# Problems with ISE logic optimization

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Hello, I'm trying to implement a simple delay line for a 1MHz clock
with an even number of inverter cells but the Synthetizer/Mapper
doesn't work!! I think the logic optimizer is simplifying my even
inverter cells in a short circuit...
Wich kind of options can I use for the Xilinx tools?
I'm using ISE7.1 and ISE8.1 with a Spartan3 device.
How can I implement a simple delay line with about 500ps of delay in a
FPGA?
Thanks a lot!!

Re: Problems with ISE logic optimization

I think this is a very bad approach. Gate delay times depend greatly
on temperature, so your 500 ps may be almost anything, but most of the
time they will not be 500 ps.

In fact, the propagation time from G to Y in a CLB varies from 530 to
610 ps, which is always more than what you want.

Do you really need such a fine delay? It is only 0.05%!

Regards

Zara

Re: Problems with ISE logic optimization
It's correct, it's a very bad approach, but I think it's the
simplest...
The delay depends on temperature, but for my application, is not so
important the absolute value of this delay if it's less than 1ns.
It's more important that two contiguous delay cells have the same
delay.

Regards

Re: Problems with ISE logic optimization

##   >Hello, I'm trying to implement a simple delay line for a 1MHz
clock
##   >with an even number of inverter cells but the Synthetizer/Mapper
##   >doesn't work!! I think the logic optimizer is simplifying my
even
##   >inverter cells in a short circuit...
##
##
##   I think this is a very bad approach. Gate delay times depend
greatly
##   on temperature, so your 500 ps may be almost anything, but most
of the
##   time they will not be 500 ps.
##

Probably the best is to use LUT1/2/3/4 and (R)LOCs to ensure that the
buffer/inverter/logic function is synthesized as one CLB only.

Zara

Re: Problems with ISE logic optimization

Also make sure the routing resources are locked too. DIRT (direct
routing) strings may help.

HTH,
Jim
http://home.comcast.net/~jimwu88/tools /

Re: Problems with ISE logic optimization
I've resolved the problem using LUT1 and (R)LOC constraints for direct
routing!!
Thanks a lot for the suggestions!!!

Daniel

Re: Problems with ISE logic optimization

This is exactly what the synthesizer is supposed to do.
You need to instantiate hard macros. You can generate the macros using the FPGA
editor and
than instantiate them as black boxes in your HDL.
As most of the delay in an FPGA is in the routing you also need to manually
place the macros.
Probably the least sensitive delay in an FPGA is the carry logic. It is a lot
faster than
500ps but this allows you to calibrate your delay in approximate 50ps steps.
Just have a reference line that measures how far a signal can go in one clock
cycle and based
on that measurement decide, where you want to tap your delay line.

See this paper for an example: