RE: Another dumb EE question

From: Scott Swazey <sswazey_at_qualcomm.com>
Date: Wed Apr 25 2001 - 02:46:38 EDT

Neil,
        Why not use a 74LS138. It's an address decoder in one chip.
The output is used as the CLK for the 74LS374 latch.

Connect as follows: 74LS138
pin 1 to +5v (or A13 if you want a finer address decode)
pin 2 to +5v (or A14)
pin 3 to A15
pin 4 to WR/
pin 5 to GND
pin 6 to +5v
pin 16 to +5v
pin 8 to GND

Output Y7/ on Pin 7 will go low when you write to addr 8000-FFFF
Connect Pin 7 of the 74LS138 to Pin 11 of a 74LS374

Connect the 74LS374 as follows:
pin 3 to data bus D0
pin 4 to D1
pin 7 to D2
pin 8 to D3
pin 13 to D4
pin 14 to D5
pin 17 to D6
pin 18 to D7
pin 1 (OE/) to GND (data is always present on the latched output)
pin 11 to pin 7 of 74LS138
pin 20 to +5v
pin 10 to GND
pins 2,5,6,9,12,15,16,19 are the latched data Q0-Q7 respectively

I hope this is what you wanted. I would like to make a small suggestion.
Use a 74LS273 instead of a 74LS374. The LS273 replaces the OE/ with a
Reset/ (active low) pin. This is VERY handy when using the latch for bank
switching. Connect pin 1 of the LS273 to the master reset/ of the board
and the output of the LS273 is guaranteed to be all zeros when the system
resets. BTW the rest of the LS723 pin out is exactly the same as the LS374

-Scott

At 11:24 PM 4/24/01 -0500, you wrote:
>
>
>Okay, on Asteroids Deluxe, I'm trying to make a write to any memory
>location in the 8000-ffff go to an 8 bit latch and latch the value on the
>data bus. So if I write 5ah to any address in 8000-ffffh, 5ah will be
>latched.
>
>I'm thinking about using a 74374 to do this. If I connected A15 coming
>from the CPU to the CLK pin and the !OE to the !WR signal coming from >the
processor, would that work?
>
>If not, what would be the least number of parts to do this in? As you can
>see by my questions, I know enough about hardware to be dangerous, but
>rather than coming up with a multichip solution, I'd like to learn from
>the experts. ;-)
>
>BTW, I plan on posting all my findings and experiments for all to
>see... For now I'm just trying to get the hardware to be a bit more
>expansive than it is. Next I'll tackle other fun things, like perhaps a
>multigame that I started last year or an entirely new game.
>
>-->Neil
>
>----------------------------------------------------------------------------
>Neil Bradley Play a song wrong once and it's a mistake.
>Synthcom Systems, Inc. Play a song wrong twice and it's Jazz.
>ICQ #29402898
>
>---------------------------------------------------------------------------

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Received on Wed Apr 25 02:51:53 2001

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