Periodic mode and oneshot mode

[Francesco Carsana]

I'm writing a real time module to generate a square wave on the parallel 
port (See attached file).
If I use periodic mode, I can generate a wave , but the period must be a 
multiple of 8254's programmed period.
I've decide to use oneshot mode, so I can change the period on the run 
with a user space program.
If, for example, I decide to use a wave period of 10 or 20 microseconds, 
I don't obtain the result, in terms of quality, that I've obtained with 
periodic mode. Infact, in oneshot mode, the square wave is much more 
unstable than in periodic mode.
If I use an higher period (100 microsecons), periodic mode and oneshot 
mode produce the same result.

The questions are:
1) Have I made a mistake in the code?
2) Why in periodic mode I obtain a so bad result? Does it matter the 
8254's reprogramming time?

Thanks.
Francesco
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