Quiz: Modeling skew requirements with data-to-data setup and hold checks

Problem: Suppose there are 'N' signals which are to be skew matched within a window of 200 ps with respect to each other. Model this requirement with the help of data setup and hold checks.

As we discussed in data setup and data hold checks, data setup check of 200 ps means that constrained data should come at least 200 ps before the reference data. Similarly, data hold check of 200 ps constrains the constrained data to come at least 200 ps after the reference data. The same is shown pictorially in figure 1(a) and 1(b).

Figure 1(a): Data setup check of 200 ps constrains the constrained signal to toggle at-least 200 ps before reference signal toggles.

Figure 1(b): Data hold check of 200 ps constrains the constrained signal to toggle at-least 200 ps after the reference signal has toggled.

Now, suppose you apply a data setup check of -200 ps instead of 200 ps. This would mean that the constrained signal can toggle upto 200 ps after the reference signal. Similarly, a data hold check of -200 ps would mean that the constrained signal can toggle from 200 ps before the reference signal. If we apply both the checks together, it would infer that constrained signal can toggle in a window that ranges from 200 ps before the toggling of reference signal to 200 ps after the toggling of reference signal. This is pictorially shown in figures 2(a) and 2(b).

Figure 2(a): Negative data setup and hold checks of 200 ps

If we combine the two checks, it implies that the constrained data can toggle upto 200 ps after and from 200 ps before the reference signal. In other words, we have constrained the constrained signal to toggle in a window +- 200 ps within the reference signal.

Coming to the given problem, if there are a number of signals required to toggle within a window of 200 ps, we can consider one of these to act as reference signal and other signals as constrained signals. The other signals can then be constrained in both setup and hold with respect to reference signal such that all of these lie within +-100 ps of the reference signal. The same is shown in figure 3 below:

Figure 3: Data checks to maintain skew between N signals

Data checks : data setup and data hold in VLSI

Many a times, two or more signals at analog-digital interface or at the chip interface have some timing requirement with respect to each other. These requirements are generally in the form of minimum skew and maximum skew. Data checks come to rescue in such situations. Theoretically speaking, data-to-data checks are applied between two arbitrary data signals, none of which is a clock. One of these is called constrained pin, which is like data pin of the flop. The other is called related pin, which is like clock pin of the flop. In the figure below, is shown two data signal at a boundary (possibly analog hard macro) having some minimum skew requirement between them.

Figure 1 : Two signals arriving at a boundary having skew requirement

Data-to-data checks are zero cycle checks: An important difference between normal setup check (between a clock signal and a data signal) and data-to-data checks is that data-to-data checks are zero cycle checks while normal setup  check is single cycle check. When we say that data checks are zero-cycle checks, we mean that these are between two data signals that have launched at the same clock edge with respect to each other.

As shown in the figure (i) below, traditional setup check is between a data signal launched at one clock edge and a clock. Since, the data is launched at one clock edge and is checked with respect to one edge later, it is a single cycle check. On the other hand, as shown in figure (ii), data-to-data check is between two signals both of which are launched on the same clock edge. Hence, we can say  data-to-data checks are zero cycle checks.

 

Figure 2 : (i) Normal setup check between a data signal and a clock signal, (ii) Data to dat setup check between two data signals

What command in EDA tools is used to model data-to-data checks: Data checks are modeled in EDA tools using ‘set_data_check’ command.

                Set_data_check –from A –to B –setup <x>

                Set_data_check –from A –to B –hold <y>

Here, A is the related pin and B is the constrained pin. The first command constrains B to toggle at least ‘x’ time before ‘A’. Second command constrains ‘B’ to toggle at least ‘y’ time after ‘A’.

Data setup time and data hold time: Similar to setup time and hold time, we can define data setup time and data hold time as follows:

• Definition of data setup time: Data setup time can be defined as the minimum time before the toggling of reference signal for which constrained signal should become stable. In the example above, <x> is data setup time.
• Definition of data hold time: Data hold time can be defined as the minimum time after the toggling of reference signal for which constrained signal should remain stable. In the example above, <y> is data hold time.

Modeling data-to-data checks through liberty file: We can model data checks through .lib also. There are constructs that can be used to model data-to-data checks. These are non_seq_setup_rising, non_seq_setup_falling, non_seq_hold_rising and non_seq_hold_falling. These commands specify setup and hold data-to-data checks with respect to rising or falling edge of reference signal respectively. E.g. ‘non_seq_setup_falling’ represents data setup check with respect to falling edge of reference signal. ‘rise_constraint’ and ‘fall_constraint’ can be used inside these to model the setup and hold checks for rising and falling edge of constrained signal. Figure 3 below shows an example of modeling data setup check through liberty file.

Figure 3 : Modeling data-to-data checks through .lib using non_seq_setup_rising

In which cases data-to-data checks are applied: Data checks are normally applied where there is a specific requirement of skew (either minimum of maximum) or race condition (where the order of arrival of two signals can affect output and the intention is to get one of the probable outputs by constraining one signal to come before the other) between two or more signals. These may be required where:

• At the digital-analog interface within a chip where analog signals at the analog block boundary are required in a specific order

• At the chip boundary, some asynchronous interface signals may have skew requirements

How data checks are useful: As we have seen above, data checks provide a convenient measure to constrain two or more data signals with respect to each other. Had these checks not been there, it would have been a manual effort to check the skew between the two arriving signals and to maintain it. Also, it would not have been possible to get the optimization done through implementation tool as these would not have been constrained for it.

References:

1.  How to constrain the data skew check in the .lib ? - http://tech.tdzire.com/how-to-constrain-the-data-skew-check-in-the-lib/ 
2. Static timing analysis for nanometer designs – a practical approach by J Bhasker and Rakesh Chadha

Also read:

• Setup time and hold time - static timing analysis
• False paths - what are they
• Propagation delay
• What is static timing analysis