digital logic Hold time of a D Flip Flop Electrical Engineering Stack Exchange December 23, 2020 – Posted in: Bookkeeping

latch setup and hold time

Time stealing is the concept of adjusting the clock phase at flip-flop2 so that data arrival time at the capture edge of flip-flop2 will not violate the timing constraints. Time stealing is used when specific logic partition needs additional time which should be deterministic at the start latch setup and hold time time. From a teaching point of view, SR latches drawn as a pair of cross-coupled components (transistors, gates, tubes, etc.) are often hard to understand for beginners. A didactically easier to understand way is to draw the latch as a single feedback loop instead of the cross-coupling.

  • Based on the level of triggering, it is of two types.
  • This circuit accepts and requantises quaternary logical currents during a SETUP clock mode and latches the input value during the HOLD clock mode.
  • Thus a gated D-latch may be considered as a one-input synchronous SR latch.
  • From a teaching point of view, SR latches drawn as a pair of cross-coupled components (transistors, gates, tubes, etc.) are often hard to understand for beginners.

The adjustment of the clock cycle for flip-flop2 as per the data arrival time is known as time stealing. The classic gated latch designs have some undesirable characteristics.


Using either terminology, the term “flip-flop” refers to a device that stores a single bit of data, but the term “latch” may also refer to a device that stores any number of bits of data using a single trigger. The terms “edge-triggered”, and “level-triggered” may be used to avoid ambiguity. But your interpretation and instincts are really on target. If the flip-flop’s setup time is 20 ns, it means that data has to be stable atleast 20ns before the capturing clock-edge. Similarly hold time is the amount of time, data has to remain stable after a clock edge has appeared. So together they define a “setup-hold-window”, in which data has to remain stable.

latch setup and hold time

Another example is hold time mentioned in negative seconds, There, the data need not be stable during clock transition. Since the data is changing within the setup time, and since setup time is a minimum amount of time before the clock that the data needs to be stable, it is impossible to tell whether the output will be a zero or a one. In fact it could even enter a metastable state and oscillate. The Timing Analyzer reports the result of clock setup checks as slack values. Slack is the margin by which a timing requirement is met or not met. Positive slack indicates the margin by which a requirement is met, and negative slack indicates the margin by which a requirement is not met. The Timing Analyzer determines clock setup slack as shown in Equation 1 for internal register-to-register paths.

Synchronizers and Metastability in Digital Logic Circuits

Setup time is the minimum amount of time the data input should be held steady before the clock event, so that the data is reliably sampled by the clock. Flip-Flops that read in a new value on the rising and the falling edge of the clock are called dual-edge-triggered flip-flops. Such a flip-flop may be built using two single-edge-triggered D-type flip-flops and a multiplexer as shown in the image.

How is wait time calculated in a call center?

It is easy to calculate the average wait time at a call center. You only need to add the total wait times for all answered calls and divide it by the number of answered calls. The metric should not be considered in isolation. Some callers may abandon the call before speaking to the live agent.

If you can understand above diagram, I can say that Latch can start sampling data from the rising edge itself and continue sampling till the respective falling edge . And Flipflop can only sample the data “at” Rising edge or Negative edge. Similarly, hold time of flip-flops are given in the timing library. Below is an example of hold times in a timing library. Well everything shwetarao said makes sense to me but iam interested in knowing whats the diffference b/w setup/hold time for a latch and a flip-flop as suggested by rakeshnunna and sree205. Yes a latch has setup and hold time in a way similar to f/f. Let us suppose the input transmission gate closes when only the first inverter has toggled its output and has reached pure voltage level .

CMOS quaternary latch

Browse other questions tagged digital-logic clock flipflop delay or ask your own question. If the data changes during the setup to hold window, then it’s likely that the output will be a solid 0 or 1, but the manufacturer makes no assertion as to which.

latch setup and hold time

The designer will take care that the next stage delay should be lesser than the difference between the clock period and phase shift. The method of borrowing time from the shorter paths of the next design stages to the bigger path is known as time borrowing.

Lockup Latches in Testing to fix Hold Failure and Clock Skew

Again, if the delay between phases is independent of frequency, hold time violations cannot be fixed by adjusting the clock frequency. If the flip-flop generally has up to 20ns of delay from the input to where the clock edge takes effect, then the output is most likely to end up being what the input was “around” 20ns before the clock edge, as your professor suggests. But the only thing that is specified for the device is that if the setup and hold times are met, then the output will follow the input. Recovery time is the minimum length of time an asynchronous control signal, for example, and preset, must be stable before the next active clock edge. The recovery slack time calculation is similar to the clock setup slack time calculation, but it applies asynchronous control signals. If the asynchronous control is registered, the Timing Analyzer uses Equation 7 to calculate the recovery slack time.

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