download cable III with spartan-3

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dsMasoud

Joined: 08 Mar 2004
Posts: 32

23 May 2007 6:51 spartan 3 parallel cable iii

as you know Xilinx download cable III circuit works with 5v and spartan-3 is 3.3v tolerant. I have a question which may seems funny.
is it possible to connect download cable III(which works with 5v) directly to program spartan-3 or should use a 3.3v kind of download cable?

thanks

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echo47

Joined: 07 Apr 2002
Posts: 4206
Helped: 566

23 May 2007 7:09 download cable III with spartan-3

According to page 2-2 of this old iMPACT User Guide, the Parallel Cable III supports 2.5V, 3.3V, and 5V.
http://toolbox.xilinx.com/docsan/xilinx4/pdf/docs/pac/pac.pdf

yego

Joined: 28 Oct 2004
Posts: 152
Helped: 14
Location: Middle of Nowhere

23 May 2007 10:19 Re: download cable III with spartan-3

dsMasoud wrote:

If you take a look at the schematic diagram of the download cable III you'll notice that the 74xx125 is getting its power from the JTAG Header pin 1 which is located on the FPGA PCB. In case of Xilinx implementation this voltage is 3.3V and is absolutely safe from the FPGA viewpoint. So why are you concerned with the issue ? FPGA will never see any voltage other than that supplied to the buffer chip (74xx125 in this case).
The cable was designed that way to easily adjust to different voltage requirements of programmed devices. Some CPLDs require 5V while S3 needs 3.3V and this cable gives them exactly what they want no matter if they are 5V tolerant or not.

The schematic diagram tells you all you need to know.
Unfortunately these days many FPGA programmers are not electronics aware. Some basic knowledge, however, would do no harm and would lets you answer some questions yourself.