5588RE: [cosmacelf] Re: How can I send a 4 bit binary code to a 4 to 16 decoder?
- Aug 4, 2008
>Randy: Thanks for the info. I am running the lovely little ELF2K V85.Some random thoughts -
You might want to look thru the schematics in the Elf2K manual - there are
plenty of examples of interfacing there, in the cards that I've already
designed. And also the RCA 1802 COSMAC manual (MPM-201, I think) has plenty
of how to decode and latch parallel I/O ports for the 1802.
You mention connecting to "D0 thru D3", but you do know that you can't
just wire up your decoder to the bus, right? You're going to have to decode
an OUT instruction and then latch the data from the bus.
And the Elf2K manual has a list of the I/O port numbers that are used for
each option. Remember that the 1802 only has 7 I/O addresses, so you can
pretty much be sure that there'll be some overlap somewhere. You'll just
have to pick a port that's not used in your configuration.
I hate to push my own stuff, but you might want to consider the Elf2K GPIO
card. It has an 8255 PPI that drives three 8 bit parallel I/O ports which
you can use as you wish. The I/O ports are isolated by the 8255 so you
won't screw up your Elf bus no matter what you do, and they can also sink or
source reasonable amounts of current for driving external gizmos. However,
it's a little harder to use from a software point of view, since you will
have to actually initialize and program the 8255 chip before you can use the
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