Using JTAG cable for general comms

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Is it possible to use a JTAG cable connection to transfer data into and
out of an FPGA during regular operations? The situation is like this:
we have a Xilinx Spartan 3 board that we are programming with a
digilent parallel cable. This works fine. We would also like to
transfer data into and out of the FPGA, for which we are currently
using serial communications (RS232), but the data rate is now becoming
a bottleneck.

The JTAG cable, connected through the parallel port, should be capable
of doing much higher speeds than RS232. It is, however, connected to
the JTAG pins of the FPGA. Can this somehow be used to transfer data in
and out without redesigning the board in any way? Is there any software
that could be a starting point for this?


Nitin Chandrachoodan

Re: Using JTAG cable for general comms
The following Xilinx Xcell article describes how you can use the JTAG port
of a Xilinx FPGA as a general comms channel to a design. One of the uses
we have for it is to replace the RS232 as the serial comms channel to designs.

The example GNAT design can be downloaded for free from here. /

It includes the GNAT design and also supplies some sample TCL illustrating
how you can pass data into and out of the FPGA.

Derek wrote:

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