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THE RETREAT OF SILICON?

by Tom Cantrell

Start ı One Gate Too Far ı Layout Lament ı Sources and PDF

LAYOUT LAMENT

Using a one-gate chip isnıt only about a penny-pinching solution to a one-gate shortage. In fact, at 10ı to 20ı, the OG chips arenıt appreciably lower priced than the traditional multigate parts.

The real virtue of OG isnıt price per gate, but rather place per gate. This becomes apparent as you meander down the path of real-world design challenges. For instance, letıs say you need two NAND gates. A standard 4-NAND gate chip doesnıt cost more, so why bother with the OG option?

First, consider that two OGs, thanks to their tiny (2 ı 2.1 mm) SOT353 package, take up less board space than the typical multi-gate 14-pin SOIC; less than 10 mmı total for two OGs versus 50 mmı or more for SOIC.

Less board space is good, but less layout headaches are even better. How often are those extra gates you need positioned nicely next to each other? Isnıt it usually true that you need one here, one there, and another way over yonder?

You know the drill, you try to shuffle parts around to make space somewhere in the middle. Or, if thatıs too much hassle, you drop the extra chip wherever itıs easiest to make room and start running traces hither and yon. Of course, in doing so, you may create a bit of unwanted noise, but thatıs what all those scopes, analyzers, and emulators are for. What could be more fun than having to seek and destroy some last-second bugs and spin a new board rev with the boss breathing down your neck? It builds character, right?

Instead, OG allows you to use a point-of-pin (POP) approach to put the exact gate you need exactly where you need it (see Figure 2). It may not cost less, but less space, power, noise, and headaches make it a good deal.

Figure 2ıEven if you need four inverters, layout issues may make four OGs the right call, instead of the typical multi-gate ı04.

SMALL IS BEAUTIFUL

Most of the attention paid to the march of silicon will be focused on the mega-wonderchips proudly strutting in the front ranks. However, every parade needs somebody to bring up the rear. Donıt be surprised to see handy helpers like OGs happily coexisting with the fancier chips that get all the applause.

Remember that old saying about not putting all your eggs in one basket? Now, thanks to OG, you donıt have to put all your gates in one chip either.

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