11.3 Spartan-6 FPGA Route - BUFG fails to work or works intermittently

AR#	33519	Search For Another Answer
Topic	Route
Last Updated	09/09/2010
Status	Active

Description

My Spartan-6 device passes simulation and has no timing issues, but it fails in hardware. One of the BUFGs in the design works intermittently.  

 

What causes this behavior?

Solution

There is a low frequency problem in Spartan-6 devices where a BUFG can be affected by an unrelated signal. The problem occurs when a signal is "bounced" off a switchbox pin that is connected to the unused "S" pin of the BUFGMUX site where the BUFG is placed. 

 

To check for this condition in FPGA Editor, trace from the "S" pin of the suspect BUFGMUX site, through the green pin-wire to the white local connection wire; this wire will end at a switchbox pin. Check to see if any routing is connected to the switchbox pin as a bounced connection. A switchbox bounce is part of a two-way connection inside a switchbox between the input and output pins. 

 

If this problem is detected, it can be avoided by forcing the signal to be routed on different resources. If this is not possible, a patch is available for ISE Design Suite 11.3. Open a WebCase and refer to this Answer Record number; (Xilinx Answer 33519).