26/10/2004 14:10:36, "Glen VanMilligan"
<
glenv@vpcvalley.com> wrote:
> Hi,
> I'm needing to add 24 I/O ports to the Mother board, but
being a new
> user,
>
> I'm not sure what lines I can use to add a device like an
8255 (
> or similar ).
> Can you offer some suggestions to do this?
I wouldn't go to the lengths of using an 8255 - it would be much
easier to use the LCD GPIOs, if you're not using all of them for LCD
purposes, and use 4 or 8 of them as a little data bus, some more
as addresses and strobes, and latch your 24 IO bits that way. Do
you want each of your 24 pins to be I/O, or are some inputs,
some outputs? If you need the more complex case, then building
what you want out of a small Xilinx device (XCR3032 or similar)
would be the way I'd approach it, to reduce the development
effort. It's very rare that I use stuff like 8255s any more. (and, for
what it's worth, the GPIO pins aren't 5Volt tolerant - if you plan to
interface to 5V stuff, then the XCR3xxx family is exceedingly
convenient, since it's 5V-tolerant, and has slew-rate control, if you
need it)
If you do want lots of bandwidth, though, then you'll need to use
the 'real' address and data buses. (The GPIOs can be switched at
many tens of MHz, though, so don't rule them out unnecessarily).
This would involve a much more complex PCB, though, since you'd
need to lay out for the backplane connector, rather than just using
the 1.25mm pitch IO connectors. It all depends on what resources
you've got, really.
Steve
(RIP John Peel, whose show has been the soundtrack to an awful
lot of Balloon design hours)
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