original 16C54". Thanks for keeping me honest. I'm still a bit confused by the pre'F84 PIC nomenclature. I agree calling the TRIS, and OPTION instructions obsolete was a bit strong. Regards, Heinz