"slow" ports on MPC8255 ?
wd at denx.de
Thu Jun 19 00:24:03 EST 2003
> 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
Software Engineering: Embedded and Realtime Systems, Embedded Linux
Phone: (+49)-8142-4596-87 Fax: (+49)-8142-4596-88 Email: wd at denx.de
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