Environmental constraints

14.4. Environmental constraints

Both DRC and optimization constraints follow environmental constraints. Setting up of operating conditions and wire load model falls under environmental constraints.

Timing Constraints

14.3. Timing Constraints

These constraints specify clock related definitions which affect synthesis and timing analysis.

Optimization constraints

14.2 Optimization constraints

Three types of optimizations are possible-area, power and timing. We have optimization constraints related to all these. Synthesis tools assign higher priority to timing constraints over area and power constraints.

Constraints: Clock, Logical DRC, Area, Power

14.    Constraints: Clock, Logical DRC, Area, Power

 

Design constraints are generally specified in “Synopsys Design Constraints” (SDC) form. SDC is very widely used and industry accepted standard for specifying design constraints.

Three types of constraints can be set for the design. They are:

1) Logical DRC constraints
2) Optimization constraints
3) Environmental constraints.

Setup and hold slack

13.    Setup and hold slack

Slack

Slack is defined as difference between actual or achieved time and the desired time for a timing path. For timing path slack determines if the design is working at the specified speed or frequency.

Setup and hold time definition

12.    Setup and hold time definition

Setup and hold checks are the most common types of timing checks used in timing verification. Synchronous inputs (e.g.  D)  have Setup, Hold time specification with respect to the clock input. These checks specify that the data input must remain stable for a specified interval before and after the clock input changes

 

Ø  Setup Time:  the amount of time the data at the synchronous input (D) must be stable before the active edge of clock

Ø  Hold Time: the amount of time the data at the synchronous input (D) must be stable after the active edge of clock.

Both setup and hold time for a flip-flop is specified in the library.

Fundamentals of Timing

1.    Fundamentals of Timing

11.1. Timing paths

Any digital circuit can be represented as a “timing path” modeled between two flip flops.

 

Setup Time and Hold Time-Story of Poor Flip-Flop !

It is always interesting to talk about setup and hold!! Don’t think that if anybody asks questions related to setup time and hold time, he or she doesn’t know about setup and hold. He or she may know everything about setup time and hold time, time being it confuses. The term “setup” and “hold” is such a word in this VLSI – ASIC design world which only creates continuous questions, hard to explain in words, at least i myself is concerned! I remember, during my MTech days my professor used to say always "whole VLSI world is depending on two pillars, setup time and hold time". It would be more realistic if i say that he used to scold us !!

Timing paths

Timing Path

Timing path is defined as the path between start point and end point where start point and end point is defined as follows:

Start Point:

All input ports or clock pins of a sequential element are considered as valid start point.

End Point:

All output port or D pin of sequential element is considered as End point.

Transition Delay and Propagation Delay

Transition Delay

Transition delay or slew is defined as the time taken by signal to rise from 10 %( 20%) to the 90 %( 80%) of its maximum value. This is known as “rise time”.

Similarly “fall time” can be defined as the time taken by a signal to fall from 90 %( 80%) to the 10 %( 20%) of its maximum value.

Transition is the time it takes for the pin to change state.

Setting Transition Time Constraints

The above theoretical definitions are to be applied on practical designs. Now, the transition time of a net becomes the time required for its driving pin to change logic values (from 10 %( 20%) to the 90 %( 80%) of its maximum value). This transition time used foe delay calculations are based on the timing library (.lib files).

Transition related constraints can be provided in Design Compiler (logic synthesis tool from Synopsys) by using below commands:

1. max_transition : This attribute is applied to each output of a cell. During optimization, Design Compiler tries to make the transition time of each net less than the value of the max_transition attribute.

2. set_max_transition: This command is used to change the maximum transition time restriction specified in a technology library.

“This command sets a maximum transition time for the nets attached to the identified ports or to all the nets in a design by setting the max_transition attribute on the named objects.

For example, to set a maximum transition time of 3.2 on all nets in the design adder, enter the following command:

set_max_transition 3.2 [get_designs adder]

To undo a set_max_transition command, use the remove_attribute command. For example, enter the following command:

remove_attribute [get_designs adder] max_transition”

(Directly quoted from Design Complier user manual)

Setting Capacitance Constraints

The transition time constraints specified above do not provide a direct way to control the actual capacitance of nets. To control capacitance directly, below command has to be used:

set_max_capacitance: This command sets the maximum capacitance constraint on input ports or designs.

In addition to set_max_transition, set_max_capacitance can also be used as this command works independent.

This command applies maximum capacitance limit to output pin or port of the design.

This command can also be used to apply capacitance limit on any net.

Eg:

set_max_capacitance 4 [get_designs decoder]

To remove the set_max_capacitance command, use the remove_attribute command.

remove_attribute [get_designs decoder] max_capacitance

Propagation Delay

Propagation delay is the time required for a signal to propagate through a gate or net.

Hence if it is cell, you can call it as “Gate or Cell Delay” or if it is net you can call it as “Net Delay”

Propagation delay of a gate or cell is the time it takes for a signal at the input pin to affect the output signal at output pin.

For any gate propagation delay is measured between 50% of input transition to the corresponding 50% of output transition.

There are 4 possibilities:

Propagation delay between 50 % of Input rising to 50 % of output rising.

Propagation delay between 50 % of Input rising to 50 % of output falling.

Propagation delay between 50 % of Input falling to 50 % of output rising.

Propagation delay between 50 % of Input falling to 50 % of output falling.

Each of these delays has different values. Maximum and minimum values of these set are very important. Maximum and minimum propagation delay values are considered for timing analysis.

For net propagation delay is the delay between the time a signal is first applied to the net and the time it reaches other devices connected to that net.

Propagation delay is taken as the average of rise time and fall time i.e. Tpd= (Tphl+Tplh)/2.

Propagation delay depends on the input transition time (slew rate) and the output load. Hence two dimensional look up tables are used to calculate these delays. How to calculate propagation delay of net and gate? Please refer below articles to find the detailed explanation.

How gate delay is calculated?

How net delay is calculated?

Contamination Delay:

Best case delay from valid input to valid output. i.e. minimum propagation delay.

Net Delay or Interconnect Delay or Wire Delay or Extrinsic Delay or Flight Time

Net delay is the difference between the time a signal is first applied to the net and the time it reaches other devices connected to that net.

It is due to the finite resistance and capacitance of the net. It is also known as wire delay.

Wire delay = function of (Rnet, Cnet+Cpin)

This is output pin of the cell to the input pin of the next cell.



Net delay is calculated using Rs and Cs.

There are several factors which affect net parasitic:

• Net Length

• Net cross-sectional area

• Resistively of material used for metal layers (Aluminum vs. copper)

• Number of vias traversed by the net

• Proximity to other nets (crosstalk)

Post-layout design is annotated with RCs extracted from layout for better accuracy. Annotated RCs override information from WLM.

Interconnect introduces capacitive, resistive and inductive parasites. All three have multiple effects on the circuit behavior.

1. Interconnect parasites cause an increase in propagation delay (i.e. it slows down working speed)

2. Interconnect parasites increase energy dissipation and affect the power distribution.

3. Interconnect parasites introduce extra noise sources, which affect reliability of the circuit. (Signal Integrity effects)

Dominant parameters determine the circuit behavior at a given circuit node. Non-dominant parameters can be neglected for interconnect analysis.

• Inductive effect can be ignored if the resistance of the wire is substantial enough-this is the case for long aluminum wires with a small cross section or if the rise and fall times of the applied signals are slow.

• When the wires are short, the cross section of the wire is large or the interconnect material used has a low resistivity, a capacitive only model can be used.

• When the separation between neighboring wires is large or when the wires only run together for short distance, inter-wire capacitance can be ignored, and all the parasitic capacitance can be modeled as capacitance to ground.

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Capacitance

Capacitance can be modeled by the parallel plate capacitor model.

C = (ε / t).WL

Where

ε --> permittivity of dielectric material (SiO2)

t --> thickness of dielectric material (SiO2)

W --> width of wire

L --> length of wire

ε --> ε_r ε_o where ε_r --> relative permittivity of SiO2

ε_o --> 8.854 x 10-12 F/m; permittivity of free space

As technology node shrinks (scaling), to minimize resistance of the wires, it is desirable to keep the cross section of the wire (WxH) as large as possible. But this increases area. Small values of W lead to denser wiring and less area overhead. In advanced process W/H ratio has reduced below unity. Under such circumstances parallel plate capacitance model becomes inaccurate. The capacitance between the sidewall of the wires and substrate called fringing capacitance can no longer be ignored and contributes to the overall capacitance.

Inter-wire capacitance become dominant factor in multilayer interconnect structures. These floating capacitors (not connected to substrate or ground) form a source of noise (cross talk). This effect is more pronounced for wires in the higher interconnect layer, as these are farther away from the substrate.

Generally higher metal layers (i.e. interconnects) have higher thickness (i.e. height) and higher dielectric layers have higher permittivity. Hence these wires display the highest inter-wire capacitance. Hence use it for global signals that are not sensitive to interference. (eg. Supply rails). Or it is advisable to separate wires by an amount that is larger than minimum spacing.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Resistance

Resistance R= (ρ.L)/ (H.W) = (ρ. L)/ Area

L --> length

W --> width

ρ --> resistivity (ohm-m)

Since H (height, thickness) is constant for a given technology we can write: R = Rs.(L/W) where Rs=ρ/H ohm/sqare is called “sheet resistance”.

At very high frequencies “skin effect” comes into play such that the resistance becomes frequency dependent. High frequency currents tend to flow primarily on the surface of a conductor, with the current density falling off exponentially with depth into the conductor.

Skin effect is only an issue for wider wires. Since clocks tends to carry the highest frequency signals on a chip and also fairly wide to limit resistance, the skin effect likely to have its first impact on these lines.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Inductance

With the adoption of low resistance interconnect materials and the increase of switching frequencies to GHz range, inductance starts to an important role. Consequences of on chip inductance include ringing and overshoot effect, reflection of signals due to impedance mismatch, inductive coupling between lines, and switching noise due to (Ldi/dt) voltage drops.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Lumped Capacitor Model

As long as the resistive component of the wire is small, and switching frequencies are in the low to medium range, it is meaningful to consider only the capacitive component of the wire, and to lump the distributed capacitance into a single capacitance.

The only impact on performance is introduced by the loading effect of the capacitor on the driving gate.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Lumped RC Model

If wire length is more than a few millimeters, the lumped capacitance model is inadequate and a resistive capacitive model has to be adopted.

In lumped RC model the total resistance of each wire segment is lumped into one single R, combines the global capacitive into single capacitor C.

Analysis of network with larger number of R and C becomes complex as network contains many time constants (zeroes and poles). Elmore delay model overcome such problem.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Elmore Delay Model

Properties of the network:

• Has single input node
• All the capacitors are between a node and ground.
• Network does not contain any resistive loops.

“Path resistance” is the resistance from source node to any other node.

“Shared path resistance” is the resistance shared among the paths from the source node to any other two nodes.

Hence,

Delay at node 1: Tow d1 = R1C1

Delay at node 2: Tow d2= (R1+R2)C2

Delay at node 3: Tow d3 = (R1+R2+R3)C3

In general:

τdi=R1C1+(R1+R2)C2+……..+(R1+R2+R3+…..+Ri)Ci

If

R1=R2=R3=….=R

C1=C2=C3=…..C then

τdi=RC+2RC+……..+nRC

Thus Elmore delay is equivalent to the first order time constant of the network.

Assuming an interconnect wire of length L is partitioned into N identical segments. Each segment has length L/N.

Then,

τd=L/N.R.L/N.C+ 2 (L/n.r+L/N.C)+……

=(L/N)2(RC+2RC+…….+NRC)

=(L/N)2. N(N+1)

or τd=RC.L²/2

=> The delay of a wire is a quadratic function of its length

=> doubling the length of the wire quadruples its delay

Advantages

• It is simple
• It is always situated between minimum and maximum bounds

Disadvantages

• It is pessimistic and inaccurate for long interconnect wires.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Distributed RC model

Lumped RC model is always pessimistic and distributed RC model provides better accuracy over lumped RC model.

But distributed RC model is complex and no closed form solution exists. Hence distributed RC line model is not suitable for Computer Aided Design Tools.

The behavior of the distributed RC line can be approximated by a lumped RC ladder network such as Elmore Delay model hence these are extensively used in EDA tools.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Transmission Line Model

When frequency of operation increases to a larger extent, rise (or fall) time of the signal becomes comparable to time of flight of the net, then inductive effects starts dominating over RC values.
This inductive effect is modeled by Transmission Line models. The model assumes that the signal is a "wave" and it propagates over the medium "net".

There are two types of transmission models:

Lossless transmission line model: This is good for Printed Circuit Board level design.

Lossy transmission line model: This model is used for IC interconnect model.

Transmission line effects should be considered when the rise or fall time of the input signal is smaller than the time of flight of the transmission line or resistance of the wire is less than characteristics impedance.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Wire Load Models

Extraction data from already routed designs are used to build a lookup table known as the wire load model (WLM). WLM is based on the statistical estimates of R and C based on “Net Fan-out”.

For fanouts greater than those specified in a wire load table, a “slope factor” is specified for linear extrapolation.

wire_load (“5KGATES”) {

resistance : 0.000271 -------------> R per unit length

capacitance : 0.00017 -------------> C per unit length

slope : 29.4005 ---------------------> Used for linear extrapolation

fanout_length (1, 18.38) ----------> (fanout = 1, length = 18.38)

fanout_length (2, 47.78)

fanout_length (3, 77.18)

fanout_length (4, 106.58)

fanout_length (5, 135.98)

}

Eg:

Fanout = 7

Net length = 135.98 + 2 x 29.4005 (slope) = 194.78 ----------> length of net with fanout of 7
Resistance = 194.78 x 0.000271 = 0.05279 units
Capacitance = 194.78 x 0.00017 = 0.03311 units

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Wire load models for synthesis

Wire load modeling allows us to estimate the effect of wire length and fanout on the resistance, capacitance, and area of nets. Synthesizer uses these physical values to calculate wire delays and circuit speeds. Semiconductor vendors develop wire load models, based on statistical information specific to the vendors’ process. The models include coefficients for area, capacitance, and resistance per unit length, and a fanout-to-length table for estimating net lengths (the number of fanouts determines a nominal length).

Selection of wire load models in the initial stage (before physical design) depends on the fallowing factors:

1. User specification

2. Automatic selection based on design area

3. Default specification in the technology library

Once the final routing step is over in the physical design stage, wire load models are generated based on the actual routing in the design and synthesis is redone using those wire load models.

In hierarchical designs, we have to determine which wire load model to use for nets that cross hierarchical boundaries. There are three modes for determining which wire load model to use for nets that cross hierarchical boundaries:

Top:

Applying same wire load models to all nets as if the design has no hierarchy and uses the wire load model specified for the top level of the design hierarchy for all nets in a design and its sub designs.

Enclosed:

The wire load model of the smallest design that fully encloses the net is applied. If the design enclosing the net has no wire load model, then traverses the design hierarchy upward until we finds a wire load model. Enclosed mode is more accurate than top mode when cells in the same design are placed in a contiguous region during layout.

Use enclosed mode if the design has similar logical and physical hierarchies.

Segmented:

Wire load model for each segment of a net is determined by the design encompassing the segment. Nets crossing hierarchical boundaries are divided into segments. For each net segment, the wire load model of the design containing the segment is used. If the design contains a segment that has no wire load model, then traverse the design hierarchy upward until it finds a wire load model.


~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Interconnect Delay vs. Deep Sub Micron Issues

Performances of deep sub micron ICs are limited by increasing interconnect loading affect. Long global clock networks account for the larger part of the power consumption in chips. Traditional CAD design methodologies are largely affected by the interconnect scaling. Capacitance and resistance of interconnects have increased due to the smaller wire cross sections, smaller wire pitch and longer length. This has resulted in increased RC delay. As technology is advancing scaling of interconnect is also increasing. In such scenario increased RC delay is becoming major bottleneck in improving performance of advanced ICs.

Here the gate delay and the interconnect delay are shown as functions of various technology nodes ranging from 180nm to 60nm. The interconnect delays shown assumes a line where repeaters are connected optimally and includes the delay due to the repeaters. From the graph it can be observed that with the shrinking of technology gate delay reduces but interconnect delay increases.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Limits of Cu/low-k interconnects

At submicron level of 250 nm copper with low-k dielectric was introduced to decrease affects of increasing interconnect delay. But below 130 nm technology node interconnect delays are increasing further despite of introducing low-k dielectric. As the scaling increases new physical and technological effects like resistivity and barrier thickness start dominating and interconnect delay increases. Introduction of repeaters to shorten the interconnect length increases total area. The vias connecting repeaters to global layers can cause blockage in lower metal layers. Thus as the technology improves material limitations will dominate factor in the interconnect delay. Increasing metal layer width will cause increase in metallization layer. This can’t be a solution for the problem as it increases complexity, reliability and cost.

Cu low-k dielectric films are deposited by a special process known as Damascene process. Adhesion property of Cu with dielectric materials is very poor. Under electric bias they easily drift and cause short between metal layers. To avoid this problem a barrier layer is deposited between dielectric and Cu trench. Even though it decreases effective cross section of interconnects compared to drawn dimensions, it improves reliability. The barrier thickness becomes significant in deep submicron level and effective resistance of the interconnect rises further. In addition to this increasing electron scattering and self heating caused by the electron flow in interconnects due to comparable increase in internal chip temperature also contribute to increase interconnect resistance.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
References

[1] Jan M. Rabaey, Anantha Chandrakasan and Borivoje Nikolic, "Digital Integrated Circuits- A Design Perspective", Prentice Hall, Second Edition
[2] Design Compiler User Manual

Delays in ASIC Design

We encounter several types of delays in ASIC design. They are as follows:

• Gate delay or Intrinsic delay
• Net delay or Interconnect delay or Wire delay or Extrinsic delay or Flight time
• Transition or Slew
• Propagation delay
• Contamination delay

Wire delays or extrinsic delays are calculated using output drive strength, input capacitance and wire load models. Other delays are intrinsic properties of each and every gate.

Delays are interdependent on different electrical properties. [Nekoogar]:

• Input capacitance of the logic gate is a function of output state, output loads and input slew rate.

• Internal timing arcs and output slew rate is a function of switching input(s).

• Capacitance of the wire is dependent on frequency.

• Internal timing arcs are a function of input slew rates.

• Output slew rate is a function of input slew rate on each input.

• Wires exhibit RLC characteristics instead of lumped RC.

Gate Delay

Transistors within a gate take a finite time to switch. This means that a change on the input of a gate takes a finite time to cause a change on the output. [Magma]

Gate delay =function of (input transition (slew) time, Cnet+Cpin).

or

Gate delay =function of (input transition (slew) time, Cload).

where Cload=Cnet+Cpin

Cnet-->Net capacitance

Cpin-->pin capacitance of the driven cell

Cell delay is also same as Gate delay.

How gate delay is calculated?

Cell or gate delay is calculated using Non-Linear Delay Models (NLDM). NLDM is highly accurate as it is derived from SPICE characterizations. The delay is a function of the input transition time (i.e. slew) of the cell, the wire capacitance and the pin capacitance of the driven cells. A slow input transition time will slow the rate at which the cell’s transistors can change state logic 1 to logic 0 (or logic 0 to logic 1), as well as a large output load Cload (Cnet + Cpin), thereby increasing the delay of the logic gate.

There is another NLDM table in the library to calculate output transition. Output transition of a cell becomes the input transition of the next cell down the chain.

• Table models are usually two-dimensional to allow lookups based on the input slew and the output load (Cload). A sample table is given below.

timing() {

related_pin : "CKN";

timing_type : falling_edge;

timing_sense : non_unate;

cell_rise(delay_template_7x7) {

index_1 ("0.012, 0.032, 0.074, 0.154, 0.318, 0.644, 1.3");

index_2 ("0.001278, 0.0046008, 0.0112464, 0.0245376, 0.05112, 0.10454, 0.212148");

values ( \

"0.225894, 0.249015, 0.285537, 0.352680, 0.484244, 0.748180, 1.279570", \

"0.231295, 0.254415, 0.290938, 0.358081, 0.489646, 0.753585, 1.284980", \

"0.243754, 0.266878, 0.303398, 0.370542, 0.502105, 0.766044, 1.297440", \

"0.267240, 0.290389, 0.326908, 0.394052, 0.525615, 0.789561, 1.320950", \

"0.307080, 0.330200, 0.366721, 0.433861, 0.565425, 0.829373, 1.360760", \

"0.380552, 0.403875, 0.440426, 0.507569, 0.639136, 0.903084, 1.434500", \

"0.497588, 0.521769, 0.558548, 0.625744, 0.757301, 1.021260, 1.552680");

}

rise_transition(delay_template_7x7) {

index_1 ("0.012, 0.032, 0.074, 0.154, 0.318, 0.644, 1.3");

index_2 ("0.001278, 0.0046008, 0.0112464, 0.0245376, 0.05112, 0.10454, 0.212148");

values ( \

"0.040574, 0.068619, 0.125391, 0.246672, 0.497688, 1.005982, 2.030120", \

"0.040570, 0.068618, 0.125390, 0.246672, 0.497688, 1.005940, 2.030240", \

"0.040565, 0.068616, 0.125389, 0.246650, 0.497770, 1.006180, 2.030120", \

"0.040532, 0.068612, 0.125387, 0.246670, 0.497710, 1.006164, 2.030100", \

"0.040578, 0.068621, 0.125392, 0.246636, 0.497688, 1.006182, 2.030040", \

"0.041763, 0.069211, 0.125662, 0.246758, 0.497726, 1.005930, 2.030000", \

"0.045813, 0.071321, 0.126671, 0.247154, 0.497846, 1.005962, 2.030180");

}

index_1 --> input transition values

index_2--> output load capacitance values

values--> delay values

Situation 1:

Input transition and output load values match with table index values

If both input transition and output load values match with table index values then corresponding delay value is directly picked up from the delay “values” table as highlighted by yellow shaded data.

Situation 2:

Output load values doesn't match with table index values

• When the actual load capacitance values does not fall directly on or at one of the load-axis index points, the delay is determined by interpolation from the closest points. Note that to carry out interpolation input transition point should match with the any one of the table index values.

• Determine the equation for the line segment connecting the two nearest points in the table.

To do this first we need to find the slope value.

Slope m = (y2-y1)/(x2-x1) where (y2-y1) is delay segment (generally in ns) on y axis and (x2-x1) is load segment (generally in pf) on x-axis.

• Solve for the delay at the load point of interest.

The linear equation is:

y = mx+c

where

y-->delay (ns)

m-->slope

x-->load capacitance (pf)

i.e. delay=slope*load point of interest (constant value is zero)

Load point of interest means load capacitance value for which delay has to be calculated.

Situation 3:

Both input transition and output load values doesn't match with table index values

• If both input transition and load capacitance values do not match exactly with the look up table index values then bilinear interpolation is used.

• Multiple linear interpolations (~3) are performed on multiple closest table data points (~4) as shown in highlighted violet color in the look up table.

Situation 4:

Output load values doesn't match with table index values and is outside the table boundary

• When the load point is outside of the boundary of the index, the delay is extrapolated to the closest known points.

• Lookup value too far out of range of the given table value could lead to inaccuracy. [Cadence]

Intrinsic delay

• Intrinsic delay is the delay internal to the gate. This is from input pin of the cell to output pin of the cell.

• It is defined as the delay between an input and output pair of a cell, when a near zero slew is applied to the input pin and the output does not see any load condition. It is caused by the internal capacitance associated with its transistor.

• This delay is largely dependent on the size of the transistors forming the gate because increasing size of transistors increase internal capacitors.

References

[Nekoogar] Farzad Nekoogar, “Timing Verification of Application Specific Integrated Circuits”, Prentice Hall

[Magma] Magma Blast Fusion User Guides

[Cadence] Cadence SOC Encounter User Guides

Dynamic vs Static Timing Analysis

Timing analysis is integral part of ASIC/VLSI design flow. Anything else can be compromised but not timing! Timing analysis can be static or dynamic. Dynamic timing analysis verifies functionality of the design by applying input vectors and checking for correct output vectors whereas Static Timing Analysis checks static delay requirements of the circuit without any input or output vectors.

Dynamic timing analysis has to be accomplished and functionality of the design must be cleared before the design is subjected to Static Timing Analysis (STA). Dynamic Timing Analysis (DTA) and Static Timing Analysis (STA) are not alternatives to each other. Quality of the Dynamic Timing Analysis (DTA) increases with the increase of input test vectors. Increased test vectors increase simulation time. Dynamic timing analysis can be used for synchronous as well as asynchronous designs. Static Timing Analysis (STA) can’t run on asynchronous deigns and hence Dynamic Timing Analysis (DTA) is the best way to analyze asynchronous designs. Dynamic Timing Analysis (DTA) is also best suitable for designs having clocks crossing multiple domains.

Example of Dynamic Timing Analysis(DTA) tool is Modelsim (from mentor Graphics), VCS (from Synopsys). DTA is also carried out on post layout netlist to verify that functionality of the design has not changed. Test vectors remain same for both.

SPICE Simulation

Device level timing analysis is carried out using SPICE simulation. SPICE simulation is very essential for full custom designs to verify the electrical properties of the designs. These are calculated based on the mathematical equations that represent electrical properties of devices. Material and some of the electrical properties of the devices, which are represented by either variables or constants, are stored in model files. Examples are threshold voltage of MOSFET, electron density etc. SPICE characterized data is tabulated in technology libraries which becomes basic delay information for the Static Timing Analysis. For example let us consider a AND gate. Several electrical properties such as input and output transition, propagation delay, output capacitance etc are evaluated by this SPICE simulation. SPICE simulated data gives maximum accuracy compared to any other form of simulation. SPICE code is manually written and simulated. Hence for a larger design SPICE simulation is cumbersome job. There are specific tools available for transistor level Static Timing Analysis (STA), (Eg. Pathmill from Synopsys) SPICE simulation being the backbone of all these tools.

What is Static Timing Analysis (STA)?

In Static Timing Analysis (STA) static delays such as gate delay and net delays are considered in each path and these delays are compared against their required maximum and minimum values. Circuit to be analyzed is broken into different timing paths constituting of gates, flip flops and their interconnections. Each timing path has to process the data within a clock period which is determined by the maximum frequency of operation. Cell delays are available in the corresponding technology libraries. Cell delay values are tabulated based on input transition and fanout load which are characterized by SPICE simulation. Net delays are calculated based on the Wire Load Models(WLM) or extracted resistance R and capacitance C. Wire Load Models(WLM) are available in the Technology File. These values are Table Look Up(TLU) values calculated based on the net fanout length.

The static timing analyzer will report the following delays (or it can do following analysis):

Register to Register delays

Setup times of all external synchronous inputs

Clock to Output delays

Pin to Pin combinational delays

Different Analysis Modes-Best, Worst, Typical, On Chip Variation (OCV)

Data to Data Checks

Case Analysis

Multiple Clocks per Register

Minimum Pulse Width Checks

Derived Clocks

Clock Gating Checks

Netlist Editing

Report_clock_timing

Clock Reconvergence Pessimism

Worst-Arrival Slew Propagation

Path-Based Analysis

Debugging Delay Calculation

and many more......!!

The wide spread use of STA can be attributed to several factors [David]:

• The basic STA algorithm is linear in runtime with circuit size, allowing analysis of designs in excess of 10 million instances.

• The basic STA analysis is conservative in the sense that it will over-estimate the delay of long paths in the circuit and under-estimate the delay of short paths in the circuit. This makes the analysis ”safe”, guaranteeing that the design will function at least as fast as predicted and will not suffer from hold-time violations.

• The STA algorithms have become fairly mature, addressing critical timing issues such as interconnect analysis, accurate delay modeling, false or multi-cycle paths, etc.

• Delay characterization for cell libraries is clearly defined, forms an effective interface between the foundry and the design team, and is readily available. In addition to this, the Static Timing Analysis (STA) does not require input vectors and has a runtime that is linear with the size of the circuit [Agarwal].

Advantages of STA:

• All timing paths are considered for the timing analysis. This is not the case in simulation.

• Analysis times are relatively short when compared with event and circuit simulation.

• Timing can be analyzed for worst case, best case simultaneously. This type of analysis is not possible in dynamic timing analysis.

• Static Timing Analysis (STA) works with timing models. STA has more pessimism and thus gives maximum delay of the design. DTA performs full timing simulation. The problem associated with DTA is the computational complexity involved in finding the input patterns (vectors) that produce maximum delay at the output and hence it is slow.

Disadvantages of STA:

• All paths in the design may not run always in worst case delay. Hence the analysis is pessimistic.

• Clock related all information has to be fed to the design in the form of constraints.

• Inconsistency or incorrectness or under constraining of these constraints may lead to disastrous timing analysis.

• STA does not check for logical correctness of the design.

• STA is not suitable for asynchronous circuits.

References

[David] David Blaauw, Kaviraj Chopra, Ashish Srivastava and Lou Scheffer, “Statistical Timing Analysis: From basic principles to state-of-the-art.”, Transactions on Computer-Aided Design of Integrated Circuits and Systems (T-CAD), IEEE.

[Agarwal] Agarwal, A. Blaauw, D. Zolotov, V. Sundareswaran, S. Min Zhao Gala, K. and Panda, R., “Statistically Delay computation considering spatial correlations,” Proceedings of the ASP-DAC 2003, pp.271-276, Jan 2003.

Companywise ASIC/VLSI Interview Questions

Below interview questions are contributed by ASIC_diehard (Thanks a lot !). Below questions are asked for senior position in Physical Design domain. The questions are also related to Static Timing Analysis and Synthesis. Answers to some questions are given as link. Remaining questions will be answered in coming blogs.

Common introductory questions every interviewer asks are:

• Discuss about the projects worked in the previous company.
• What are physical design flows, various activities you are involved?
• Design complexity, capacity, frequency, process technologies, block size you handled.

Intel

• Why power stripes routed in the top metal layers?

The resistivity of top metal layers are less and hence less IR drop is seen in power distribution network. If power stripes are routed in lower metal layers this will use good amount of lower routing resources and therefore it can create routing congestion.

• Why do you use alternate routing approach HVH/VHV (Horizontal-Vertical-Horizontal/ Vertical-Horizontal-Vertical)?

Answer:

This approach allows routability of the design and better usage of routing resources.

• What are several factors to improve propagation delay of standard cell?

Answer:

Improve the input transition to the cell under consideration by up sizing the driver.

Reduce the load seen by the cell under consideration, either by placement refinement or buffering.

If allowed increase the drive strength or replace with LVT (low threshold voltage) cell.

• How do you compute net delay (interconnect delay) / decode RC values present in tech file?
• What are various ways of timing optimization in synthesis tools?

Answer:

Logic optimization: buffer sizing, cell sizing, level adjustment, dummy buffering etc.

Less number of logics between Flip Flops speedup the design.

Optimize drive strength of the cell , so it is capable of driving more load and hence reducing the cell delay.

Better selection of design ware component (select timing optimized design ware components).

Use LVT (Low threshold voltage) and SVT (standard threshold voltage) cells if allowed.

• What would you do in order to not use certain cells from the library?

Answer:

Set don’t use attribute on those library cells.

• How delays are characterized using WLM (Wire Load Model)?

Answer:



For a given wireload model the delay are estimated based on the number of fanout of the cell driving the net.



Fanout vs net length is tabulated in WLMs.



Values of unit resistance R and unit capacitance C are given in technology file.



Net length varies based on the fanout number.



Once the net length is known delay can be calculated; Sometimes it is again tabulated.

• What are various techniques to resolve congestion/noise?

Answer:

Routing and placement congestion all depend upon the connectivity in the netlist , a better floor plan can reduce the congestion.

Noise can be reduced by optimizing the overlap of nets in the design.

• Let’s say there enough routing resources available, timing is fine, can you increase clock buffers in clock network? If so will there be any impact on other parameters?

Answer:

No. You should not increase clock buffers in the clock network. Increase in clock buffers cause more area , more power. When everything is fine why you want to touch clock tree??

• How do you optimize skew/insertion delays in CTS (Clock Tree Synthesis)?

Answer:

Better skew targets and insertion delay values provided while building the clocks.

Choose appropriate tree structure – either based on clock buffers or clock inverters or mix of clock buffers or clock inverters.

For multi clock domain, group the clocks while building the clock tree so that skew is balanced across the clocks. (Inter clock skew analysis).

• What are pros/cons of latch/FF (Flip Flop)?

Answer: Pros and cons of latch and flip flop

• How you go about fixing timing violations for latch- latch paths?
• As an engineer, let’s say your manager comes to you and asks for next project die size estimation/projection, giving data on RTL size, performance requirements. How do you go about the figuring out and come up with die size considering physical aspects?
• How will you design inserting voltage island scheme between macro pins crossing core and are at different power wells? What is the optimal resource solution?
• What are various formal verification issues you faced and how did you resolve?
• How do you calculate maximum frequency given setup, hold, clock and clock skew?
• What are effects of metastability?

Answer: Metastability

• Consider a timing path crossing from fast clock domain to slow clock domain. How do you design synchronizer circuit without knowing the source clock frequency?
• How to solve cross clock timing path?
• How to determine the depth of FIFO/ size of the FIFO?

Answer: FIFO Depth

STmicroelectronics

• What are the challenges you faced in place and route, FV (Formal Verification), ECO (Engineering Change Order) areas?
• How long the design cycle for your designs?
• What part are your areas of interest in physical design?
• Explain ECO (Engineering Change Order) methodology.
• Explain CTS (Clock Tree Synthesis) flow.

Answer: Clock Tree Synthesis

• What kind of routing issues you faced?
• How does STA (Static Timing Analysis) in OCV (On Chip Variation) conditions done? How do you set OCV (On Chip Variation) in IC compiler? How is timing correlation done before and after place and route?

Answer: Process-Voltage-Temperature (PVT) Variations and Static Timing Analysis (STA)

• If there are too many pins of the logic cells in one place within core, what kind of issues would you face and how will you resolve?
• Define hash/ @array in perl.
• Using TCL (Tool Command Language, Tickle) how do you set variables?
• What is ICC (IC Compiler) command for setting derate factor/ command to perform physical synthesis?
• What are nanoroute options for search and repair?
• What were your design skew/insertion delay targets?
• How is IR drop analysis done? What are various statistics available in reports?
• Explain pin density/ cell density issues, hotspots?
• How will you relate routing grid with manufacturing grid and judge if the routing grid is set correctly?
• What is the command for setting multi cycle path?
• If hold violation exists in design, is it OK to sign off design? If not, why?

Texas Instruments (TI)

• How are timing constraints developed?
• Explain timing closure flow/methodology/issues/fixes.
• Explain SDF (Standard Delay Format) back annotation/ SPEF (Standard Parasitic Exchange Format) timing correlation flow.
• Given a timing path in multi-mode multi-corner, how is STA (Static Timing Analysis) performed in order to meet timing in both modes and corners, how are PVT (Process-Voltage-Temperature)/derate factors decided and set in the Primetime flow?
• With respect to clock gate, what are various issues you faced at various stages in the physical design flow?
• What are synthesis strategies to optimize timing?
• Explain ECO (Engineering Change Order) implementation flow. Given post routed database and functional fixes, how will you take it to implement ECO (Engineering Change Order) and what physical and functional checks you need to perform?

Qualcomm

• In building the timing constraints, do you need to constrain all IO (Input-Output) ports?
• Can a single port have multi-clocked? How do you set delays for such ports?
• How is scan DEF (Design Exchange Format) generated?
• What is purpose of lockup latch in scan chain?
• Explain short circuit current.

Answer: Short Circuit Power

• What are pros/cons of using low Vt, high Vt cells?

Answer:

Multi Threshold Voltage Technique

Issues With Multi Height Cell Placement in Multi Vt Flow

• How do you set inter clock uncertainty?

Answer:

set_clock_uncertainty –from clock1 -to clock2

• In DC (Design Compiler), how do you constrain clocks, IO (Input-Output) ports, maxcap, max tran?
• What are differences in clock constraints from pre CTS (Clock Tree Synthesis) to post CTS (Clock Tree Synthesis)?

Answer:

Difference in clock uncertainty values; Clocks are propagated in post CTS.

In post CTS clock latency constraint is modified to model clock jitter.

• How is clock gating done?

Answer: Clock Gating

• What constraints you add in CTS (Clock Tree Synthesis) for clock gates?

Answer:

Make the clock gating cells as through pins.

• What is trade off between dynamic power (current) and leakage power (current)?

Answer:

Leakage Power Trends

Dynamic Power

• How do you reduce standby (leakage) power?

Answer: Low Power Design Techniques

• Explain top level pin placement flow? What are parameters to decide?
• Given block level netlists, timing constraints, libraries, macro LEFs (Layout Exchange Format/Library Exchange Format), how will you start floor planning?
• With net length of 1000um how will you compute RC values, using equations/tech file info?
• What do noise reports represent?
• What does glitch reports contain?
• What are CTS (Clock Tree Synthesis) steps in IC compiler?
• What do clock constraints file contain?
• How to analyze clock tree reports?
• What do IR drop Voltagestorm reports represent?
• Where /when do you use DCAP (Decoupling Capacitor) cells?
• What are various power reduction techniques?

Answer: Low Power Design Techniques

Hughes Networks

• What is setup/hold? What are setup and hold time impacts on timing? How will you fix setup and hold violations?
• Explain function of Muxed FF (Multiplexed Flip Flop) /scan FF (Scal Flip Flop).
• What are tested in DFT (Design for Testability)?
• In equivalence checking, how do you handle scanen signal?
• In terms of CMOS (Complimentary Metal Oxide Semiconductor), explain physical parameters that affect the propagation delay?
• What are power dissipation components? How do you reduce them?

Answer:

Short Circuit Power

Leakage Power Trends

Dynamic Power

Low Power Design Techniques

• How delay affected by PVT (Process-Voltage-Temperature)?

Answer: Process-Voltage-Temperature (PVT) Variations and Static Timing Analysis (STA)

• Why is power signal routed in top metal layers?

Avago Technologies (former HP group)

• How do you minimize clock skew/ balance clock tree?
• Given 11 minterms and asked to derive the logic function.
• Given C1= 10pf, C2=1pf connected in series with a switch in between, at t=0 switch is open and one end having 5v and other end zero voltage; compute the voltage across C2 when the switch is closed?
• Explain the modes of operation of CMOS (Complimentary Metal Oxide Semiconductor) inverter? Show IO (Input-Output) characteristics curve.
• Implement a ring oscillator.
• How to slow down ring oscillator?

Hynix Semiconductor

• How do you optimize power at various stages in the physical design flow?
• What timing optimization strategies you employ in pre-layout /post-layout stages?
• What are process technology challenges in physical design?
• Design divide by 2, divide by 3, and divide by 1.5 counters. Draw timing diagrams.
• What are multi-cycle paths, false paths? How to resolve multi-cycle and false paths?
• Given a flop to flop path with combo delay in between and output of the second flop fed back to combo logic. Which path is fastest path to have hold violation and how will you resolve?
• What are RTL (Register Transfer Level) coding styles to adapt to yield optimal backend design?
• Draw timing diagrams to represent the propagation delay, set up, hold, recovery, removal, minimum pulse width.

About Contributor

ASIC_diehard has more than 5 years of experience in physical design, timing, netlist to GDS flows of Integrated Circuit development. ASIC_diehard's fields of interest are backend design, place and route, timing closure, process technologies.

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