The only way in which an 8051 can be of use to someone not aware of the masked-rom content is as an 8031, i.e. with the /EA line tied low. That means it uses external memory.

The only way "extend" applies is if he's adding ports to whatever's there. The O/P's query is, indeed, quite vague, but no more so than many of the replies.

Yes, we could all clean up our references to make them more specific, and very well should do so.

This question comes up every year at about this time, though, and I suppose it's because, wherever they teach the use of the 805x-class of MCU, they get to the business of expanding the circuit at about this point in the curriculum. Every year we have the same series of "don't ever use the 8255" type responses without ever having a suggestion as to how to replace it with something that actually extends the ports. Some people suggest using an IIC device, but IIC requires considerable support. Some people suggest using a CPLD, which has promise, but involves considerable effort as well as a decision as to how the port has to function (the 8255 has several operating modes of which one has to be chosen). Some people suggest using a part with enough on-board I/O to handle the requirements. All these can be made to work, but none fill the bill, largely because we don't know what the actual requirement is.

"Extend the ports" pretty clearly implies make more out of what's there. The O/P's reference to 8051 pretty clearly indicates that he doesn't really know what he wants to use, but, taken in the context of what comes up every year, suggests he has either a textbook or an application circuit/evb in front of him, at least figuratively. We all know that an 8051, irrespective of which manufacturer produced it, is a masked-ROM part that can only be used in 8031 mode by anyone not aware of the specifics of the masked code.

Given that this comes up every year, couldn't we, for once, come up with a reasonable response. The negative ones, e.g. "don't do this" or "don't use that" which immediately popped up this time, don't help a bit. What's needed is a positive construct. There are ways of extending the I/O capabilities of the 805x-series MCU's, irrespective of what they have or haven't got on-board already. I think Jan's suggestion of an FAQ, since it has come up repeatedly, is an excellent one. How about avoiding the negatives and providing some "real" solutions. However, let's start with the notion that the guy actually has an 8051/8751 in his possession and wants to extend its port structure. The O/P's in Asia, where 8255's are common and the latest, greatest derivatives aren't. He's clearly a learner, and probably doesn't want to buy lots of stuff, least of all expensive stuff that requires a PCB be made. Let's keep that in mind.

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