AR #3813 - 4.1i XC4000XL/Spartan PAR - Router duplicates registers for use as "output-to-output route-thrus."
4.1i XC4000XL/Spartan PAR - Router duplicates registers for use as "output-to-output route-thrus."
| AR# | 3813 |
| Part | SW-Route |
| Last Modified | 2008-10-19 00:00:00.0 |
| Status | Archive |
| Keywords | replicate, duplicate, flop, register, FF, sync, asynchronous, route-thru, routethru |
Description
Keywords: replicate, duplicate, flop, register, FF, sync, asynchronous, route-thru, routethru
Urgency: Standard
General Description:
PAR will sometimes duplicate flops to drive separate sections of a net. This is considered to be an "output-to-output route-thru." This can be a problem when the input net is asynchronous, as there is a chance that the flip-flops will be clocked while the input is changing, and the two FFs could end up in different states.
This issue affects all 4000-derivative parts, including Spartan and Spartan-XL.
Solution 1
The following Perl script can be used to detect occurrences of duplicated registers in XC4000X* designs. The routed .ncd file is used as a command line argument. A valid Xilinx environment must be present for the script to run successfully. It may be necessary to modify the first line to the correct location, perl5, on your system.NOTE: It has been found that this script does not give accurate results for cases where CLBs contains multiple route-thrus. A patch for XDL is available to correct. this. See (Xilinx Answer 13636) for details.
Sample output:
swissx [186] dup_flop.pl test.ncd
A register has been replicated for net "DONE", which is driven by CLB "ACTIVE_N" at site CLB_R9C14:
dup_flop.pl:
#!/usr/local/bin/perl5
@ncd_root=split(/\./,$ARGV[0]);
`xdl -nopips -ncd2xdl $ncd_root[0]`;
open (FILE,"$ncd_root[0].xdl");
while(<FILE>){
chomp;
@fields=split(/\s+/);
if ($fields[0] eq "inst" && $fields[2] eq "\"CLB\""){
$comp_name=$fields[1];
$site_name=$fields[6];
}
if (/ROUTETHROUGH-XQ-YQ/) {
open (FILE2,"$ncd_root[0].xdl");
while(<FILE2>){
chomp;
@fields2=split(/\s+/);
if ($fields2[0] eq "net") {
$net=$fields2[1];
}
if ($fields2[1] eq "outpin" && $fields2[2] eq $comp_name && ($fields2[3] eq "YQ" || $fields2[3] eq "XQ")) {
$net_name=$net;
}
}
print "\n";
print "A register has been replicated for net $net_name,\n";
print "which is driven by CLB $comp_name at site $site_name.\n";
print "\n";
}
}
exit;
Solution 2
An environment variable (CM_EXCLUDE_XQYQ) has been created that disables the use of CLB flip-flop outputs for output-to-output route-thrus.To set the variable:
Workstations:
setenv CM_EXCLUDE_XQYQ
PCs:
set CM_EXCLUDE_XQYQ=TRUE