True enough, but that's because, as you always like to do, you jumped in with your "Oh, My God! Don't use the 8255!" without any justification. So far you've thrown down a number of "red herrings" but provided no persuasive argument that the 8255, limitations and all, is not suitable for expanding the I/O of an 805x. It might not be easy to use with YOUR 805x, though I would guess it can be done.


that is the skew between transmit and receive that's introduced by disassembling and reassembling the bytes/words before they can be interpreted. In a parallel situation, you might be required to acknowledge a byte within, say, 1 microsecond. A hardware handshake, as provided by an 8255, many other hardware arrangements, or which you can easily provide with a relatively fast MCU, could manage that. However, if you have to transfer a word containing the payload byte and the control byte, or bits, required to effect the handshake, there would be considerable delay as the data is serialized and deserialized before it could be interpreted and acted upon.



Sorry about that, I didn't mean you, personally, I meant the general "you" in the sense of "whatever it's called" since I'm not a big user, hence, tend to mix 'em up.




All you've done for the past 100 kBytes is argue that the 8255 is a poor choice for this purpose. You've not presented even ONE reasonable argument why that would be, for the typical case, though I can see why you'd not want to use one yourself.

True, the 8255's slow, but its' not too slow to use with many of the currently marketed parts that fit the 805x model. It's old, but then, being quite old myself, I don't find that a problem, at least not for the 8255. The thing still works as it did nearly 30 years ago.

Now ... why do we keep having this argument? ... Well, it's because you CATEGORICALLY state that it's wrong to use it. You tell people who have ready access to them that they're too hard to get. You tell people using 25-year old MCU's that they're too slow. Most of the time you don't even ask what sort of MCU they're using. You just tell them that they're stupid and they should do what you say.

I don't like that.

If you gave them an informed choice, after telling them what's involved and how to find out about that, I'd be fine with that. If a guy wants to expand the I/O on his 805x, you really can't answer without first finding out which MCU he's using, what kind of I/O he wants to expand, and other things like that. Have you ever asked such questions?

Those ST parts to which you referred are not fast enough to keep up with every type of 805x, even the ones in standard package, without some tweaking. I'd have to slow down the bus to make 'em work with my Maxim parts. However, they would work with the bus timing extended.

Unfortunately, you have to buy the internal FLASH and RAM in order to get the I/O. Since they cost twice what the 8255 costs, and availability is slow and in quantity only, the choice is easy if I'm building just one of something.

There's nothing wrong with the IIC parts if they work for what's required, but they don't seem to have the instantaneous (well, not quite) auto-handshaking that might be needed. The package of the IIC part will probably present a problem, though. If I ever get to using a part like the ones you use, I'll certainly consider the option of using the 8255 as a port-mapped peripheral, but only as an option. I've never liked 'em.

RE