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Parent page: High Speed Design
Two of the core challenges with routing a high-speed design are controlling the impedance of the routes, and matching the lengths of critical nets. Impedance-controlled routing ensures that the signal that leaves an output pin is correctly received by the target input pins. Matching the route lengths ensures that timing-critical signals arrive at their target pins at the same time. Tuning and matching route lengths is also essential to differential pair routing.
Accordion patterns have been added to the routing to ensure that the differential pairs have matched lengths.
The Interactive Length Tuning and Interactive Diff Pair Length Tuning commands (launched from the Route menu, or the button on the Active Bar) provide a dynamic means of optimizing and controlling net or differential pair lengths by allowing variable amplitude tuning patterns to be inserted, according to the available space, rules, and obstacles in your design.
Three styles of tuning patterns are available: Accordion, Trombone, and Sawtooth.
Three styles of tuning patterns are available, press Tab after launching the Interactive Length Tuning command to select the pattern.
Length tuning properties can be based on design rules, properties of the net, or values you specify. Controls for these wave patterns are accessed through the Properties panel - press Tab during length tuning to open the panel.
The elegance of the length tuning feature is that it cleverly combines sophisticated software algorithms with intuitive user control. Length tuning segments are added by simply wiping the cursor along the route path, with the dimensions and positions of the various tracks and arcs that make up the tuning segments automatically calculated and inserted by the length tuning algorithm. Keyboard shortcuts give control over the style and properties of the tuning segments, as they are being added.
The process of interactive length tuning is following:
Tuning segments are automatically added as the cursor moves along the route path.
The sections below describe these steps in more detail.
There are two design rules that are obeyed during length tuning, the Matched Length rule and the Length rule, both are in the High Speed category in the PCB Rules and Constraints Editor. Either or both of these rules may be important in your design, it all depends if your potential issues are related to skew (signals arriving at different times - use the Matched Length rule), or the overall signal delay (use the Length rule).
The Matched Length design rule specifies that the target nets must all be routed to the length of the longest net in the set, within the specified tolerance (show image). The set of nets that are targeted is defined by the rule scope or query.
The length tuning tool will find the longest net in the set of target nets and give you a valid range and target length (Value) of:
TargetLength = Longest routed net in set
MinLimit = LongestNet - MatchedLength Rule Tolerance
MaxLimit = TargetLength
Complimenting the Matched Net Lengths rule, the Length design rule specifies the minimum and maximum permissable routed length of a net, or set of nets. Targeted nets must have a length within the specified Minimum and Maximum lengths (show image).
The length tuning tool will find the longest net in the set of target nets and give you a valid range and target length (Value) of:
TargetLength = Longest routed net in set
MinLimit = Rule Minimum
MaxLimit = Rule Maximum
Either or both of these rules may be important in your design, it all depends if your potential issues are related to skew (signals arriving at different times - think Matched Net Lengths rule), or the overall signal delay (think Length rule).
If there is an applicable Length rule and a Matched Length rule, the length tuning tool considers both rules and works out the tightest set of constraints.
The valid range and target length (Value) are determined as follows:
TargetLength = Longest routed net in set, or lowest MaxLimit from rules
MinLimit = (LongestNet - MatchedLength Rule Tolerance), or highest MinLimit from rules
MaxLimit = TargetLength
ValidRange = Highest MinLimit to Lowest MaxLimit
(most stringent combination of Length and Matched Length rules)For example, if the maximum length specified by the Length rule is shorter than the longest existing route length identified by the Matched Length rule, then the Length rule wins and its shorter length is used during tuning. The panel displays the calculated Min Limit and Max Limit for each rule, use these to check that the target lengths are what you are expecting.
In the image shown just above, a Length rule and a Matched Length rule apply to the target nets. Note that the most stringent values come from the Matched Net Length rule (tolerance 0.5mm
), the Max Limit value shows that the current length of the longest net in the target set of nets is 46.836mm
(which is less than the maximum allowed by the Length rule). In this example, the tightest allowable tolerance in the range of lengths is the tolerance defined in the Matched Length rule (0.5mm
), so it is used to calculate the ValidRange
. The target length is always the more stringent maximum length.
After launching the Length Tuning command and before clicking a route for length tuning in the design space, press Tab to open the Properties panel where the Accordion, Trombone and Sawtooth buttons are available. Click the appropriate button to choose the required pattern, then click the design space pause button overlay () to resume placement.
Press Tab after launching the Interactive Length Tuning command to select the pattern.
Mitered Lines
or Mitered Arcs
.Style – style of the accordion corners, choose between Mitered Lines
, Mitered Arcs
or Rounded
.
Mitered Lines
or Mitered Arcs
.Style – style of the trombone corners, choose between Mitered Lines
, Mitered Arcs
or Rounded
.
When the required tuning pattern is chosen in the Properties panel, click a route in the design space to start its length tuning. Filtering will be applied to the design space to allow better visibility of just that net. Move the cursor along the route in the direction that the accordion is to be added. A tuning pattern will appear and continue to grow as the cursor moves. The animation below shows an example of placing an accordion tuning pattern.
Accordions are placed when the Interactive Length Tuning command is run, and the route length needs to be increased.
During length tuning, press Tab to open the Properties panel, where the properties of the selected tuning pattern (target length and geometry properties) can be changed on-the-fly. Click the design space pause button overlay () to resume placement.
There are three approaches for specifying the target length: manually defined, based on a net that has already been routed, or defined by design rules.
During interactive length tuning, the Target section of the Properties panel includes options for selecting the required Target Length mode. The lower section of the panel includes options that define the shape and dimensions of the tuning pattern, which can also be controlled interactively using shortcuts.
From Rules – applicable Length and Matched Length design rules. The software will then obey the most stringent combination of these rules. Double-click on a rule in the list in the panel to examine its properties in detail. To learn more about how the Length and Matched Length design rules are applied when tuning a net, refer to the Configuring the Design Rules section.
The applied rule is highlighted in blue. You can change the applied rule as you tune by clicking that rule's entry – it will become the rule highlighted in blue, and the target length (and descriptive text) will change accordingly.
The Value / Target Length field shows the overall length that the Interactive Length Tuning or Differential Pair Length Tuning command is attempting to achieve with the addition of the tuning pattern being placed. If the Clip to Target option is enabled, the tuning pattern length is automatically clipped once the target length (when tuning in Manual or From Net / From Diff. Pairs mode) or the length slightly below the target but within the defined tolerance (when tuning in From Rules mode) is achieved. That can be used to prevent adding too many tuning segments, which could result in passing the target length and exceeding the maximum permissible length.
Press Tab during length tuning to open the Properties panel, where you can select the target length mode.
When the PCB panel is set to Nets mode, it displays the current length of the routed signals. The default mode of the panel is to display the Name, Node Count, Routed length, and Un-Routed (Manhattan) length. Right-click in the column headings region of the panel to display a menu, where you can select extra columns, as well as hide existing columns.
If there are Length design rules configured, then the routed state of each net targeted by the rule is also colored, highlighted in yellow if the route length < rule minimum
, clear if the net passes the rule
, or red if the route length > rule maximum
.
Three of the nets fail the Length design rule, two are short and one is too long.
If there is a Length rule and/or a Matched Length rule defined, then you can monitor the length during both interactive routing and interactive length tuning, by displaying the Length Tuning Gauge. While you are routing or tuning, use the Shift+G shortcut to toggle the Gauge on and off.
The Gauge shows the current Routed Length as a number, and the red/green slider shows the Estimated Length. If you are length tuning an existing route then the Estimated Length is the sum of all of the placed tracks and arcs (the actual physical length). If you are using the Length Gauge while you are interactively routing a net, then the Estimated Length will be the sum of the placed routing plus the remaining distance to the target pad (the length of the connection line).
During interactive routing, it might seem confusing that the Routed Length has not even reached the rule minimum value, but the Gauge slider is somewhere between the rule minimum and maximum - as it is in the image below. That is because during interactive routing the slider represents the Estimated Length, where:
Estimated Length = Routed Length + distance to target (length of connection line)
The Gauge displayed as a Length design rule is being obeyed during Interactive Routing - it shows the current Routed length as a number, the slider shows the current Estimated Length.
The Gauge functions as follows:
The Gauge settings are calculated from the constraints defined by the applicable rules.
MinLimit
)MaxLimit
)MinLimit
) is 46.58MaxLimit
) is 47.58 (obscured by the green bar in the image above)TargetLength
) is 47.58 (route length of the longest net in the set, equal to MaxLimit
)During interactive length tuning, the pattern geometry properties can be changed on the fly from the Properties panel or using the shortcuts. The shortcuts available during length tuning include:
Use the shortcut keys to control the shape and amplitude of the tuning pattern during placement.
The tuning engine builds tuning patterns according to the current geometry settings. There are combinations of these settings, along with the current track width, that can make it impossible for the tuning engine to create a pattern shape. If you are attempting to length tune and the patterns do not appear, try these steps:
Mitered Lines
mode.If you follow these steps you should see rectangular-shaped tuning segments as you length tune. Keep these additional points in mind as you work with accordion patterns:
Amplitude > Radius + Route Width
.To modify a placed tuning pattern, click once to select it and display the editing handles. Click and drag on an edge or vertex to resize the pattern bounding region – the pattern sections are automatically resized to suit the new updated shape of the bounding region.
The Properties panel can be used (it will display an associated mode when a tuning pattern is selected). Use the controls to change the properties of the selected pattern.
The shortcuts can be also used during interactive editing. Click and hold a selected tuning pattern to use them.
Below is a video demonstrating how an Accordion pattern can be resized.
Resize the accordion bounding box to change the Amplitude or length, click and hold to move, edit the Style in the Properties panel.
As you tune the lengths of multiple nets, it can become difficult to fit additional tuning patterns in. If you are using Accordion patterns they can be rotated, which can help fit them between obstacles or add additional patterns. The video below demonstrates how to rotate a placed accordion pattern.
To rotate a selected accordion, hold Ctrl then:
For the Trombone and Sawtooth patterns, the polygonal area that the pattern is constructed within an envelope that can be thought of as a sleeve. Click to select a placed pattern and display the sleeve.
The trombone and sawtooth patterns are built within a sleeve shape, which supports a variety of shape-change behaviors.
There a number of different movement and size-change behaviors available, depending on where you click and hold on the sleeve. There are three zones where you can click and drag, these are shown in the image above.
The Shift and Ctrl shortcuts can be included during a click and drag action, modifying the behavior in the following ways:
The Properties panel of a placed tuning pattern selected in the design space includes the Layer drop-down in its Properties region. Use this drop-down to quickly change the signal layer where the tuning pattern is placed.
Note that you can select multiple routing objects (tuning pattern, tracks, arcs) to change their signal layer in a single action. Learn more about Strategies for Selecting the Routing.
The length of a differential pair can also be tuned against the length of other differential pairs, using the Interactive Differential Pair Length Tuning command (Route menu). As with differential pair routing, this command operates on the two nets in the pair simultaneously.
If you plan to length tune differential pairs, create the following rules to target the set of differential pairs:
A good approach to tune the lengths of differential pairs is to:
The PCB editor also provides you with automatic length/delay tuning (or multi-tuning) functionality. The functionality supports both single traces and differential pairs. Regular traces and odd angles (except differential pairs) are also supported.
The functionality is used as follows:
Choose the Route » Automatic Length Tuning command from the main menus (shortcut: Ctrl+Alt+T).
Configure the accordion-based pattern and its attributes as required in the Auto Tuning Process dialog that opens. Refer to the Accordion Pattern Geometry Properties section to learn more.
The Auto Tuning Process dialog
After clicking OK in the dialog, the tuning patterns will be created.
A length tuning pattern, being a union, is a group object – comprised of primitive track and/or arc segments, with full control over the amplitude, gap, and corner radius (or miter). As with other group objects, such as components, dimensions, and polygons, a length tuning pattern can be exploded. In other words, it can be converted into its constituent-free primitives, which can then be modified independently. Use the Explode Length Tuning to Free Primitives command to do this, available from the main Tools » Convert sub-menu, or the right-click Unions sub-menu.
Note that exploding any object in the PCB editor is a one-way process, once an object has been exploded it cannot be converted back into that object-kind. You can only use the Undo command to achieve this.
The Tools » Equalize Net Lengths command from the main menus of the PCB editor can be used to match the length of nets identified by a defined Matched Net Lengths design rule. After launching the command, the Equalize Nets dialog will open.
Use this dialog to define the style and sizing of the accordion segments that will be added by the software to equalize the lengths of the target nets. After clicking OK, track segments will be added to all nets in the set covered by the design rule that are shorter than the longest net in the set. The command will attempt to add track to these shorter nets until the specified tolerance condition in the relevant Matched Net Lengths rule has been met.
A design rule check will be performed for all defined (and enabled) Matched Net Lengths rules only and the Design Rule Verification Report (Design Rule Check - <PCBDocumentName>.html) will be opened as the active document. The report will list any violations of these rules. For information about how far outside of the tolerance each net in the applicable set is, refer to the relevant message in the Messages panel, an example of which is shown below:
Matched Net Lengths: Between Net LCD_RW And Net LCD_RS Length:85.061mm, outside tolerance by 7.564mm
In this case, the longest net in the set targeted by the applicable Matched Net Lengths rule is LCD_RS. The net LCD_RW has a routed length of 85.061mm, which is outside of the tolerance defined by the rule by 7.564mm.
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