From: Ethan Dicks (ethan.dicks_at_gmail.com)
Date: 2007-05-22 23:03:01
On 5/22/07, Antitrack@networld.at <Antitrack@networld.at> wrote:
> I still don't know how to "select", in my circuirty, what memory location my
> cartridge ought to use. What if I want to build a cart that uses memory from
> $deb6 to $deba? And how does the cart "know" when I poke something in these
> memory locations to do something?
After re-reading your query, let me try a less abstract example...
With TTL...
When the 6510 reads/writes any location between $DE00 and $DEFF,
there's a line on the cartridge port (if I've remembered the port
correctly), that is "asserted" or goes to 0V (you have to check the
port assignment to see what logical True and logical False are, but
commonly, with TTL, True is 0V). If you run just that signal to your
chip, it will appear at any address between $DE00 and $DEFF. Since
you want five specific addresses that's aren't on any even boundary
(are you _sure_ you have 5 registers to read/write, not 4 or 8?),
it'll be a bit tricky. Let's try $DEB0 to $DEBF to put things cleanly
on a powers-of-two boundary.
To decode $xxB0 to $xxBF, what you are saying is that you will be
hooking A0 through A3 to your peripherals, giving you 16 different
bytes to read/write, so you need to decode A4-A7 to be exactly $B, or
A4 on, A5 on, A6 off, A7 on. You can either build a nest of 2-input
logic gates to match that exact set of conditions (A4 + A5 + /A6 + A7)
or you could use a single 74LS154 attached to A4-A7 and then tap the
12th output of 16, then invert it (to match the sense of your $DExx
select), then OR it with your $DExx line to get your (low-true)
select...
I'll try this in a 4-line block of text since I think an ASCII art
table will get skewed...
$DExx select = 1 (false) OR '154 line 12 = 1 (false) is 1 (false)
$DExx select = 1 (false) OR '154 line 12 = 0 (true) is 1 (false)
$DExx select = 0 (true) OR '154 line 12 = 1 (false) is 1 (false)
$DExx select = 0 (true) OR '154 line 12 = 0 (true) is 0 (true)
So by hanging a '154 off of 4 address bits you want to decode, then
inverting the select (since address bits are high true), and ORing it
with your low-true block select, you get a select line that is only
asserted (low-true) when any of those 16 addresses appear on the
address bus. The same goes for a '138, but it's 3 address lines
decoded and 32 selected address lines.
You can also pipe $DExx and A0-A7 into single 16V8 GAL and by coding
the GAL equations right, have your output select trigger on just about
any combination of address lines you like (and change your cards
address later by changing the equations), but you'd need a GAL
programmer to do that. The chips are cheap - the last batch of 16V8s
I bought was no more than $1.25 per chip and you only need one socket
and a small amount of board space.
You can also employ address comparators like the 74LS688 to compare up
to 8 address bits on the bus to 8 DIP switches to have a
user-selectable address. The '688 was commonly used in ISA
peripherals to map the ISA card to anywhere in a block of addresses on
the bus. If the card needed 8 addresses, then A3-A10 would be routed
to the '688 and the user could put the card anywhere inside a chunk of
2048 addresses, aligned to an 8-byte boundary (i.e., $xxx0 or $xxx8).
- ('688 pinout at
http://www.xs4all.nl/~ganswijk/chipdir/giicm/74688.txt)
Hope this clears things up.
-ethan
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