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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.
The Interactive Length Tuning and Interactive Differential Pair Length Tuning commands (launched from the Route menu) provide a dynamic means of optimizing and controlling net or differential pair lengths by allowing variable amplitude wave patterns (accordions) to be inserted according to the available space, rules, and obstacles in your design.
Length tuning properties can be based on design rules, properties of the net, or values you specify. Controls for these wave patterns, also known as accordion sections, 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 complete control over the style and properties of the tuning segments as they are being added.
Launched from the Route menu, the Interactive Length Tuning commands 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. No need to worry if you drift off course, as soon as you bring the cursor back over the route, the algorithm will add tuning segments up to that point.
There are 2 key ingredients to mastering the length tuning tool: knowing the shortcuts, and understanding how to read the Net Length Gauge.
|Spacebar||Cycle through the 3 tuning patterns|
|, (comma)||Decrease amplitude by the amount specified for the Amplitude Increment|
|. (full stop)||Increase amplitude by the amount specified for the Amplitude Increment|
|3||Decrease pitch by the amount specified for the Gap Increment|
|4||Increase pitch by the amount specified for the Gap Increment|
|1||Decrease corner miter|
|2||Increase corner miter|
|Y||Toggle starting direction|
|Tab||Open the Interactive Length Tuning mode of the Properties panel|
|Shift+G||Toggle Length Tuning Gauge on/off|
There are three approaches for specifying the target length:
To choose which of these methods is being used, press Tab during length tuning to open the Properties panel in Interactive Length Tuning mode. The lower sections 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. The Target Length section of the panel includes options for selecting from the three Target Length modes.
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 - think Matched Length rule), or the overall signal delay (think Length rule).
The Matched Length design rule specifies that the target nets must all be routed to the same length, within the specified tolerance. The set of nets that are targeted is defined by the rule scope, or query. The length tuning
tool will then find the longest net in the set of target nets and give you a valid range of
MinLimit = LongestNet - Tolerance, through to
MaxLimit = LongestNet.
Complimenting the Matched Length rule, the Length 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.
If there is an applicable Length rule and Matched Length rule, the length tuning tool considers both of these rules and works out the tightest set of constraints. So if the maximum length specified by the Length rule is shorter than the longest length targeted by the Match Length rule, then the Length rule wins and its length is used during tuning.
The Valid Range and Target Length are determined as follows:
Matched Net Length MinLimit = LongestNet - RuleTolerance
Matched Net Length MaxLimit = LongestNet
ValidRange = Highest MinLimit to Lowest MaxLimit (most stringent combination of Length and Matched Length rules)
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 above, the target length is being defined by the rules. Note that the most stringent values come from the Matched Net Length rule, the Max Limit value shows that the current length of longest net in the target set of
53.479mm (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 (
1mm), 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 interactive routing or length tuning by displaying the Length Tuning Gauge. While you are routing, use the Shift+G shortcut to toggle the Gauge on and off.
The Gauge shows the current Routed Length as a number, while the green slider shows the Estimated Length. At first glance at the Gauge it might seem confusing that the Routed Length has not even reached the rule minimum value, but the green slider is somewhere between the rule minimum and maximum - as it is in the image below. That is because the green slider represents the Estimated Length, where
Estimated Length = Routed Length + distance to target.
The Gauge functions as follows:
Matched Net Length MinLimit)
Matched Net Length 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 on the column headings 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.
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 one matched length rule that defines the pair-to-pair length matching requirements (with the Group Matched Lengths option enabled), and a second, higher priority matched length rule that defines the within-pair length matching requirements (with the Within Differential Pair Length option enabled), as this is typically a tighter requirement.
A good approach to tune the lengths of differential pairs is to:
To modify an existing accordion section, 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.
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. To do so, use the Explode Length Tuning command, available from the main Tools » Convert sub-menu, or the right-click Unions sub-menu.
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