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> with the input 0 (NEW = 0) and FF = 1 - K.  It's not clear to 
> me whether James just wants to show decay of a capacitor 
> discharging thru a resistor, or model a full R/C filter where 
> the input could change.


The latter, but that code is done and working. If I hand it a correct value
of K, it does it's thing just fine. 

Actually, the code is based on the diagram I've attached to this email. It
uses two constants, one is the original attenuation represented by at (a=RC)
and the second attenuation is e^(-at). There is a routine that does
fractional multiplication:

;For PIC Microcontrollers
fracmul
    CLRW            ;W=0
fracmul1
    BCF status,c               
    RRF arg1        ;ARG1:=ARG1/2
    BCF status,c    ; don't copy the carry
    RLF arg2        ;CY:=MSB(ARG2),
                    ; ARG2:=ASL(ARG2,1)
    BTFSC status,c  ;IF CY=0 SKIP THE ADD
     ADDWF arg1,w   ;ELSE W:=W+ARG1
    MOVF arg2                  
    BTFSS status,z  ;IF ARG2 NE 0 REITERATE
    GOTO fracmul1
    RETLW 0h        ;ELSE RETURN RESULT IN W

So to attenuate the sample by at, we compute at as a fraction of 256 and put
the sample in arg1, the fraction in arg2 and call this routine. That result
is then set aside. Next we calculate the feedback from the last value which
is attenuated by e^(-at)

This is just the previous output value in arg1 and the fraction e^(-at) in
arg2 and a call to the same routine, the results of the two calls are summed
and that is the new output value. 

If I calculate the two values for arg2 and put them in, the code does
exactly as expected and simulates a low pass RC filter having the values of
R and C specified. I can get the assembler to calculate the first value with
the code:

at EQU 256* 360000/1000000
	movlw at
;for a digital equivalent to an RC filter with a 
;360kOhm resistor and a 1uF capacitor

That puts 92 in w which is 92/256 = 0.360 = RC

So that students may alter the R and C values somewhat and see the results.
But I can't get it to calculate the second value e^(-at)

Olin was exactly right in a previous post. You can't scale it by 256 (or any
other constant) and then de-scale it by the same amount at the end.

There may very well be more efficient ways to do this, but this way is
directly derived from the diagram above and that diagram is very easily
derived from the operation of the standard low pass RC filter circuit.

Again, the point is education, not optimal operation.

---
James.
 

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