The guy who wants to know about how to expand the I/O on an 805x-core MCU is using it in 8031-mode. So, even if it says DS89C450 on it, if he's got the nEA pin tied low, it's an 8031.

True, 8031's are just junked 8051's, where there was a ROM error or the customer decided to cahnge the masked ROM, but once you start discussing expansion of the memory or I/O of an 805x-core MCU, you have to consider its 803x mode of operation. If there's cause to rule it out, e.g. the part doesn't present an external bus, then you have to move on to other options. If, however, it has the ability to work in 803x mode, then there's a world of possibility.

Students whose instructors have assigned them a project generally have a board in mind. Often it's a board with an 8255 site on it, together with the 8031-configured 805x-core MCU.

Whether the learner learns on a 33 MHz 1-clocker or a 12 MHz 12-clocker doesn't seem to matter. The learning seems to work the same. The fast part is a bit fussier, i.e. RAM has to be pretty fast and FLASH or EPROM is out of the question. My DS89C4x0's have to copy their EPROM into SRAM and then change their clock rate in order to use external memory. Then, just to make the operation complete, they have to change the demultiplexing hardware so that the MCU can operate in "Page" mode, which cuts the external memory cycle time in half so long as the high address byte doesn't change. Needless, to say, I can't use an i8255, no matter how fast, with that arrangement. The LCD has to be interfaced by means of a pair of MSI registers, as does the keypad.

Any standard 805x-core MCU in the standard package with the standard features, can be used as an 803x. You know that. We all know that.

RE