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From my understanding, most modern consumer CPU's are based on synchronous logic. Some high-speed applications (signal processing, etc.) use ansync logic for its higher speed.

However, in today's market, speed in consumer products is one of the main selling points (see AMD vs Intel.) Is the development of more complicated lithography faster than adoption of fully asynchronous logic? Or is ansync logic too complicated/impractical for VLSI applications?

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closed as primarily opinion-based by Elliot Alderson, pipe, Finbarr, Dmitry Grigoryev, Lior Bilia Feb 6 at 15:59

Many good questions generate some degree of opinion based on expert experience, but answers to this question will tend to be almost entirely based on opinions, rather than facts, references, or specific expertise. If this question can be reworded to fit the rules in the help center, please edit the question.

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    \$\begingroup\$ I think it is very hard to program them in a procedural manner. \$\endgroup\$ – mehmet.ali.anil Feb 4 at 16:33
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    \$\begingroup\$ The current architectures are hybrid. Synchronous blocks are interconnected in asynchronous way. And yes, asynchronous logic is much more complicated. \$\endgroup\$ – Eugene Sh. Feb 4 at 16:34
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    \$\begingroup\$ Because we have no good tools yet for asynchronous timing closure. \$\endgroup\$ – Oldfart Feb 4 at 16:37
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    \$\begingroup\$ Systems are becoming increasingly asynchronous. On modern CPUs things like RAM, cache, and the CPU cores all run with independent clocks. The new Intel HEDT processors take this a step further and allow each of the many cores in the chips to run at clocks independent from each other and from the RAM, cache, and external busses, etc. \$\endgroup\$ – J... Feb 4 at 17:59
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    \$\begingroup\$ One thing to remember, synchronous clocks introduce a standardized heartbeat for the system, whereas if you go with a pure async design, otherwise trivial timing differences like propagation delay may have to be accommodated through some other means. Also it is relatively efficient to implement pure functions in async designs, but stateful functions (or harder operations like square root) can get complicated. Routing signals can also be difficult (no buses, you may get stuck connecting everything to everything). \$\endgroup\$ – jrh Feb 4 at 23:22
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I spent some years in a startup commercialising async design technology, so I'm familiar with the reasons:

  • async isn't intrinsically faster. The worst-case path delay remains the same. It's just that sometimes you get to take advantage of a faster path executing.

  • async has overhead of completion detection too.

  • Design tools. This is the really big one: there isn't really a full async "flow" of tools available to the same quality as synchronous design.

  • Training. You'd effectively have to retrain all your designers on the new paradigm and tools.

  • Risk and conservativism. So much of the industry is "produce something similar to the last one, but a bit different". This has a very high chance of working. Companies are much more reluctant to build something totally different since it has much more chance of being a total writeoff costing tens of millions of dollars.

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  • \$\begingroup\$ It could be intrinsically cheaper but slower. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Feb 4 at 18:00
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    \$\begingroup\$ I remember thinking about this once, and also realized: A synchronized chip is trivially underclocked (for battery/temp/etc), whereas an asynchronized chip is probably less so? \$\endgroup\$ – Mooing Duck Feb 4 at 21:09
  • \$\begingroup\$ In terms of design tools/paradigm, synchronous logic is able to totally ignore glitches in output the combinatorial logic, and only require the clock signals to be glitch free. Also asynchronous circuits are more likely to require adding additional delay for some signals, to ensure a ready signal cannot arrive at the next block before the data arrives, etc. In synchronous circuits artificial delay is rarely needed. One of the few cases to spring to mind is to satisfy hold time requirements, which can often be satisfied by natural delay. Also some cross clock domain situations can need delay. \$\endgroup\$ – Kevin Cathcart Feb 4 at 21:48
  • \$\begingroup\$ I would think that in many situations using a two-phase non-overlapping clock could offer many advantages over a single clock. When using a single clock, any runt pulses can cause a system to jump the rails, but when using a two-phase clock generated by feeding a clock input through two comparators, runt pulses on a clock phase would be harmless if preceded or followed by a valid-length clock pulse without an intervening pulse on the opposite clock phase. \$\endgroup\$ – supercat Feb 5 at 16:46
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    \$\begingroup\$ @MooingDuck It depends on the logic paradigm. I worked on some Null Convention Logic, and it would run as fast as the conditions (process, voltage, temperature) would allow. You wouldn't need to under-clock it because of those conditions, it would do it automatically. \$\endgroup\$ – W5VO Feb 5 at 19:19
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It is very tempting to design asynchronous integrated circuits. The other answers already cover many reasons to think twice before doing it. Here is one more:

IC development is not finished with the design. Verification and test are equally important. Not only the design tools are very advanced for synchronous circuits, but it is the same with simulation tools and test equipment.

Verification

It is not sufficient to have the circuits working at lab conditions. They need to be robust with respect to the operating voltage (V) range, the operating temperature (T) range, and the variation due to the manufacturing process (P). For synchronous logic this can be guaranteed with the help of static timing analysis. The circuit is broken down to all timing paths, from flip-flop to flip-flop. Setup and hold times are checked for every single timing path, and for different combinations of P, T, and V. These PTV combinations are the so called simulation corners.

A similar verification could be done for asynchronous circuits, but it is much more difficult and much less supported by the design tools. It also restricts the designer to asynchronous constructs that actually can be verified. There is no reliable verification for arbitrary asynchronous circuits.

Test

Similar difficulties are there when it comes to testing the hardware. Testing synchronous logic is fully supported by testing standards and equipment. Testing asynchronous circuits not only is more complicated, but because of the lack of timing abstraction, it is not even sufficient to prove that the circuit will work for all PTV corners. The circuit could fail due to race conditions at some PTV combination, that is not covered by the corners.

Summary

IC designers have not given up the asynchronous paradigm, but asynchronous logic comes with heavy disadvantages during verification and validation. In an industrial context, asynchronous IC design needs to be restricted to construct that can be proven to work over the whole parameter space of process variation, as well as the operating ranges for temperature and voltage.

The so called "Locally synchronous globally asynchronous" design is one way to get more benefits and less disadvantages of both timing paradigms.

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Async binary counters are simpler because they only use 1 memory cell or T flip flop per divide by two. Hence the old CD and 74HC4020 and 4040 offer many binary stages cheaply. The prop delay in each stage means it cannot be used without race conditions or glitches with gate decoding of the binary addresses unless the prop delay is less than 1/2 input clock cycle using the trailing edge to latch the result. The output latency is then multiplied by N stages.

Synchronous binary counters use an extra memory cell to D FF to delay but minimize the delay to 1 value for any length of counters so it consumes more area.

Hence all CPU’s use complementary clocks to optimize expected latency in address and memory read /writes to maximize thruput but not exceed prop delay, setup and hold times.

Memory now uses many phases such as DDR, 3DR, 4DR, 5DR especially for graphics memory but with CPU clocks going much faster than the single cycle RAM rate so that read and write address delays may be timed by single or multiple or half counts of the superclock ( eg 100MHz xN) designated by T fractional or integers counts for each parameter. These prop delays increase with temp. For CMOS and reduce with higher Vram voltage which if properly cooled might reduce latency or other increase Pd and temp rise and make it worse (slower). So cooling, V, f, T all are critical for optimal latency whether it is used for Async or Sync operations.

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Early logic designs were asynchronous. Often times they were mixed with analog circuits too. However, as time progressed designs became more complicated, thus demanding more engineers to work on a single design. The timing diagrams of async. logic can be cumbersome when there are a lot of states. Now add a team of engineers trying to interface different blocks of logic together but the timings between them are not aligned very well. The designs become brittle to changes in unforeseen ways.

Enter synchronous logic design. Now all the state flows are aligned to a clock signal that allows the interfaces to be very well defined, and more robust against design changes. Yes, it might use more gates, but it is (usually) stable by design; as opposed to stable as long as no one touches it!

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    \$\begingroup\$ There are asynchronous design methodologies as well that guarantee "correct by design" and "well-defined interfaces." These are simply different from the simplifying assumptions made for synchronous designs. \$\endgroup\$ – Edgar Brown Feb 5 at 19:11

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