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Trenz JTAG adapter

Sun, 2008-05-18 18:39 — Peufeu

I was having problems using JTAG on the Spartan-3E FPGA.
Sometimes it would work, but not reliably, and most of the time it would not work. The JTAG waveforms looked good on the scope.
iMpact would complain that the ID number of the chip was wrong.
It was also very senditive to the cable used. I tried various flat cables with some limited success.

I am using this JTAG adapter (click image for link).

After lots of head scratching, here came the fix : remove the 3 capacitors shown below in the Trenz JTAG adapter. The purpose of those caps besides corrupting signal integrity is a mystery.

Result : now it works flawlessly.

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Fri, 2008-06-06 14:33 — Anonymous Coward

jtag troubles - fixes

I tracked my problem down to a long cable from
the JTAG device to the FPGA.

Apparently the Xilinx inputs are either VERY corrupted by
noise on the chip's ground or have some sort of negative
impedance characteristic -- they oscillate by themselves
when the input changes state.

A "SLOW" change in state on the "clock" line and the "problem" appears.

Either results in the same sort of problem.

Thus all the series resistors on FPGA traces.... In MOST
of the FPGA designs you look at !!!!!