Post by thread:BTFSS help

Hi, I am a PIC newbie. I am working on 16f877. I use MPLAB 4.12 for Windows. I have a value stored in the FileRegister 0x27 < General Purpose Register >. That value is always between 0 and 7 (both inclusive). This value will be dynamically varying during program execution. I need to use this value in the instruction BTFSS. How do I use it as required? Are there any other methods that can be used to skip instructions based on a bit, with the bit position dynamically varying during program execution? Regards, Somasundaram.

I don't understand your question. What is it you want to test in the register? Do you want to test the position of the bit and then do instructions based on its position in the register? Or do you want to do instuctions based on the value in the register? You ask if there are other instructions based on a bit. Depends what you want to do. Other ways of testing for bits set is the MOVLW K (constant) and SUBWF 0x027,W routine Then you test Status for Z, C or DC (BTFSS STATUS,Z) as one example. There are many more. Better if you explain to us what you need to do. Maybe with some short code examples. Quentin Somasundaram wrote: {Quote hidden}> > Hi, > > I am a PIC newbie. I am working on 16f877. I use MPLAB 4.12 for Windows. > > I have a value stored in the FileRegister 0x27 < General Purpose Register > >. That value is always between 0 and 7 (both inclusive). This value will > be dynamically varying during program execution. I need to use this value > in the instruction BTFSS. How do I use it as required? Are there any other > methods that can be used to skip instructions based on a bit, with the bit > position dynamically varying during program execution? > > Regards, > Somasundaram.

Q> I don't understand your question. What is it you want to test in the Q> register? Do you want to test the position of the bit and then do Q> instructions based on its position in the register? Q> Or do you want to do instuctions based on the value in the register? Could it be that Somasundaram wants to dynamically change the bit thats being tested, like this?: BTFSS 0x27,V ; 'V' is a variable -DS >> I have a value stored in the FileRegister 0x27 < General Purpose Register >> >. That value is always between 0 and 7 (both inclusive). This value will >> be dynamically varying during program execution. I need to use this value >> in the instruction BTFSS. How do I use it as required? Are there any other >> methods that can be used to skip instructions based on a bit, with the bit >> position dynamically varying during program execution?

Sorry, I didn't put it clear. test_for_bit equ 0x27 value_to_test equ 0x26 Say value 5 is stored in the file register 0x27 I would like to test the <test_for_bit> bit of <value_to_test>. BTFSS value_to_test,[value stored in test_for_bit] -> Test the bit5 in the value stored at 0x26 But the value stored in 0x27 will keep changing with program execution. Say, after sometime( may be while in a loop), 0x27 holds the value 3, then the same statement must test the bit3 of the value stored at 0x26. Something like: I would like to count the number of 1's in the value ( at 0x26) using btfss instruction. - Somasundaram. At 02:09 PM 10/16/99 +0200, you wrote: {Quote hidden}>I don't understand your question. What is it you want to test in the >register? Do you want to test the position of the bit and then do >instructions based on its position in the register? >Or do you want to do instuctions based on the value in the register? > >You ask if there are other instructions based on a bit. Depends what you >want to do. >Other ways of testing for bits set is the MOVLW K (constant) and SUBWF >0x027,W routine >Then you test Status for Z, C or DC (BTFSS STATUS,Z) as one example. >There are many more. > >Better if you explain to us what you need to do. Maybe with some short >code examples. > >Quentin > >Somasundaram wrote: >> >> Hi, >> >> I am a PIC newbie. I am working on 16f877. I use MPLAB 4.12 for Windows. >> >> I have a value stored in the FileRegister 0x27 < General Purpose Register >> >. That value is always between 0 and 7 (both inclusive). This value will >> be dynamically varying during program execution. I need to use this value >> in the instruction BTFSS. How do I use it as required? Are there any other >> methods that can be used to skip instructions based on a bit, with the bit >> position dynamically varying during program execution? >> >> Regards, >> Somasundaram. > >

Somasundaram wrote: > Sorry, I didn't put it clear. *I* knew what you were getting at! > Say value 5 is stored in the file register 0x27 > I would like to test the <test_for_bit> bit of <value_to_test>. This is a FAQ. The PIC does not provide for this (and I don't recall any other processor doing so either). If that's *all* you need and you want to do it fast (bad luck - it isn't!), then the suggested method is to use a table either of the masks (DB 1,2,4,8,16,32,64,128) which you access via a ADDWF,PCL call as a subroutine, or you use an "action table" accessed in the same fashion, but with the index multiplied by two or four and separate test procedures in each entry. Use macros! > Something like: I would like to count the number of 1's in the value > (at 0x26) using btfss instruction. Ah! That's a separate FAQ. I was thinking of the parity calculation that usually goes like: SWAPF value,W XORWF value,W MOVWF temp RRF temp,W RRF temp,W XORWF temp,F RRF temp,W XORWF temp,F and now you test bit 0 of temp for the parity. If however you want the overall sum of the bits, that is slightly different. I'm sure there is a *terribly* clever way to do that too, but I think I better go to bed now before I send any more private messages such as the one I thought I sent to Russell McMahon, to the list! -- Cheers, Paul B.

Somasundaram wrote: > > Sorry, I didn't put it clear. > > test_for_bit equ 0x27 > value_to_test equ 0x26 > > Say value 5 is stored in the file register 0x27 > I would like to test the <test_for_bit> bit of <value_to_test>. > > BTFSS value_to_test,[value stored in test_for_bit] -> Test the bit5 in the > value stored at 0x26 Try this: MOVF TEST_FOR_BIT,W MOVWF TEMP1 MOVF VALUE_TO_TEST,W MOVWF TEMP2 ;ABOVE ONLY NEEDED IF YOU WANT TO RETAIN ORIGINAL VALUES MOVF TEMP1,F BTFSC STATUS,Z GOTO TEST2 ;IF TEMP1 =0, CHECK ONLY LSB TEST BCF STATUS,C RRF TEMP2,F DECFSZ TEMP1,F GOTO TEST TEST2 BTFSS TEMP2,0 GOTO IF_0 GOTO IF_1 > Something like: I would like to count the number of 1's in the value ( at > 0x26) using btfss instruction. I take it this is part of your first question so change above to: MOVF TEST_FOR_BIT,W MOVWF TEMP1 MOVF VALUE_TO_TEST,W MOVWF TEMP2 CLRF TEMP3 ;ABOVE ONLY NEEDED IF YOU WANT TO RETAIN ORIGINAL VALUES INCF TEMP1,F ;EXTRA COUNT TO CHECK LSB TEST BTFSC TEMP2,0 INCF TEMP3,F ;CONTAINS NUMBER OF "1" BITS BCF STATUS,C RRF TEMP2,F DECFSZ TEMP1,F GOTO TEST GOTO NEXT..... This clear for you? I did not take any "short cuts" here so that you can see what can be done. Quentin

> > Sorry, I didn't put it clear. > > test_for_bit equ 0x27 > value_to_test equ 0x26 > > Say value 5 is stored in the file register 0x27 > I would like to test the <test_for_bit> bit of <value_to_test>. > > BTFSS value_to_test,[value stored in test_for_bit] -> Test the bit5 in the > value stored at 0x26 There's no instruction that can do that. As posted before there's a couple of ways of solving the problem. 1. Shift the bit to bit 0 then test bit 0. 2. Use a jump table to convert the bit number into a bit mask then apply the mask to the original value. 3. Use a jump table with two instruction each that does the appropriate bit test then a jump to the end. I use this technique in my NPCI interpreter. > > But the value stored in 0x27 will keep changing with program execution. > Say, after sometime( may be while in a loop), 0x27 holds the value 3, then > the same statement must test the bit3 of the value stored at 0x26. > > Something like: I would like to count the number of 1's in the value ( at > 0x26) using btfss instruction. Now that's a different problem. The fastest way to do this is to have a 256 byte jump table that takes the number and returns the number of bits. Probably even better would be to half the table an test, then zero, the high bit of the value under test. Then the table till only be 128 bytes long. The shortest way is to use technique 1 listed above. Instead of using a counter to variably test the bits of value_to_test, keep the same loop and shift the value_to_test (vtt) 1 bit to the right/left each time through the loop, testing the single bit at the end each time. So if the vtt is 10110101 You test bit 0, see that it's a one an increment the number of ones (noo) count. Then you shift one bit to the right getting 01011010 Test bit 0 again. Now the bit to test is fixed, which the PIC does no problem. Now since it's 0, the noo stays put. Repeat until the vtt is 0 (or exactly 8 counts if you like). When done the noo has the number of ones. Hope this helps, BAJ

I presume that you sort of parse the possible value in your general purpose register and have various actions taken on various values? Is that right? in that case you could do two things 1. create a lookup or branche off table of the 8 possible actions. 2. load the value in the genral purpose register in W and test its value with xorlw movfw GenPurpReg, W ; suppose this value is 5 xorlw 5 ; if the value is 5 then the result is 0 skpnz ; test the zero flag (MPLAB assembler short for bftsc STATUS, Z) goto ...... ; do something start the next tet sequence...... Henk Tobbe VK2GWK / PA0ADC {Quote hidden}> >I have a value stored in the FileRegister 0x27 < General Purpose Register >>. That value is always between 0 and 7 (both inclusive). This value will >be dynamically varying during program execution. I need to use this value >in the instruction BTFSS. How do I use it as required? Are there any other >methods that can be used to skip instructions based on a bit, with the bit >position dynamically varying during program execution? > >Regards, >Somasundaram.

Hi, Somasundaram -----Original Message----- From: Somasundaram <spam_OUTmeisomsTakeThisOuTMD4.VSNL.NET.IN> To: .....PICLISTKILLspam@spam@MITVMA.MIT.EDU <PICLISTKILLspamMITVMA.MIT.EDU> Date: Saturday, October 16, 1999 2:30 PM Subject: BTFSS help >Hi, > >I am a PIC newbie. I am working on 16f877. I use MPLAB 4.12 for Windows. > >I have a value stored in the FileRegister 0x27 < General Purpose Register>. >That value is always between 0 and 7 (both inclusive). This value will >be dynamically varying during program execution. I need to use this value >in the instruction BTFSS. How do I use it as required? Are there any other BTFSS is required to have a constant bit position. But you can try reprogramming the instruction at run-time (takes some time :) >methods that can be used to skip instructions based on a bit, with the bit >position dynamically varying during program execution? > There are no one-instruction way to check a bit with a variable position. Try these methods: Method 1: Rotate the register you are checking several times, till the bit in question will be copied to carry bit in STATUS. Then simply check carry. Method 2: Make a look-up table with 8 entries. Each entry will contain mask X depending on a bit position N, which is also address to the table. (X = 2 ^ N) When you need to check a specific bit, find the corresponding mask, AND the mask with the register you are checking. Then check zero bit. z = 1 if bit N = 0, z = 0 if bit N = 1. Method 3: Keep additional variable that contains dynamically changing mask. Then apply the mask when needed. I hope it helps.

Hi, directly not. Indirectly, see the following snippet MaskVal EQU 0x27 ; the value you have given i EQU 0x28 ; or whatever Data EQU 0x29 ; the register to be BTFSS-ed Mask EQU 0x2A ; or whatever W EQU 0 ; for the sake of readibility F EQU 1 ; same ;............ movlw 1 ; initiate mask movwf Mask ; same movf MaskVal,W bz done ; mask already done movwf i ; set up loop clrc loop rlf Mask,F ; shift mask into position decfsz i ; loop administration b loop ; same done movf Mask,W ; fetch mask value andwf Data,W skpnz ; this is that skip you wanted Some mnemonics below may appear strange for you (e. g. clrc) but they're supported by MPASM. I hope this helps. But check double before using! Regards, Imre On Mon, 16 Oct 2000, Somasundaram wrote: {Quote hidden}> Hi, > > I am a PIC newbie. I am working on 16f877. I use MPLAB 4.12 for Windows. > > I have a value stored in the FileRegister 0x27 < General Purpose Register > >. That value is always between 0 and 7 (both inclusive). This value will > be dynamically varying during program execution. I need to use this value > in the instruction BTFSS. How do I use it as required? Are there any other > methods that can be used to skip instructions based on a bit, with the bit > position dynamically varying during program execution? > > Regards, > Somasundaram. > >