Re: Carry-chain based tapped delay line in Spartan3 - resolution? PVT variability?

John_H wrote:
[snip]
I do use a controlled injection of the source into the 8 LUTs at the
bottom of my chain giving pretty strong repeatability there. I used a
method from XAPP 671 to get another half-LUT delay but I added it at the
front end. Bottom line: I have 0-7.5 LUTs worth of programmable delay
averaging just over 200 ps for each half step along with about 100 ps
per tap up the carry chain.

This is an important point. I tried to create a variable delay by
injecting the input into each mux element of the carry chain and
controling the muxes to select the source point. In simulation I
saw that changing the delay parameter (mux control) made almost
no difference in the delay. I first thought something was wrong with
the logic, then found that the routing delays to the various carry
mux inputs very nearly matched the carry chain delay, causing
the the net effect of the delay selection to be almost zero.

Regards,
Gabor

.

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