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I read that the propagation delay of CMOS logic gate is longer than TTL; therefore they are not used where speed of processing is important. I thought computers or computer cpu is using CMOS. According to the text I read it says CMOS is not used in computers. Is that true?

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  • \$\begingroup\$ Could you give the name of the book? I suspect this book dates back to Stone Age of computers. Or, maybe, you misinterpret the text in some way? \$\endgroup\$ – Vasiliy Nov 7 '13 at 17:45
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First off, take that book, douse it in gasoline/petrol and burn it. Secondly, saying that something is CMOS is akin to saying that something is a circuit. There are so many variants ... Even 25 to 30 years ago CMOS ASIC flows (Gate arrays, or Sea of Gates) approaches were already way faster than any of the discrete forms of logic (AS, LS etc.). You first saw discrete logic chips that then integrated more transistors, these became known as MSI (Medium Scale integration) designs (like UARTS etc.) that may have been built on similar CMOS processes. But by the time that LSI (large scale integration) designs started to come out the processes for discrete and integrated CMOS had bifurcated and diverged. Probably at the 3 um node LSI and VLSI (Very large scale integration) design never looked back. Microprocessor processes (also CMOS) had their own design methodology and techniques.

Even at the time of publishing that book had it wrong. We were in the midst of the latest 0.13um process technology, worrying about scaling effects and yield in these new radically smaller transistors. Going from 2 um to now 20 nm (arms fully waving now) which is a factor of 100X and a scaling according to Moore's law of sqrt(2) roughly translates into 14 generations of scaling /changes.

Just to give to perspective, in 0.18 um process technology you can build a pretty sweet op-amp that has a unity gain bandwidth of about 1.2 GHz (0.8 ns) which is good for 14 bits SNR. This is CMOS technology that dates back to the late 1990's.

If I recall correctly, you could build 2 GHz logic designs in 0.13um processes with full clock trees and fan out quite nicely.

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It's CMOS, and has been for years; there was an intermediate process NMOS which was used up to the early 80s, but everything modern is now CMOS. Your text is very out of date.

TTL does not miniturise as well and has much higher power consumption. So the more transistors you have the worse TTL does. Modern designs are to a great extent power-limited.

Conversely, discrete logic chips contain only one or two transistors between input and output, and are driving the comparatively large capacitance of pads and PCB traces, so TTL and unusual processes like AS TTL are faster.

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The propagation delay of CD4k gates is greater than that of TTL. The propagation delay of modern CMOS designs, e.g. AC, LVC, rivals everything short of advanced Schottky TTL (and to be fair, a few low-voltage CMOS families are actually faster than AS).

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  • \$\begingroup\$ so what is used in PC computers? \$\endgroup\$ – user16307 Nov 7 '13 at 10:55
  • \$\begingroup\$ @user16307: CMOS. Freescale tried a bipolar PPC CPU once, but it wasn't found to be much faster than the current CMOS designs, and it used considerably more power. \$\endgroup\$ – Ignacio Vazquez-Abrams Nov 7 '13 at 10:57
  • \$\begingroup\$ but look at this link at the bottom of page 40 (it is a book published and says CMOS is not used in computers): books.google.dk/… \$\endgroup\$ – user16307 Nov 7 '13 at 11:01
  • \$\begingroup\$ @user16307: They're definitely talking about CD4k there (on page 40). \$\endgroup\$ – Ignacio Vazquez-Abrams Nov 7 '13 at 11:03
  • \$\begingroup\$ When was the first edition of the book published? \$\endgroup\$ – pjc50 Nov 7 '13 at 11:05

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