Z0 and Zdiff Planning

Two new tabs on the enhanced 'Layer Stack Editor' have been implemented to enable the creation of Impedance Templates to control the impedance of single ended signals (Z0 Planning) and differential signals (Zdiff Planning);

Z0 Planning Tab

The 'Z0 Planning' tab is shown in the figure below:

Impedance Template Section

This section of the dialog allows you to add, delete and rename templates. You can select the current template from the drop down list. Note that the name of the impedance template will be displayed in red if the impedance template is illegal. This will be so in either of the following cases:

•     The impedance, width or spacing for one or more layers cannot be calculated

•     The calculated width is smaller than the minimum width allowed for the design

The layers that this applies to will not have values displayed in the spreadsheet, as shown in the following figure;

The OK button will be disabled if any templates are illegal. You can add, rename and delete templates

Template Solver Section

There are two options, 'Solve Current' and 'Solve All'. 'Solve Current' Calculates the currently displayed template, taking into account any changes that have been made. The 'Width', ' Z0' and 'Velocity' values are updated in the spreadsheet, as appropriate. 'Solve All' calculates all templates, taking into account any changes that have been made.

If you change the layer stack in the 'Layer Stack' tab, you are prompted to update the impedance templates when you select the 'Solve' option.

Spreadsheet Section

The spreadsheet displays the 'Width', 'Z0' and 'Velocity' values for each conductor layer in the layer stack. You will typically define an impedance template in one of two ways:

• Specify width values and calculate the impedance:

• Specify impedance values and calculate the width:

When an impedance template is first created, the 'Width' value is typically set at the 'All' level to be the board level value of net width attribute. If you select the 'Calculate Template' option without making any other changes, impedance values will be calculated using this width value. However, if you explicitly type in a target impedance value, the default width value is ignored:

In either mode, 'Velocity' values are calculated and displayed, these values cannot be changed. You can specify the required width or impedance at two levels:

• Values set at 'All' level apply to all layers that don't have an explicit value set, as shown in the figures above

• Values set on a particular layer are used to calculate the values for that layer only

Note that it is also possible to specify both the width and impedance values. In this case, no values are calculated.

Zdiff Planning Tab

This tab allows you to define the values required to control the impedance of differential pairs.

The tab works in a similar way as the 'Z0 Planning' tab, described above, with the exception that the information in the spreadsheet is different.

User editable values.

There are three user editable values in the 'Zdiff Planning' tab, 'Width', 'Spacing' and 'Z_diff' (differential impedance). In general, you must specify two out of the three values, and the third value will be calculated. As with 'Z0 Planning tab', default values can be specified at the 'All' level, and explicit values set on individual layers.

If you specify fewer than two values for any layer, the third value will not be calculated

The following result values are calculated, and are not editable by the user:

• Z_odd - Odd mode impedance

• Z_even - Even mode impedance

• Z_common - Common mode impedance

• v_diff - Differential velocity

• v_odd - Odd mode velocity

• v_even - Even mode velocity

• v_common - Common mode velocity

Applying templates to design items

Once an impedance template has been defined, it can be set on a design item in one of the following ways:

• Using the 'Impedance Template' column in the 'Constraint Manager' spreadsheet. This is in the 'Impedance' tab by default.

• Using the 'Edit', 'Set Impedance Template' option in the 'Constraint Manager'          

• Using the 'Set Impedance Template' on the tree view context menu in the 'Constraint Manager'

• Using the 'Set Impedance Template' option of the 'Routing' menu in 'Physical Editor'

When an impedance template has been set on an item, the spacings and widths are copied onto the target item, as appropriate, and the router will use these values to ensure that the required impedance values are achieved. In addition, impedance templates can be set hierarchically.