AR# 11382

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Virtex/-E/-II/-II Pro, Package file - Pin-out tables do not list VCCO and GND pins in their physical PAD order (SSO)

Description

General Description:

The Virtex-II pin-out data/package file does not list the pins in the die pad order with the VCCO/GND pads between the I/O. (Virtex/Virtex-E pin-out tables do have this information.)

For example, compare the following two package files:

http://support.xilinx.com/products/virtex/package/2v500fg256.txt

(This Virtex-II file does NOT include the VCCO/GND pad locations.)

http://support.xilinx.com/products/virtex/package/v300efg256.txt

(This shows where VCCO/GND are inserted among the pins.)

The Simultaneously Switching Output Guidelines for Virtex-II in the "Virtex-II Platform FPGA User Guide" (see link below) list the following:

- Maximum number of simultaneously switching outputs per Power/Ground pin in Table 2-37

- Equivalent Power/Ground pairs per bank in Table 2-38

http://www.xilinx.com/xlnx/xweb/xil_publications_display.jsp?category=/User+Guides/FPGA+Device+Families/Virtex-II/&iLanguageID=1

Select Design Considerations -> Using Single-Ended Select I/O Ultra Resources -> Design Considerations -> Simultaneous Switching Output (SSO) Guidelines.

How do I follow the SSO guidelines and distribute the outputs properly without information on the location of the Power/Ground pairs among the I/O?

Solution

(For a detailed discussion about handling SSOs, please see (Xilinx XAPP689): "Managing Ground Bounce in Large FPGAs.")

The "equivalent" Power/Ground pairs are not the physical number of Power/Ground pairs. Physically, many more Power/Ground pairs are located between the I/Os to ensure short paths to Power and Ground. However, the package parasitic and inductance, as well as process variation, also contribute to the overall effect of SSO. The "equivalent" Power/Ground pair numbers are generated with all of these effects taken into consideration.

To determine the maximum SSO per bank, use the two tables with the correct I/O standard. For example, if you are using GTL with a maximum SSO of 4 per Power/Ground pair, and 2v1000fg256 with 3 equivalent Power/Ground pairs per bank, your maximum SSO per bank is: 3 x 4 = 12.

There are no further restrictions on the placement of I/Os within a bank, and the physical placement of Power/Ground on the die is NOT needed to follow the SSO guideline. Also, it is important to note that a good decoupling scheme is A MUST. Reference (Xilinx XAPP623): "Power Distribution System (PDS) Design: Using Bypass/Decoupling Capacitors" for more information on PDS decoupling.

AR# 11382
Date 12/15/2012
Status Active
Type General Article
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