Hey ppl
Is there a way we can have a control over the input order of the LUTs...I see that eventhough we specify the inputs in the VHDL code (I0, I1, I2 and I3), during the mapping process, the tool automatically makes modifications and the routed design has a different order than the one specified in the VHDL code....
Thanks in Advance
Mr.B
Yes, there is with a little bit of hand-waving. The PAR software does not lock inputs for a standard LUT, however it does lock inputs for an SRL16E, because there the order of the inputs are important. The SRL16E behaves like a LUT4 as long as the write enable input is held low. The trick, therefore to locking the LUT pin assignments is to replace the LUTs with SRL16E's with the enable control grounded. I haven't tried this on recent versions of ISE, but I know it worked in versions up through 6.3.
The "LOCK_PINS" constraint should take care of this. Check it out in the constraint user guide. Depending on ISE version, you may need to set it to different values.
HTH, Jim
snipped-for-privacy@gmail.com wrote:
When I was trying to lock down only 2 pins out of 4, I had no problems for a while, then I couldn't get them to work; it might have been a software change. For this delay-matching circuit (to a first order approximation) I had to use the LOCK_PINS:ALL constraint to map I0-I3 to A1-A4... all 4 pins even though only 2 were critical.
There are very few applications that benefit from locking down the pins, but they do exist.
Jim Wu wrote:
Well, depending on how you define "best routing", swapping LUT pins may or may not be a godd thing.
Cheers, Jim
Peter Alfke wrote:
Yep, and normally you also need to lock your routing for these rare apps. Why not enter the twilight zone of 'directed routing'? :-)
thank u folks....
John, which version of xilinx wer u working on? I am currently working on 8.1 and was trying to use the lock pins constraint, INST "lut_name" LOCK_PINS = "I0:A1, I1:A2, I2:A3, I3:A4"; it pops up with a syntax error in da UCF file. I dont know wer I am goin wrong. could u suggest me smthin on this....
thank u
Mr.B
and was trying to use the lock pins constraint, INST "lut_name" LOCK_PINS = "I0:A1, I1:A2, I2:A3, I3:A4"; it pops up with a syntax error in da UCF file. I dont know wer I am goin wrong. could u suggest me smthin on this....
Hi Pete
I am tryin to permute the input order for that I need to fix the inputs in a certain fashion.
Mr.B
Peter Alfke wrote:
8.1 and was trying to use the lock pins constraint, INST "lut_name" LOCK_PINS = "I0:A1, I1:A2, I2:A3, I3:A4"; it pops up with a syntax error in da UCF file. I dont know wer I am goin wrong. could u suggest me smthin on this....8.1 and was trying to use the lock pins constraint, INST "lut_name" LOCK_PINS = "I0:A1, I1:A2, I2:A3, I3:A4"; it pops up with a syntax error in da UCF file. I dont know wer I am goin wrong. could u suggest me smthin on this....
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