Post by thread:Direct, Indirect when referencing BTFSC FILE, BIT

John Considine wrote: > > I am trying to have a variable be decremented and that result be used as > the BTFSC test bit > > For example > > movlw H'07' > movwf TestBit > decf TestBit, 1 > btfsc PORTB, TestBit > > Is this possible, I tried using indirect referencing like > > movlw TestBit > movwf FSR > btfsc PORTB, INDF > > but that will not work. Any suggestions. > Thanks You cannot use BTFSC like that. (?) You may need to try something like... movlw b'10000000' movwf TestBit - - - - bcf status,carry ; shift bit one position to right rrf TestBit btfsc status,carry ; if carry = 1 then reset bit 7 = 1 rrf TextBit - - - - movf PORTB,w ; do bit comparison on port andwf TestBit,w btfsc status,Z goto BitIsOne -- Best regards Tony http://www.picnpoke.com Email spam_OUTsalesTakeThisOuTpicnpoke.com

At 12:13 1/11/99 +1100, you wrote: {Quote hidden}>John Considine wrote: >> >> I am trying to have a variable be decremented and that result be used as >> the BTFSC test bit >> >> For example >> >> movlw H'07' >> movwf TestBit >> decf TestBit, 1 >> btfsc PORTB, TestBit >> >> Is this possible, I tried using indirect referencing like >> >> movlw TestBit >> movwf FSR >> btfsc PORTB, INDF >> >> but that will not work. Any suggestions. >> Thanks > >You cannot use BTFSC like that. (?) > >You may need to try something like... > > movlw b'10000000' > movwf TestBit > > - - - - > > bcf status,carry ; shift bit one position to right > rrf TestBit > btfsc status,carry ; if carry = 1 then reset bit 7 = 1 > rrf TextBit > > - - - - > > movf PORTB,w ; do bit comparison on port > andwf TestBit,w > btfsc status,Z > goto BitIsOne > >-- >Best regards > >Tony > >http://www.picnpoke.com >Email .....salesKILLspam@spam@picnpoke.com > > I realy can't see what is going on here, If you are decementing to see what the results is you can have either a 1 or a 0, no matter what. Or is it that you are trying to use this as a mask for testing of each potential bit location. Ah yes the latter If the latter is true, then there is the problem in that the compiler + opcodes expect this to be a litteral value, hence the method used by Tony. E.G BTFSC myval, 0-7, where the 0 to 7 are fixed. If this is the case then perhaps you could waste a bit of space and use a table type function ;Test byte is the location to be compare d with the counter ;value. This makes the code resuable for testing of bit ;compares for any direct RAM location movf PORTB movwf test_byte,w ;do this bit on pre entry counter ;the current bit location to look at ;the counter is 0-7 and indicates the bi t that we want to look ;at. Note that this vale must be preload ed and maintained by ;the calling code ;function body ;*********** bit compare: ;*********** ; Perequsites ; =========== ; Counter must be set with the bit to test ; w must contain the table offset ; test_byte must be the byte to test to ; LOCAL variable :- test_byte -> effectiv e only, not to be preserved movlw 3 ;so that we get to the correct location in the table addw counter add pcl,w ;point to the location to jump to TABLE nop nop nop btfsc test_byte,0 goto return_true goto return_false btfsc portb,1 ... return_true do stuff return_false do other stuff I know that there is stuff missing in the table but you can get the general idea. Tonys stuff also works, but resides on the assumption that the value (As you requested) is dectmented by one each time, where the table value lets you attack any bit at any time. On further thinking Tonys way on bit test will work at all times, so ignore the rubbish I just came up with Dennis

John Considine wrote: > I am trying to have a variable be decremented and that result be used > as the BTFSC test bit This is a VFAQ. There is no bit pointer function on the PIC or AFAIK *any* microcontroller. Tony's reply noted; but I suspect you don't really need, possibly even want, to read PORTB each time. Would this not do?: movlw 8 movwf TestBit ; count from 8 movf PORTB,W movwf shiftr ; copy of PORTB nexbit: btfsc shiftr,7 ; test MSB goto bitset ; the bit denoted by TestBit is set rlf shiftr ; roll the bits decfsz TestBit,F ; look at next bit goto nexbit noneset: ; fall through if none set. bitset: ; the bit denoted by TestBit is set If you use 0 and 1 rather than W and F for the "destination" you are really *asking* for trouble! -- Cheers, Paul B.

> > I am trying to have a variable be decremented and that result be used as > the BTFSC test bit > > For example > > movlw H'07' > movwf TestBit > decf TestBit, 1 > btfsc PORTB, TestBit > > Is this possible, Nope. > I tried using indirect referencing like > > movlw TestBit > movwf FSR > btfsc PORTB, INDF > > but that will not work. Any suggestions. There was a discussion of this a while ago on the list. The bit number for a btfsc instruction is fixed. Two possible ways of handling it. 1. Use a jump table to translate the bit number into a bit mask. Then use the mask to test the required bit (using and) then btfsc off the zero flag. 2. Build a jump table with 8 btfsc's and their associated no jumps in it. But you cannot perform the task they way you like unfortunately. BAJ

--- John Considine <jconsiKILLspamMAGICNET.NET> wrote: > I am trying to have a variable be decremented and > that result be used as > the BTFSC test bit > > For example > > movlw H'07' > movwf TestBit > decf TestBit, 1 > btfsc PORTB, TestBit Here are a couple of ideas for getting 1<<TestBit 10 cycles: 2^n: clrw btfsc TestBit,2 movlw b'11111100' addwf TestBit,f incf TestBit,w btfsc TestBit,1 iorwf TestBit,f incf TestBit,f skpnc swapf TestBit,f ......... another way (7 cycles [including the call]) two_to_n: andlw 7 addwf pcl,f retlw 1 retlw 2 retlw 4 retlw 8 retlw 0x10 retlw 0x20 retlw 0x40 retlw 0x80 ............ or probably more to the point movf portb,w movwf portb_shadow btfss TestBit,2 swapf portb_swadow,f btfss TestBit,0 rlf portb_shadow,f movlw b'10000000' btfss TestBit,1 movlw b'00100000' andwf portb_shadow,f skpnz goto _the_bit_is_set .lo ===== __________________________________________________ Do You Yahoo!? Bid and sell for free at http://auctions.yahoo.com