SETUP Time and SETUP Violation in a Single D Latch

latch setup and hold time

This time is such that it goes into the memory of latch; i.e., before input transmission gate closes, Data should traverse both the inverters of the loop. So, setup time of the latch involves the delay of input transmission gate and the two inverters. Figure 5 below shows the setup time for the latch. A comprehensive static timing analysis includes analysis of register-to-register, I/O, and asynchronous reset paths.

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This series we are starting for Latch based Timing Analysis. In case of Latch, there are lot of basic concepts which are similar to Flipflop based Timing but still we get confuse a lot of time, I am going to try my best to clarify that. To understand more about flip-flop schematic and operation, read here. Therefore we must ensure that no data arrives during that interval.

What is the setup time and hold time for a latch?What is the setup time and hold time for a latch?

The reason is that D must arrive at Z before the clock goes high switching OFF the input transmission gate and ON the second TG so that the new data can correctly be latched in the master latch. Therefore, D must be stable long enough to travel from the input through the three inverters/input transmission gate and reach Z before the clock goes high. If D arrives too late only at A before the input transmission gate is OFF, the new data has not even arrived inside the latch so that the new data lost is obvious. If D arrives late and only reaches node B or C in the master latch before the rising edge of the clock, node Z still stores the previous data and the contention between new data at B/C and previous data at Z occurs at the master loop. That’s why setup time is required so that the new data can be reliably stored in the flip-flop without metastability.

latch setup and hold time

The Timing Analyzer removal time slack calculation is similar to the clock hold slack calculation, but it applies asynchronous control signals. If the asynchronous control is registered, the Timing Analyzer uses the equations shown in Equation 9 to calculate the removal slack time. Another min-delay problem occurs between ϕ2 transparent latches and ϕ1 pulsed latches or pulsed domino latches.

How to fix the hold timing check?

That is, for some ICs you can actually release the data before the clock edge arrives. But there remains a window in which the data must be stable. That some of the hold values in the fig are negative. This normally happens when the path from the pin of the Flop to the internal latch point for the data is longer than the corresponding path for the clock. Thus a negative hold timing check implies that the data pin of the flop can change before the clock pin and still meet hold time check. The setup time for a sequential cell is the minimum length of time during which the datainput signal must remain stable before the active edge of the clock to ensure correct functioning of the cell.

  • With this method and the appropriate parameters of degradation, we characterize point B in Figure 4, whereas by adding fixed margin to the determined setup and hold time, the pessimistic point A of Figure 4 is obtained.
  • Are most likely to occur in the first flip-flop of subcircuit c.
  • Formalisms and tools are meant to include automata theory, HDLs, RTL models, gate-level netlists , simulation software, and automated test equipment .
  • It can be removed or reduced by changing the frequency of the clock.
  • The analysis in digital domain, in Reg to Reg system is very popular but the root cause of Setup and Hold time is often not taken care of in the education system.

In addition to tolerating clock skew, good systems minimize the skew that impacts each path. By considering the components of clock skew and dividing a large die into multiple clock domains, we can budget smaller amounts of skew in most paths than we must budget across the entire die. For example, if clock skew were greater than a quarter cycle, min-delay problems could occur between ϕ1 and ϕ2 transparent latches. Because it is very difficult to design systems when the clock skew exceeds a quarter cycle, we will avoid such problems by requiring that the clock have less than a quarter cycle of skew between communicating elements. The setup values of a FF can also be negative and this means that at the pins of the flip flop, the data can change after the clock pin and still, meet the setup check. The setup and hold timing constraints on the input pin D with respect to the rising edge of clock CK of the FF. If data makes transition within this setup window and before the hold window, then the flip-flop output is not predictable, and flip-flop enters what is known as meta stable state.

Clock Setup Check

So-called metastable-hardened flip-flops are available, which work by reducing the setup and hold times as much as possible, but even these cannot eliminate the problem entirely. This is because metastability is more than simply a matter of circuit design. When the transitions in the clock and the data are close together in time, the flip-flop is forced to decide which event happened first. However fast the device is made, there is always the possibility that the input events will be so close together that it cannot detect which one happened first. It is therefore logically impossible to build a perfectly metastable-proof flip-flop. Flip-flops are sometimes characterized for a maximum settling time .

  • These timing checks verify the data input is stable before and after the clock.
  • It is therefore logically impossible to build a perfectly metastable-proof flip-flop.
  • The solutions proposed by Dolphin Integration provide the best compromise between circuit speed and reliability.
  • After this, the EDA tool will be able to time the two segments as one complete path.
  • That is the clock pulse doesnot suddenly disappear at falling edge arrival time or doesn’t suddenly appear at the rising edge arrival time due to slew tranistion time.
  • A gated SR latch circuit diagram constructed from AND gates and NOR gates .
  • I suppose this would be analogous if L2 and L4 were negative latches all on a common clock.

At 4ns, the transparency ends so the data at the input needs to be stable before and after the edge at 4ns. I think the above cases are the very worst case, but Tempus or PrimeTime or the like should give back some credit if time borrowing was not needed. The image near the middle of this page shows 4 positive latches, with L2 and L4 connected to the inverted clock. I suppose this would be analogous if L2 and L4 were negative latches all on a common clock.


Latch have recovery and removal times but not setup and hold time. In reality, the timing when the latch is enabled is the same as if the latch were simply a transparent delay element (Figure 5.) Figure 5. When the latch is enabled, it essentially becomes a passive delay. In an ideal scenario, time given to the startpoint should be equal to the time borrowing of the latch. The operation relies on the present and past input bits along with the past output and clock pulses. The operation depends on the present and past input along with the past output binary values.

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The clock transition used is determined by the timing_type which specifies whether the cell is rising edge-triggered or falling edge-triggered. The hold time for a sequential cell is the minimum length of time during which the data-input signal must remain stable once the edge of the clock is activated to ensure correct functioning of the cell. Hold time is the time for which the data for the next clock cycle shouldnot arrive or when put in other way, it is the time the data for the previous clock cycle has to be held for it to be latched. A violation of hold time occurs when the data intended for the next cycle arrives early and there is a probability for the latch to store this data for the current cycle . A lock-up latch is a transparent latch used to avoid large clock skew and mitigate the problem in closing hold timing due to large uncommon clock path. Setup time is the amount of time required for the input to a Flip-Flop to be stable before a clock edge.

Difference between lockup latches and lockup registers.

The capacitances that create hold time requirements can be in multiple places. It can be in the clock buffers, perhaps delaying the clock to state holding circuits. Or it can be in transistors preventing or enabling feedback to hold the state. The hold time needed for most of the will be mentioned as 0 seconds. It doesn’t mean the devices are infinitesimally faster but they have logics which doesn’t need the data to be stable after clock edge. It’s only likely to be a solid 0 or 1, it’s not guaranteed. This means there could be a finite and unpredictable extra time, beyond the normal propagation delay quoted in the data sheet, for which two gates driven by this output could make different decisions about whether it was a 0 or 1.

Assume that hold time, set-up time and propagation delay time are negligible. Complete the timing diagram below for an SR latch 2.

Similar to Clk-to-q delay, library setup and hold time

You can argue that non-inverting paths should be allowed. The same argument is valid for combinational loopback as well. Very good question, yes latches are also prone to metastability similar to flip-flops. You can go through below post to understand the concept.

latch setup and hold time

FREE 60-day trial to the world’s largest digital library. The design and application of a “flip-flap-flop” using tunnel diodes (Master’s thesis). Alternatively, the restricted combination can be made to toggle the output. If S is pulsed high while R is held low, then the Q output is forced high, and stays high when S returns to low; similarly, if R is pulsed high while S is held low, then the Q output is forced low, and stays low when R returns to low. Transitioning from the restricted combination to leads to an unstable state. I have been unable to find detailed timing specs in the datasheet, app notes, forums, or internet search. Making statements based on opinion; back them up with references or personal experience.

Which is the maximum time we can borrow from the latch?

If we want the data not to propagate to Out, we have to ensure that it does not cross input transmission gate. So, Data should not be latch setup and hold time present at the transmission gate’s input at time (T – T). In other words, it has to be held stable this much time after CLK edge.

  • An exception is that some flip-flops have a “reset” signal input, which will reset Q , and may be either asynchronous or synchronous with the clock.
  • These lockup latches are used in scan-based designs, i.e., in between to scan flip flops which have large probability of hold failure.
  • The reason is that D must arrive at Z before the clock goes high switching OFF the input transmission gate and ON the second TG so that the new data can correctly be latched in the master latch.
  • Recently, some authors reserve the term flip-flop exclusively for discussing clocked circuits; the simple ones are commonly called transparent latches.
  • Scan flip-flop , icon , and overall circuit structure .
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