Designers can select one or more objects in the [Clock Regions] window, [I/O Ports] window or [Package Pins] window, or click the button to cancel all selections, and then run the [Tools] → [Clear Placement Constraints] command. Open the window shown in Figure 10-48. There are three options. [Instance placement] clears the layout plan of the instance, [I/O port placement] clears the I/O port layout plan, and [Both] both objects are cleared.
In this example, we select a specific Pin foot in the [Package Pins] window to select [I/O port placement] as an example.
![[Clear Placement Constraints] window](http://i.bosscdn.com/blog/0F/01/2S/28_0.png)
Figure 10-48 [Clear Placement Constraints] Window
Click the [Next] button to open the dialog box shown in Figure 10-49. Select the [Unplace selected port] option, click [Next], open the dialog box shown in Figure 10-50, select [Unplace one fixed port], click the [Next] button, click [Finish] in the window that appears, clear An I/O constraint.
It should be noted that the operation process here will select several objects depending on whether the designer has selected the object, and the operation interface is slightly different.
Figure 10-49 Specifying I/O constraints to be cleared
Figure 10-50 prompts to clear a fixed type of port
Sixth, establish and configure I / O portSelect [Create I/O Ports...] in the right-click menu of the [I/O ports] window, and the dialog box shown in Figure 10-51 will pop up. Enter the port name such as port_1 to change the port properties or create a bus type port. . Click the [OK] button to see that a port has been added to the [I/O ports] window.
Figure 10-51 Establishing an I/O port
The configuration of the I/O port is similar. Select [Configure I/O Ports...] from the right-click menu.
Seven, establish a management interface for I / O portsMaximize the [I/O Ports] window, select all ports of *USB_0*, run the right-click menu command [Create I/O Port Interface...], enter usb0_interafce in the pop-up dialog box, as shown in Figure 10-52, click [ OK], you can see that these *USB_0* ports have a management interface named usb0_interface, as shown in Figure 10-53. Using this method, PCB management can be facilitated.
Figure 10-52 Establishing an I/O port interface
Figure 10-53 Established I/O Port Window
Eight, allocate I / O portPlanAhead provides a variety of methods for allocating I/O ports, which can be automatically assigned, as well as several semi-automatic allocation methods, such as assigning I/O ports to an I/O Bank [Place I/O Ports in an I/O Bank ], assign I/O ports to a specified area [Place I/OPorts in an Area], and sequentially assign I/O ports [Place I/O Ports SequenTIally].
It should be noted that in order to ensure the correctness of I/O allocation, it is necessary to turn on the automatic DRCs check function of I/O allocation before performing I/O allocation. Execute [Tools] → [OpTIons] → [General] to open the [AutomaTIcally enforce legal I/O placement] function.
1. Automatically assign I/O ports.
You can complete the automatic assignment of I/O ports by running [Tools] → [Autoplace I/O Ports].
2. Place the I/O port in an I/O Bank.
In the [I/O ports] window shown in Figure 10-53, select the usb0_interface port, run [Place I/O Ports in an I/O Bank] in the right-click menu, or click the button in the [Package] window. Paste the pin associated with usb0_interafce to the mouse, drag the mouse in the [Package] window, find the appropriate location and click the left mouse button to complete the I/O port assignment.
Clear the allocation to the usb0_interface port to facilitate the following operations.
3. Place the I/O port in a zone.
In the [I/O ports] window shown in Figure 10-53, select the usb0_interface port, run [Place I/O Ports in an Area] in the right-click menu, or click the button in the [Package] window. Package] Drag the mouse to draw a rectangular box suitable for all I/O ports, and release the mouse to complete the I/O port assignment.
Clear the allocation to the usb0_interface port to facilitate the following operations.
4. Place the I/O ports in sequence.
In the [I/O ports] window shown in Figure 10-53, select the usb0_interface port, run [Place I/O Ports SequenTIally] in the right-click menu, or click the button in the [Package] window. At this time, you can use [Package]. Drag the mouse over the window to place all I/O ports in order until all I/O ports are allocated. It should be noted that the description here is the synthesis of the pre-I/O port allocation, the application is a CSV file, there is no comprehensive netlist, the clock pins, high-speed transceivers and differential pairs are not automatically processed by the tool, therefore, such Special attention is required when assigning pins.
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