3.3V configuration of Spartan-3?

There are a lot of details there that will take some real work to understand. I just dealt with configuring a Spartan 3 and dealt with the whole thing by just using a separate JTAG port for the FPGA. I have spent some significant time looking into using JTAG for both development purposes and for boundary scan. The one thing I got from all corners is that you are much better off avoiding the various problems you may encounter (not just interface voltage issues) in a single scan chain by not connecting everything in one chain. Our in-house boundary scan expert does not like using the Xilinx parts because of various problems when doing the tests, even though they are not in the same scan chain. I have been told by nearly every chip vendor that they don't support chaining their product with other brands of chips if they support chaining at all!

I think Xilinx software may actually be a bit better behaved when working with other chips in the scan chain, but I still would not recommend this depending on the other chips. TI says they support scan chains in their literature and was a major proponent a decade or so ago. But when I contacted support about using their DSP chip emulation software in a scan chain they recommended not to.

It seems to be a common practice to disregard the chaining capabilities of JTAG unless you really have to save the board space. Instead give each chip a separate connector. During development this will let you use multiple tools at the same time and if you need boundary scan in manufacturing they make tools that will drive multiple chains.

Evan,

Yes, you are being paranoid.

Unlike others, our app notes have been tested, and reviewed by senior engineers...

Aust> I've got a Spartan-3 which is configured (in master serial mode) from

Thanks Rick. We've actually got 3 chains on this board, covering 17 devices. The company doing the JTAG kit and software seem to be happy with the arrangement, but we'll have to wait and see how it turns out.

A single 2.5V JTAG chain for the Spartan only solves half of this problem, though, since there are still 5 pins connected between the spartan and the PROM. It seems to me to make more sense to run Vcco on the PROM at 2.5V (rather than 3.3V, which is the official line from XAPP453), for the simple reason that the XCF04S is much more tolerant to high input voltages than the Spartan. I'll give it a go and see.

Evan