John Payson wrote:
> [4] One style of "PWM" that I've not seen implemented in hardware
>     even though a hardware implementation would be easy works
>     as follows [the MSB of the output value is bit -1; the LSB
>     is bit -n]

>     Code to implement this looks like this [assuming that "count"
>     runs from 0 to 255, incremented somewhere else]
>
>                 decf    count,w
>                 xorlw   255
>                 andwf   count,w ; Note: only one bit in W will be set!
>
>                 andwf   value,w
>                 ; Z flag now indicates desired PWM state

After much thought and a bit of fiddling, I'm not sure I understand
this.

I've whipped up a FORTH algorithm that expresses the current PWM value
by displaying a series of asterisks or spaces ... but the output I see
suggests either that I've misunderstood the algorithm or misunderstood
the concept.

I agree that the DECF followed by XORLW and ANDWF will yield only one
bit set in the result.

Here is the FORTH rendition of the PIC code above ...

variable value
: test
  255 0 do                              ( vary count 0 to 254 )
    i 1-                                ( decf count,w    )
    255 xor                             ( xorlw 255       )
    i and                               ( andwf count,w   )
    value @ and                         ( andwf value,w   )
    255 and                             ( force to 8 bits )
    0= if ." *" else space then         ( test Z flag     )
  loop ;

And here is the output I get with values ranging from 0 to 4.

0 value ! test
********************************************************************************
********************************************************************************
********************************************************************************
***************
1 value ! test
* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
* * * * * * * * * * * * * * * * * * * *
2 value ! test
** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** ***
*** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** ***
*** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** ***
*** *** *** *** *** *** *** *** *** ***
3 value ! test
*   *   *   *   *   *   *   *   *   *   *   *   *   *   *   *   *   *
*   *   *   *   *   *   *   *   *   *   *   *   *   *   *   *   *   *
*   *   *   *   *   *   *   *   *   *   *   *   *   *   *   *   *   *
*   *   *   *   *   *   *   *   *   *
4 value ! test
**** ******* ******* ******* ******* ******* ******* ******* *******
******* ******* ******* ******* ******* ******* ******* ******* *******
******* ******* ******* ******* ******* ******* ******* ******* *******
******* ******* ******* ******* ******* **

--
James Cameron                                    (cameron@stl.dec.com)
Digital Equipment Corporation (Australia) Pty. Ltd. A.C.N. 000 446 800