title: constraining delay rules product: orcad pcb ... · this document gives an overview of how...

Application Note

Constraining Delay Rules of 14

Title: Constraining Delay Rules Product: OrCAD PCB Designer Professional 16.6-2015 Summary: Shows how to constrain delay rules Author/Date: Pascal Willems / 15. October 2015

Table of Contents

1 Introduction ................................................................................................................. 2 2 Relative Propagation Delay for H-structure ................................................................. 2

2.1 Net Scheduling ................................................................................................................................ 2 2.1.1 How to Place a Tpoint ................................................................................................................................................... 2

2.2 Define a Pin Pair .............................................................................................................................. 3 2.2.1 How to build a Pin Pair? ................................................................................................................................................ 3 2.2.2 How to Change a Pin Pair? ........................................................................................................................................... 4 2.2.3 How to Delete a Pin Pair? ............................................................................................................................................. 4

2.3 Relative Propagation Delay ............................................................................................................. 4 2.3.1 How to define a Match Group from Pin Pairs ................................................................................................................. 4

2.4 Constraining the Relative Propagation Delay .................................................................................. 5 2.4.1 Pin Pairs ....................................................................................................................................................................... 6 2.4.2 Scope ........................................................................................................................................................................... 6 2.4.3 Relative Delay / Delta : Tolerance ................................................................................................................................. 6 2.4.4 Relative Delay / Actual .................................................................................................................................................. 6 2.4.5 Relative Delay / Margin ................................................................................................................................................. 7 2.4.6 Relative Delay / +/- ........................................................................................................................................................ 7 2.4.7 Length ........................................................................................................................................................................... 7 2.4.8 Delay ............................................................................................................................................................................ 7

2.5 Routing the Constrained Connection............................................................................................... 8 3 Min/Max Propagation Delay ........................................................................................ 8

3.1 Settings in Constraint Manager ....................................................................................................... 8 3.1.1 Pin Pairs ....................................................................................................................................................................... 8 3.1.2 Delays ........................................................................................................................................................................... 8

4 Total Etch Length ........................................................................................................ 9 4.1 Information in the Constraint Manager ............................................................................................ 9

5 Additional Info about Relative Propagation Delay ..................................................... 10 5.1 Define a match group when no Pin Pairs are defined ................................................................... 10

5.1.1 How to add a net to an existing Match Group .............................................................................................................. 11 5.1.2 How to remove a net from a Match Group ................................................................................................................... 11 5.1.3 How to add a net to different Match Groups ................................................................................................................ 11

5.2 Constraint Settings of a Match Group ........................................................................................... 12 5.2.1 Pin Pairs ..................................................................................................................................................................... 12

5.3 How to define the Reference Net in a Match Group...................................................................... 12 5.4 Routing the Connections from the Match Group ........................................................................... 13

6 Bibliography .............................................................................................................. 14

Application Note


1 Introduction

This document gives an overview of how delay and length rules for high-speed requirements from the logic component manufacturer can be implemented in a PCB using OrCAD Professional. Topics like same length for connections and absolute propagation delays and relative propagation delays are covered by this Application Note. For example DDR2 memory components are known as the component which needs for certain signals a Cline (Connection line, wire) from a split-up point to one memory component that has the same length as from the split-up point to the other memory component. These “Tree” structure is often named Y, T or H structure. This constraining might also be needed for other components. The split-up point is called Tpoint or Ts in PCB Editor.

2 Relative Propagation Delay for H-structure

2.1 Net Scheduling

Or: How to define the sequence of the pins in a net and the length between certain pins.

2.1.1 How to Place a Tpoint

Activate the command net schedule by clicking on or Edit -> Net Schedule. Select the Rats or routed net or a pin from the net where the Tpoint should be added. Select a pin in this net. RMB -> Insert T and click with the mouse on the point where the Tpoint shall be placed. Click on Pins and Tpoints in the corresponding sequence to the desired structure.

Step by step explanation for an H-structure example:

Unscheduled nets: Rats as usual

Tpoint will be placed RMB -> Insert T

Application Note


Note: The visibility of the ratsnest Tpoint after routing the net is controlled by the environment variable "ratt_off_if_connected". Setup -> User Preferences -> Display -> Visual -> ratt_off_if_connected = unhighlighted.

2.2 Define a Pin Pair

To constrain the length of a segment of a net it is essential to define where the segment starts and where it ends. A Pin Pair defines the two endpoints of a segment of a net.

2.2.1 How to build a Pin Pair?

Open Constraint Manager by clicking on or Setup -> constraints… Select the worksheet Electrical -> Net -> Routing -> Relative Propagation Delay (or Impedance or Min/Max Propagation Delay depending on which type of constraint you want to define) On the Net click RMB -> Create -> Pin Pair…

1. Select net

2. RMB -> Insert T

3. RMB -> Insert T

4. Select Pad

5. Select Tpoint again

6. Select Pad


8. RMB -> Insert T

9. Select Pad


11. Select Pad and RMB -> Done

Application Note


The Pin Pairs are shown in the Constraint Manager:

Do not close the Constraint Manager. Start directly with constraining the Pin Pairs or building match groups. Pins pairs without defined constraints will be deleted when closing Constraint Manager

2.2.2 How to Change a Pin Pair?

Delete the unwanted Pin Pair and create the desired Pin Pair as needed.

2.2.3 How to Delete a Pin Pair?

In Constraint Manager right click on Pin Pair and select delete: RMB - > Delete Or: Pin pairs with no constraints will be deleted when the constraint Manager is being closed.

2.3 Relative Propagation Delay

To set the Relative Propagation Delay a match Group must be defined.

2.3.1 How to define a Match Group from Pin Pairs

Select the Pin Pairs with [Shift] or [Ctrl] and mouse click. RMB -> Create -> Match Group…

When not selected it is possible to

have different Pin Pairs in

other Worksheets.

Select the first endpoint of your net segment

Select the second

endpoint of your net segment

Click on [Apply] to

create the Pin Pair

Tpoints

Pins

PPr = Pin Pair

Application Note


Example from 2.1.1

2.4 Constraining the Relative Propagation Delay

Locate the Match Groups (MGrp) in the Constraint Manager: Electrical -> Net -> Routing -> Relative Propagation Delay

T1

T2

T3

1 1

2

2

3

3 4

4

5

5

6

6

Sender

Application Note


2.4.1 Pin Pairs

As the Pin Pairs are set manually no setting is required for Pin Pair.

2.4.2 Scope

Choose Global to be on the safe side when unsure about this setting.

Global All segments of this match group are compared to each other.

Local All segments (Pin Pair) of this Match Group from the same net are compared to each other. The segments (Pin Pairs) from one net in this Match Group are not compared to segments (Pin Pairs) from another net in the same Match Group. The Local setting only makes sense if the Group has Net members and the Pin Pairs column is set to All Drivers: All Receiver.

2.4.3 Relative Delay / Delta : Tolerance

Should all connections of a Match Group have the same length, set delta to zero and define only a tolerance. Do some of the connections need to have a certain difference to the reference, use the delta to define the difference. Positive values define the connection to be longer than the target. Negative Values define the connection to be shorter than the target. A Tolerance is needed. It can be defined as length, time or as percentage. The units of the tolerance have to be the same as of the delta. The percentage can be used together with all units.

2.4.4 Relative Delay / Actual

Here the actual difference of the Cline to the target in the units selected at tolerance is shown. If the connections are routed but no result is shown or the cells are yellow: Select the worksheet Electrical -> Net -> Routing -> Wiring In the column Topology/Verify Schedule select yes for the net which uses the Pin Pairs and/or Net Scheduling

Application Note


Or: RMB on Actual -> Analyze

Or: Analyze -> Analysis Modes… -> Electrical Modes -> Relative propagation delay -> On

2.4.5 Relative Delay / Margin

The Margin shows the difference to the maximum tolerance.

2.4.6 Relative Delay / +/-

A plus (+) is shown when the connection is longer as the target. A minus (-) is shown when the connection is shorter as the target. Define the Target manually to make sure the relative delay is referenced on the desired value.

2.4.7 Length

It shows the length of the connection (between the Pin Pair).

2.4.8 Delay

It shows the time that is needed for a signal on this connection (between the Pin Pair).

Application Note


2.5 Routing the Constrained Connection

Route the nets as usual with add connect. For more detailed description on tuning length constrained clines please see chapter 5.4 Routing the Connections.

3 Min/Max Propagation Delay It is possible to define the length of a delay of signal from one pin to another pin in a net. It is called Propagation Delay. It can be defined in either time or length units. It is possible to define the minimum and maximum delay on a net in ns or in mm. Electrical -> Net -> Routing -> Min/Max Propagation Delays

3.1 Settings in Constraint Manager

3.1.1 Pin Pairs

Choose Generates pin pairs based on...

Longest/Shortest Pin Pair The longest Pin Pair and the shortest Pin Pair is compared to the constraint settings.

Longest/Shortest Driver/Receiver The longest driver to receiver Pin Pair and the shortest driver to receiver Pin Pair is compared to the constraint settings.

All Drivers/All Receivers All Pin Pair combination between drivers and receivers in this net will be listed in CM and compared to the constraint settings.

3.1.2 Delays

Min and max Propagation Delay Example

If the connections are routed but no result is shown or the cells are yellow: RMB on Actual or Margin-> Analyze

Application Note


Or Analyze -> Analysis Modes… -> Electrical Modes -> Propagation delay -> On

4 Total Etch Length Electrical -> Net -> Routing -> Total Etch Length Total Etch Length sums up all Clines in one net and compares it to the Min and Max value entered in the Constraint Manager. If a net has only one start and one end pin it would show the same result as Min/Max Propagation Delay. If the net has another branch, for example to a pull-up resistor this constraint does not help to control the length between driver and receiver, therefore use: Min/Max Propagation Delay described in chapter 3.

4.1 Information in the Constraint Manager

If the connections are routed but no result is shown or the cells are yellow: RMB on Actual or Margin -> Analyze

Or Analyze -> Analysis Modes… -> Electrical Modes -> Total etch length -> On

Margin = Actual - Min

Margin = Actual - Max

Air Distance Actual Total Etch

Length / (X length + Y length)

Application Note


5 Additional Info about Relative Propagation Delay

5.1 Define a match group when no Pin Pairs are defined

Open the Constraint Manager:

Setup -> Constraints or click on Select the electrical worksheet Open the Net folder and Routing folder if not already open Select Relative Propagation Delay Choose the nets which should have the same length, with [Shift] and or [Crtl] and by clicking on a net the needed nets can be selected RMB -> Create -> Match Group

Give the Match Group a meaningful name and click on [OK].

Application Note


5.1.1 How to add a net to an existing Match Group

Select the net -> RMB -> Add to… -> Match Group -> Select the Match Group you want to add the net to and click [OK]

Or select the Match Group -> RMB -> Match Group Members…

5.1.2 How to remove a net from a Match Group

5.1.3 How to add a net to different Match Groups

Use ‘Preserve existing membership’ option during creation or editing of Match Groups to keep the net also in the existing Match Group. If the ‘Preserve existing membership’ option is not selected during creation or editing of Match Groups the net will be moved to the new Match Group FAQ: Why is the net twice in CM? Why is the net visible in the Match Group as well as on its original position in the Constraint Manager?

Application Note


The match group includes only Pin Pairs from a net not the net itself even though the Pin Pairs are not explicit selected.

5.2 Constraint Settings of a Match Group

Sometimes all connections from a data or address Bus or certain segments from a net have to have all the same length. To achieve this goal the match group can be used. Most settings are explained in chapter 2.4 Constraining the Relative Propagation Delay.

5.2.1 Pin Pairs

Choose Generates Pin Pairs based on...

Longest Pin Pair The longest Pin Pair on the net.

Longest Driver/Receiver The longest Driver-Receiver Pin Pair uses the longest Pin Pair when there are no drivers or receivers on the net.

All Drivers/All Receivers The relative propagation delay restriction is applied to all Driver-to-Receiver Pin Pairs on the net.

The longest Pin Pair might change when moving a component. Please consider also the way to create Pin Pairs manually. Therefore see chapter 2.2. Which Pins are driver or receiver can be set with the Property Pinuse on a pin or with SI-models for the components.

5.3 How to define the Reference Net in a Match Group

The reference is called target. As long as the target is not manually defined, the system will take automatically the longest connection as reference. After editing the length of the nets and reopening the CM the reference changes automatically. Defining the target manually works as follows: In column Relative Delay/Delta:Tolerance -> RMB -> Set as target

Application Note


5.4 Routing the Connections from the Match Group

With or Route -> Connect design the shortest possible connections.

To easily adjust the net length you can use the delay tune or Route -> Delay Tune. How the Cline elongation (accordion) should look like can be set in the Options

When editing one of the connections of the match group the difference to the target plus tolerance is displayed:

Head-up display Explanation and numeric example Info in Constrain Manager

The actual connection is longer as the target and it is out of tolerance.

The displayed value shows how much the connection exceeds the

limit

Target+Tolerance+RDly=Length 21.18 + 5% => 22.24 22.24 + 2.19 = 24.43

RDly=Length–(TARGET+Tolerance) 24.43-1.05*21.18 = 2.19

Relative Delay -> +/- is now + (positive) because the length is longer

as target

The actual connection is longer as

the Target, but is inside of the tolerance.

The displayed Value is the margin

RDly=Length-(TARGET+Tolerance) 21.85-(21.18*1.05) = -0.39

Actual = (Length-Target)/Target (21.85-21.18)/21.18 = 3.162096%

Actual shows the difference to the target in percent

The actual connection is shorter as

the Target, but is inside of the tolerance.

RDly=Length-(TARGET-Tolerance) 20.57-(21.18*0.95) = +0.45

Margin = RDly / Target 0.45 / 21.18 = 2.1244% +/- ? =Target – Length

Application Note


The displayed value is the margin. The value jumps from right to left

when the margin becomes bigger (or smaller) than half the tolerance

21.18 – 20.57 < 0

Relative Delay -> +/- is now – (negative) because the length is

smaller than target. Margin is the rest of tolerance.

The actual connection is shorter than the target and it is out of tolerance.

The displayed value shows how much the connection exceeds the

limit

RDly=Length-(TARGET-Tolerance) 19.29-(21.18*0.95) = -0.83

Margin = RDly/Target 0.83/21.18=3.92%

Actual = (Target-Length)/Target (21.18-19.29)/21.18 = 8.923455%

Actual shows the difference to the target in percentage

During editing the target Cline are in the head-up display the differences to the longest and the shortest connection in the match group shown.

Note: Whether the head-up display follows the mouse pointer or is in the right bottom corner, can be set under: Setup -> User Preferences… -> Route -> Connect -> allegro_dynam_timing_fixedpos Note2: The quantity of the decimal places shown in the head-up display an CM is according to the accuracy setting of the design. Setup -> Design Parameters… -> Design ->Accuracy

6 Bibliography All Cadence Help Documentation. For further explanations refer to the Constraint Manager User Guide: %cdsroot%\doc\cmug\cmug.pdf

The T stands for Target

title: constraining delay rules product: orcad pcb ... · this document gives an overview of how...

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