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AR# 11378

Virtex/-E/-II/-II Pro/-4, Spartan-II/-IIE/-3 - Are all of the VCCINT, VCCAUX, VCCO, or GND pins in the device connected internally?

Description

General Description:

Are all of the VCCINT, VCCAUX, VCCO, or GND pins in the Virtex device connected internally? My PCB board contains unsoldered/unconnected VCCINT and VCCAUX pins. Will the device work properly?

Solution

VCCINT

VCCINT inputs feed a common metalization inside the chip, so they are connected internally. However, all VCCINT balls should be soldered to ensure good decoupling capacitors and to minimize IR drop and noise on the power rails.

The distribution of VCCINT (or ICCINT) is not perfect with incomplete connections. Although your design might work, it is exposed to uncontrolled internal VCC drop and VCC noise. Xilinx does not recommend this approach.

VCCAUX

NOTE: VCCAUX pins are present only in Virtex-II, Virtex-II Pro, and Spartan-3. There are no VCCAUX pins in Virtex/-E and Spartan-II/-IIE devices.

All VCCAUX bumps on the die go to a common metal interconnect and to a common connection on the laminate package.

As with VCCINT, DO NOT leave any VCCAUX pins unconnected, since it increases the inductance to the bypass caps and slightly increases noise and jitter. VCCAUX pins must be well bypassed.

See (Xilinx Answer 11182) for more information on VCCAUX.

VCCO

VCCO pins are only tied together for pins in the same bank.

GND

On the die, all ground inputs (relative to VCCAUX, VCCOs , VCCINT) feed a common metalization.

In addition, some packages have multiple ground planes that are closer to VCCO, VCCAUX, and VCCINT. These ground planes are designed for Signal Integrity (SI) reasons (i.e., to minimize the return path inductance to the balls for each supply), as well as the return path for the I/O pins themselves.

For Virtex-II or Virtex-II Pro, refer to additional information on PCB layout related to VCC and GND planes requirements in the "Virtex-II Platform FPGA User Guide," or the "Virtex-II Pro Platform FPGA User Guide," respectively at:

http://www.xilinx.com/xlnx/xweb/xil_publications_index.jsp?category=User+Guides

1. Select Virtex-II or Virtex-II Pro under the FPGA Device Families section.

2. Select the "Virtex-II Platform FPGA User Guide" or the "Virtex-II Pro Platform FPGA User Guide."

3. Select PCB Design Considerations -> Printed Circuit Board Considerations -> Layout Considerations.

For Virtex-4, refer to the Virtex-4 PCB Designer's Guide :

http://www.xilinx.com/xlnx/xweb/xil_publications_display.jsp?category=/User+Guides/FPGA+Device+Families/Virtex-4/&iLanguageID=1
Select Virtex-4 PCB Designer's Guide.

AR# 11378
Date Created 08/29/2007
Last Updated 12/15/2012
Status Active
Type General Article