I'm interfacing to a microprocessor system, and purposefully not using programmable logic. Feel free to call me a masochist. There's a lot of address decoding etc. happening and I may be able to cut costs and save board space if I use some diode logic. I'd probably also be saving in propagation delay — some of the gates are three levels of cascaded 74HC32s or 74HC08s involving 8 to 16 inputs.

The functions I'm considering replacing sit between HC (possibly some AC) family ICs, all running at 5V. They always sit between an HC IC and another HC IC (usually a '138), so signal restoration shouldn't be an issue. If necessary, I can change those to HCT to give more leeway. They're going to be running at clock periods of 500ns to 250ns at most (that's 2–4 MHz).

Can I use wired gates with relative impunity?

Do you have a suggestion on the type of diode to use (ideally available as through-hole and surface-mount), or shall I go with my all-time favourite, the 1N4148?

  • 2
    \$\begingroup\$ 70 ns (40MHz) is really not a low frequency for wired logic and it's though also for HC logic. Consider parasitic capacitances that come using discrete diodes and eventually transistors. \$\endgroup\$
    – clabacchio
    Feb 5 '12 at 10:31
  • \$\begingroup\$ Er, I apologise for the braino. I have NO clue how ‘70ns’ came about. I think my Sunday morning coffee was far weaker than it should be, because the number is completely unrelated to what I was thinking. Go figure. The actual clock period is somewhere in the order of 250-500ns. I'll update the answer. \$\endgroup\$
    – Alexios
    Feb 5 '12 at 11:01

The main problem with diode logic is slow rise time due to the (relatively) weak pull-up in combination with the ever-present stray capacity in your circuit (assuning a wired AND circuit). The use of a buffer transistor might get you into slow switching due to saturation. I don't think you will get into (addiotional) trouble from the less-than-perfect switching characteristics of your diode.

If you want to cut PCB size consider using some of the very small one-gate-per-package chips. OTOH, if you realy are a masochist, go for all the trouble you can find! But that does not match with asking here...

So impunity: NO. Check your timing margins, if you have a few us to spare I guess brew-it-yourself logic might work. Below 100ns I would not try it.

  • \$\begingroup\$ Thanks! I knew There Ain't No Such Thing As A Free Lunch, but it was worth asking. I love the 74x1G ICs, but they're not much use on 2.54mm prototyping boards so I'd like to avoid using them until I have PCBs made. Clearly, even masochism has its limits. :) \$\endgroup\$
    – Alexios
    Feb 6 '12 at 14:35

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