Many kinds of sequential logic require that the output of one register be fed into the input of another register which is strobed by the same clock. In such logic, it's necessary to ensure that a changed output from the first register cannot affect the input of the second until after the hold time of the second register has been satisfied. If data flows in one direction, it may be possible to delay the clock of the downstream device, but if data needs to flow bidirectionally it may be difficult to ensure that all devices have sampled their data before any changes can percolate to any device's data input; getting reliable behavior from high-speed logic devices would require sharp clock edges even when using very slow data rates.

It would seem that in many cases this problem could be eased considerably if flip flops' clock inputs defined two distinct levels with non-overlapping tolerance bands: a "sample" level (perhaps 15%-30% VDD) and a "propagate" level (e.g. 45%-60% vdd), such that setup time would be required before the clock signal reaches 15% VDD, hold time would be measured after it reaches 30% VDD, minimum propagation would be measured after it reaches 45%, and maximum propagation after it reaches 60%.

Such specifications would ensure that provided every device sees the clock signal reach 30% of VDD before any device sees it reach 50% of VDD, then setup/hold times would automatically be met for any devices with a zero hold time; positive hold times could be met by limiting the rate of rising clock edges.

It would seem that this approach should allow system designers great flexibility in balancing clock edge speed with clock rate requirements, and would allow much better noise immunity than Schmitt trigger inputs (unless all registers are clocked by the output of the same Schmitt trigger, they may switch at different times, and even when using one Schmitt trigger, logic may get sunk if a fuzzy clock signal makes the output go metastable). Are there any logic families that use such behavior to improve the performance and reliability of their edge-triggered logic functions?

  • \$\begingroup\$ I don't think any current logic families do this, but if you search I think you can find some similar ideas executed by fiddling with complementary transistor sizes on the chip. \$\endgroup\$ Sep 7, 2015 at 20:45

1 Answer 1


I dunno what the modern chips do internally, but for instance the 8080 used a non-verlapping two-phase clock:

enter image description here

I think the problem with your approach is

  • the levels at which CMOS logic is active is not easily controlled, and/or controlling it accurately would put extra demands on other aspects (geometry, size, yield, etc.)

  • there is not realy a level at which a FF samples the signal, rather there is a level 1 at which the first (master?) latch stops sampling, and a level 2 at which the second (slave?) latch starts sampling the master's output. When level 1 << level 2 (wide band between them) and the clock edge is not too steep you automatically have a 'deadband' as you want.

  • \$\begingroup\$ As I understand it, an FF has one latch which samples the input as long as the clock is "low", and a second latch which samples the output of the first when the clock is "high". Many chips effectively produce a two-phase non-overlapping clock, but do so by using delay circuits rather than by using buffers that are triggered by separate input logic levels. While use of two-phase clock signals seems to be regarded as a remnant of the 1970s, I would think that it would solve a whole lot of difficulties related to clock noise, skew, etc. and that it shouldn't be too hard to... \$\endgroup\$
    – supercat
    Sep 7, 2015 at 20:15
  • \$\begingroup\$ ...effectively use a single wire to encode both clock phases (above upper threshold means phase 1 high, phase 2 low; below lower threshold means phase 2 high phase 1 low; between thresholds would mean both phases low). Some devices specify Schmitt-trigger clock inputs, but I would think split levels for primary/secondary phases would be in just about every way better. \$\endgroup\$
    – supercat
    Sep 7, 2015 at 20:24
  • \$\begingroup\$ But you (almost) never can have something for nothing. Accurate split levels would take up some resources. Aparently chip engineers thing those resources are better used in otherw ays. \$\endgroup\$ Sep 7, 2015 at 20:37
  • 1
    \$\begingroup\$ There is also "domino logic" \$\endgroup\$
    – TEMLIB
    Sep 7, 2015 at 20:46

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.