Electronics > Beginners
Best practices for glue logic for surface mount components?
MrD:
I'm designing my first surface mount board (used to through-hole hand soldered projects) and I've got a question about glue logic.
In my schematics, I've got my signals from node to node combined through logic gates: address decoding, device selection, transceiver direction selection etc. My first project has a few 3-input OR gates, some 2-input NANDs, some 2-input ANDs, and runs at 3.3V.
Is it best to implement these with single gate devices like 74LVC1G00GW,125, minimizing trace length as much as I can, resolving signals into their final useful form as early and as locally as possible, etc? Or should I be trying to look for surface mount versions of the multi-gate 74 series devices and minimizing the component count instead of having lots of small gates dotted about the board?
Advice or links to reading material appreciated :)
tautech:
Routing for multi gate SMD packages is always the issue as you don't have the room to slip traces between pads like you do with DIP however reassigning gates to best suit routing can be the savoir with SMD.
Using multi gate packages will certainly keep your board more compact if you're prepared to spend the time on routing.
I break it down to circuit blocks and then arrange them to get best routing and when that doesn't work reassign gates to get the result required. I'll even drop in a TH resistor so to provide pathways for routing but you can do the same with big SMD resistors or zero ohm links, even a wire jumper if you're desperate.
T3sl4co1l:
Just use however many you need. If the required gates pack into quads effectively, go for quads. Note you can get duals of 2-in's and triples of 1-in's in the "tiny" formats. (And maybe there's duals of 3-in's and triples of 2-in's, I don't recall offhand.)
Singles are irritating for the relative wasted space and extra power/ground routing (which goes away if you're using 4-layer with VDD/VSS internal layers) so strike a balance between both extremes.
Tim
MrD:
Thank you both, I'll let you know how it goes. It did seem strange to be able to select single gates, but if that's how it goes... :)
NivagSwerdna:
As above. Try and minimise number of packages unless there are cost reasons not to. Where you have unused gates follow the datasheet rather than just let them float.
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