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Parent page: The Routing
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 an essential ingredient of differential pair routing.
Accordion patterns have been added into 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 Interactive Length Tuning mode of the Properties panel - press TAB during length tuning to open the panel in this mode.
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.
Launched from the Route menu (or via the button on the Active Bar), the Interactive Length Tuning command prompts you to select a route. After clicking on a net or differential pair (or any free line or track for that matter), you simply slide, or wipe the cursor along the path of the route. If you drift off course simply bring the cursor back over the route, as soon as you do the algorithm will add tuning segments up to that point.
Tuning segments are automatically added as the cursor moves along the route path.
Length tuning supports all three popular tuning patterns: Trombone, Sawtooth, and Accordion. Press Tab after launching the Length Tuning command to open the Properties panel in Interactive Length Tuning mode, click the appropriate button to choose the required Pattern, then start length tuning a net.
Press Tab after launching the Interactive Length Tuning command to select the pattern.
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:
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:
There are two key ingredients to mastering length tuning: knowing the shortcuts, and understanding how to read the Net Length Gauge.
The shortcuts available during length tuning include:
Use the shortcut keys to control the shape and amplitude of the tuning pattern during 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.
To select which of these methods is going to be used, press Tab during length tuning to open the Properties panel in Interactive Length Tuning mode. The Target Length section of the 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 the shortcuts detailed above.
Press TAB during length tuning to open the panel in Interactive Length Tuning mode,
where you can select the target length mode and adjust the accordion parameters.
The tuning engine builds the accordions according to the current Max Amplitude, Space, Miter and Style settings. There are combinations of these settings, along with the current track width, that can make it impossible for the tuning engine to create an accordion shape. If you are attempting to length tune and the accordions 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:
Amplitude > Radius + Route Width
.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 ( ). The set of nets that are targeted is defined by the rule scope, or query. 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
(vertical green bar in Gauge)MinLimit = LongestNet - MatchedLength Rule Tolerance
(left yellow bar in Gauge)
MaxLimit = TargetLength
(right yellow bar in Gauge, obscured by the green bar in above image)Red/green slider = current total route length
The Length design rule specifies the overall 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
(vertical green bar in Gauge)MinLimit = Rule Minimum
(left yellow bar in Gauge)MaxLimit = Rule Maximum
(right yellow bar in Gauge)Red/green slider = current total route length
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
(vertical green bar in Gauge, obscured by the right yellow bar in above image)MinLimit = (LongestNet - MatchedLength Rule Tolerance), or highest MinLimit from rules
(left yellow bar in Gauge)MaxLimit = TargetLength
(right yellow bar in Gauge)ValidRange = Highest MinLimit to Lowest MaxLimit
(most stringent combination of Length and Matched Length rules)Red/green slider = current total route length
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 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.
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
)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.
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:
To modify an existing accordion, click once to select it and display the editing handles, as shown in the animation below. Click and drag on an edge or vertex to resize the accordion bounding region — the accordion sections are automatically resized to suit the new updated shape of the bounding region.
Resize the accordion bounding box to change the Amplitude or length, click and hold to move, edit the Style in the Properties panel.
A length tuning accordion, being a union, is a group object - comprised of primitive track and/or arc segments. As with other group objects, such as components, coordinates, dimensions and polygons, a length tuning accordion object 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 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.
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