"slow" ports on MPC8255 ?

[Wolfgang Denk]

Hello,

I wrote:

> has anybody of you experienced problems with accesses  to  port  data
> registers on a MPC8255 yet?
...
> - writing port A data register (as 32  bit  word)  and  then  looping
>   reading  it back will show that it takes some time for the write to
>   take effect, i. e. we can read the old  value  for  some  time  and
>   then, bit for bit, the new value becomes visible.

Problem solved. The solution was simple. Looking closer I found  that
the  effect  showed only up for bits changing from 0 to 1. Then I saw
that ports pins in  question  are  configured  as  open  drain.  When
switching  a  pin  to 1 the level would rise slow enough that reading
the data register would pick up a low (0) for several cycles.

So actually it was no problem at all, just some slow circuitry and  a
fast  CPU.  Don't  know why the MPC8260 behaves differently (internal
pull-ups?).


Wolfgang Denk

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Software Engineering:  Embedded and Realtime Systems,  Embedded Linux
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