74HCxx vs 74LSxx

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tcat
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74HCxx vs 74LSxx

Postby tcat » Wed Mar 07, 2018 11:33 am

Hi,

I plan to build a simple FPU board for MC68000, based on a DIY design. I need to decode many address lines. Am I safe to replace original 74HCxx chips with 74LSxx ones, as these seem easier to get here?

I cannot tell any difference between LS and MC technology, and when to use one or the other. The FPU is MC68881@16MHz, and the FPU board should piggy back on standard CPU MC68000@8MHz using standard ATARI STFM power supply.

Many thanks.

Tomas


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Pr0f
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Re: 74HCxx vs 74LSxx

Postby Pr0f » Wed Mar 07, 2018 11:52 am

The main things to watch for are slew rate, propagaton delay and fanout

The LS devices will drive more devices than the standard HC, but there are a raft of variants. It does depend a little on what you are driving, and what if any are your speed restrictions. There is a Motorola tech arcticle for using the FPM on a 68000 / 68008 as a peripheral chip.

http://www.google.co.uk/url?sa=t&rct=j& ... roV5VKxQM7


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Re: 74HCxx vs 74LSxx

Postby tcat » Wed Mar 07, 2018 5:24 pm

Hi PrOf,

Perfect, the document even mentions some macro definitions, that could help to provide FPU library. 74HCxx are used to drive chip select, would it fail to function with 74LSxx?

CIRCUIT.PNG
FPU-CPU peripheral wiring
CIRCUIT.PNG (8.54 KiB) Viewed 431 times

Many thanks.

Tomas


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Pr0f
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Re: 74HCxx vs 74LSxx

Postby Pr0f » Wed Mar 07, 2018 10:36 pm

I can't see why LS wouldn't work in that design - all those logic gates are only driving one load each, so LS will easily handle that. Speed should be ok at that clock speed. If you use sockets, it would be easy enough to swap out chips if you needed to.


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Re: 74HCxx vs 74LSxx

Postby Dave » Thu Mar 08, 2018 2:43 am

74HCT would seem to be the easiest fit for this.

Be aware that with the data bus size difference, you’ll probably want to latch half the data bus to present 16 bits to the copro. And the same in reverse. A good look at the Qubide design will show you one way to do it.


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Pr0f
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Re: 74HCxx vs 74LSxx

Postby Pr0f » Thu Mar 08, 2018 7:13 am

That's not needed on the 68881/2 as these chips can accept 8 and 16 bit data too, it's just a question of putting the correct signals in place on the chip so it knows how you are talking to it.

It was quite clever of Motorola to allow the FPU to be used as a peripheral as well, as it opens it's use up to the other family members and even other processors.

It made me wonder if it would be possible to come up with some 'graphics' instructions and potentially interface a graphics co processor to the 68K (020 and above) - it's a lot of work though.



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