5 Feb
2010
5 Feb
'10
3:04 p.m.
That being said, apparently few think that it's an
issue, as a web
search doesn't turn up much in the way of grumbling. But how many
8255s are installed in an application where mode switching is used?
Darned few, I would guess.
The problem is still there even if you don't want to change modes after
inintialisation. After a hardware reset all pins you want to use as
outputs (that is pins you have connected to inputs of your peripheral
logic) are going to appear floating, so if your peripheral logic is TTL
(a good bet at the time the 8255 was designed), they will appear high. As
soon as you program the mode register, all those lines go low. You can
then set them back to your desired states. But your logic has to be
designed to cope with that.
I am trying to think of where the 8255 was used. OK, on the 5150 and 5160
motherboard to handle the keyboard interface and DIP switches, where
glitches don't matter. What other major uses did it have? Very frw of my
classic computers use one.
The 8255 was obsolete years ago. It's slow and most of the desired
functionality can be gotten more easily with other means, either with
programmable logic or even SSI.
So? This is classiccmp
So we're really beating a dead horse.
I was simply pointing out that Intel even managed to mis-design a
parallel port chip. Most of their other chips were equaliy inellegant.
-tony