RE: [N8VEM-S100:4650] hot rodding (really 74ls682)

Curt,

For board select, P>Q is useless.
But if you were to use this chip to create pulse width output, then it would be used.
Or if the chip was used in some sort of digital feedback system.

I suppose it could also be used to select different kinds of memory over a certain range,
ie, say you overlap RAM and ROM, but wish to make a flexible amount of ROM available.
Thinking in the 64K world, now you can have... say 55 pages of ROM and 201 pages of RAM.

Just a thought... don't know how practical those ideas would be.

Cheers,.
Josh

Date: Mon, 21 Jul 2014 00:09:45 -0700
From: cu...@zen-room.org
To: n8vem...@googlegroups.com
CC: mon...@vitasoft.org
Subject: Re: [N8VEM-S100:4650] hot rodding (really 74ls682)

argh. to think my parts box has unobtainium in it! nobody sells f520's any more... I've looked into the 8 bit comparator thing pretty hard, and the ALS520's still exist at ti, and they have a significant edge on the ls682's, although not at much as the F520's. if you had external pull-ups, then the F521s could be used, at a smoking 7ns typical. mouser had both F521's and ALS520's.

when do you use the P>Q output?

On Sunday, July 20, 2014 3:02:31 PM UTC-7, monahanz wrote:

Good point Curt, and definitely not a whine. I had not really looked at the F520’s before. To some extent you are comparing apples to oranges.

For P=Q on the LS682’s the typical delay is 13 ns, max 25 ns (TI datasheet). For P=Q for the F520 its 5 and 5.5. However that’s a 74Fxx part, I cannot find a corresponding LS520 datasheet.

The delay is more a function of the 74LS/74F part rather than the internal chip layout. A brief look on the web did not turn up 74F682’s or 74ALS682’s. If they exist, I bet they would be comparable to the 520.

Where are you getting your F520’s from BTW.

Going forward, since I really use the LS682 pin 1 (used for P>Q), we could have a jumper to allow either chip on the board.

John

From: n8ve...@googlegroups.com [mailto:n8vem...@googlegroups.com] On Behalf Of curt mayer
Sent: Sunday, July 20, 2014 2:09 PM
To: n8ve...@googlegroups.com
Subject: [N8VEM-S100:4644] hot rodding (really 74ls682)

A lot of these designs want to be pushed a little for clock rate, and It feels really odd sticking a slow part like a 'ls682 in my critical address decode path, especially when it's cascaded like in the rom/ram board. sticking 35 ns memory in there and then blowing another 30 ns just to decode the address lines to get a chip select will keep you from running at 10mhz without wait states. an 'F520 will do the same job in half the time, if you ground the unused p>q output; so all my '520s in '682 sockets have pin 1 lifted and wired to ground with a scrap of kynar. and my chip selects come out 10ns earlier.

it this just a whine? I really dislike the '682, since you can't get any faster parts with this pinout that have the internal pullup.

--curt, on a quest for a 12mhz s100 0WS z80.

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