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:* [email protected] <javascript:> [mailto: > [email protected] <javascript:>] *On Behalf Of *curt mayer > *Sent:* Sunday, July 20, 2014 2:09 PM > *To:* [email protected] <javascript:> > *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. > > -- > You received this message because you are subscribed to the Google Groups > "N8VEM-S100" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to [email protected] <javascript:>. > For more options, visit https://groups.google.com/d/optout. > -- You received this message because you are subscribed to the Google Groups "N8VEM-S100" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. For more options, visit https://groups.google.com/d/optout.
