Hi Nima,

One of the reasons to write in assembly language rather than a high level language like C is to make your programs small and fast. Once you get your PWM program working properly, you might want to think about ways to optimize it. Sometimes you'll need to do this sort of thing for practical reasons. If you're a bit of a geek, you might also find that it's just plain fun.

As an example, here's a snippet that works about the same as your program does, but with only eight instructions in the main loop:

MainLoop:   CLR   RI0               ; Acknowledge any recent Rx bytes

            SETB  LED               ; Turn output on
StillOn:    DEC   A                 ; Count down from OnTime (in SBUF)
            CJNE  A,#0,StillOn      ;  to zero

            CLR   LED               ; Turn output off
StillOff:   DEC   A                 ; Count down from 255 to OnTime
            CJNE  A,SBUF,StillOff   ;  (in SBUF)

            JMP   MainLoop          ; Repeat forever

(Folks here are geeky enough that somebody will probably pop up and show how to do it with six instructions, or four :)

Here are a couple of other things to think about once you get your program working properly:

• Because of the subroutine calls and loop overhead and whatnot in your program, the duty cycle of the output waveform won't be exactly what you'd expect from your TON and TOFF numbers.
• Since the output waveform depends on the timing loops in your program, it would be really difficult to make the microprocessor do anything else besides generate the PWM output.

You can solve both of these problems by using the hardware timers and their associated interrupts. I'll bet Craig's book explains these things in detail. Or, as Erik mentioned here, if your microcontroller has a Programmable Counter Array (PCA), it will generate PWM signals all day long with no help at all from the processor once you get it iniitalized.

It would be interesting to know what you are doing. What made you want to generate a PWM signal in the first place?

-- Russ