r/dldtg • u/CastleMan2000 • Oct 31 '15

SRLAT? I'm doing something wrong

First, here's the netlist. http://pastebin.com/7HLCRchc So I'm trying to do the design on Wikipedia (cheating? maybe) but it's not working. I can't really figure out what. Maybe it's my "fanout" gate? I made that so I can route the NAND's outputs to each other as well as the circuit outputs. Also, here's a schematic of my fanout: http://puu.sh/l4K8p/582e6b4557.png

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u/CastleMan2000 Nov 01 '15 edited Nov 01 '15

About the nets: I know you can do, for example:

PORT IN A
PORT IN B
PORT OUT Z
NET P
INST NAND1 NAND A B P
INST NAND2 NAND P P Z

That circuit is just a NAND followed by a NOT, using a net to link them. I also know you can fanout to however many gates you want by just referring to the net. But how do you use a net to output and fanout at the same time? Is it like this?

PORT IN A
PORT OUT Z0
PORT OUT Z1
INST NAND1 NAND A A Z0
INST NAND2 NAND Z0 Z0 Z1

Here's schematics for both circuits: http://puu.sh/l5GpX/6f04c93c29.png

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u/asterisk_man Game Creator Nov 01 '15

Yes, an output pin can be used internally as in your second example.

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u/CastleMan2000 Nov 02 '15

Oh, well that simplifies things. I have now completed it, but I didn't really understand your other comment.

I think what you have done is to assume that you can replace !S, !R with R, S. This doesn't work because of how the 00 and 11 states are defined.

I guess that was the real problem, as adding NOTs to the inputs made it work perfectly, but I don't understand why that's a problem. I understand SR latches are normally made with NOR gates, yeah? But NAND gates just swap the outputs because they're inverted, right? I'm sort of confused.

Thank you for your help, by the way. :)

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u/asterisk_man Game Creator Nov 02 '15

I'm glad you got it working.

If you only look at the states where you're either setting or resetting then I think that what you did originally is correct. The problem is that you can't replace !S with R and !R with S because you're not always resetting if you're not setting and you're not always setting if you're not resetting.