wbOutputData, which feeds external logic. Your objective is to
precisely control timing skew by manually routing the nets of this bus.
You can use the Report Datasheet command to analyze the current timing of members of
the output bus,
wbOutputData. The Report Datasheet command lets you
analyze the timing of a group of ports with respect to a specific reference
- From the main menu, select .
- Select the Groups tab in the Report Datasheet dialog box, as seen in
the following figure, and enter the following:
- Click OK.
In this case, you are examining the timing at the ports carrying the
wbOutputDatabus, relative to the first bit of the bus,
wbOutputData. This allows you to quickly determine the relative timing differences between the different bits of the bus.
- Click the Maximize button to maximize the Timing - Datasheet window and expand the results.
- Select the
You can see from the report that the timing skew across the
wbOutputDatabus varies by almost 660 ps. The goal is to minimize the skew across the bus to less than 100 ps.
section, as seen in the following figure.
- Click the Restore button so you can simultaneously see the Device window and the Timing - Datasheet results.
- Click the hyperlink for the Max Delay of the source
This highlights the path in the Device window that is currently open.Note: Ensure the Autofit Selection is highlighted in the Device window so you can see the entire path, as shown in the following figure.
- In the Device window, right-click on the highlighted path and select
Schematic from the popup menu.
This displays the schematic for the selected output data bus. From the schematic, you can see that the output port is directly driven from a register through an output buffer (
If you can consistently control the placement of the register with respect to the output pins on the bus and control the routing between registers and the outputs, you can minimize skew between the members of the output bus.
- Change to the Device window.
To better visualize the placement of the registers and outputs, you can use the
mark_objectscommand to mark them in the Device window.
- From the Tcl Console, type the following
mark_objects -color blue [get_ports wbOutputData[*]] mark_objects -color red [get_cells wbOutputData_reg[*]]
Blue diamond markers show on the output ports, and red diamond markers show on the registers feeding the outputs, as seen in the following figure.
The outputs marked in blue are spread out along two banks on the left side starting with
wbOutputData(on the bottom) and ending with
wbOutputData(at the top), while the output registers marked in red are clustered close together on the right.
To look at all of the routing from the registers to the outputs, you can use the
highlight_objectsTcl command to highlight the nets.
- Type the following command at the Tcl
highlight_objects -color yellow [get_nets -of [get_pins -of [get_cells \ wbOutputData_reg[*]] -filter DIRECTION==OUT]]
This highlights all the nets connected to the output pins of the
In the Device window, you can see that there are various routing distances between the clustered output registers and the distributed outputs pads of the bus. Consistently placing the output registers in the slices next to each output port eliminates a majority of the variability in the clock-to-out delay of the
- In the main toolbar, click the Unhighlight All button and the Unmark All button.