We have detected your current browser version is not the latest one. Xilinx.com uses the latest web technologies to bring you the best online experience possible. Please upgrade to a Xilinx.com supported browser:Chrome, Firefox, Internet Explorer 11, Safari. Thank you!

AR# 38447

Design Assistant for PCI Express - Memory reads result in 0xFFFFFFFF


The core is correctly detected by they system but memory read accesses returns 0xffffffff.

Note: This Answer Record is a part of the Xilinx Solution Center for PCI Express (Xilinx Answer 34536). TheXilinx Solution Center for PCI Express is available to address all questions related to PCIe. Whether you are starting a new design with PCIe or troubleshooting a problem, use the Solution Center for PCIeto guide you to the right information.


Usually the system will report all 1's for a read if some type of error condition occurs. On a memory read this could be either that the memory enable bit in the command register is not set or, more likely, there was a BAR miss. Both of these will cause the endpoint to return a completion with UR status which will be reported back to the application as all 1's.

Specifically, if using the Endpoint Block Plus core for PCI Express, this may happen if the user has incorrectly tied the cfg_rd_en_n signal to ground, which is the asserted state, or left it floating in the RTL, which means synthesis probably tied it off the ground. By doing this, the user is inadvertently preventing the block plus wrapper from obtaining the information it needs from the block to perform BAR decodes, which will result in BAR misses.

Revision History:
10/08/2010 - Initial Release

Linked Answer Records

Associated Answer Records

Answer Number Answer Title Version Found Version Resolved
34538 Xilinx Solution Center for PCI Express - Design Assistant N/A N/A
AR# 38447
Date 12/15/2012
Status Active
Type General Article
  • Endpoint Block Plus Wrapper for PCI Express
  • Virtex-6 FPGA Integrated Block for PCI Express ( PCIe )
  • Spartan-6 FPGA Integrated Endpoint Block for PCI Express ( PCIe )