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I have had issues with detecting a JTAG chain with 2 XC9500XL CPLDs and 1 Spartan 3 FPGA on a custom pcb.

I made a mistake thinking the Spartan 3's JTAG was 3v3 instead of 2v5 and connected the FPGA directly to the 3v3 JTAG.

Xilinx wrote an Application Note about using a 3v3 JTAG interface with the Spartan 3 [here] where they add resistors to limit the reverse current induced by the overvoltage protection diode.

Shouldn't by chain work anyways even though I haven't added the resistors ?

Clément Chaine
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2 Answers2

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Somewhat.

There are two interesting paths here: the TMS signal from the JTAG adapter and the TDI signal, either from the JTAG adapter or from the previous element on the chain.

TMS will be driven high most of the time while the adapter is connected and idle, to keep the JTAG state machine in reset state, so this will supply 3.3V to TMS, which then powers Vccaux through the clamping diode.

TDI will usually be driven low while the adapter is idle, which is harmless.

The adapter likely uses a current-limited driver for TMS, to avoid damage from incorrect wiring, so it is likely that all you are doing here is destabilize your Vccaux rail.

The same applies to the TDI connection if the Spartan3 is the first element in the chain.

If the Spartan3 is later in the chain, then the TDI signal is driven by the previous IC -- if that is a 3.3V IC, then the current through the diode may exceed the ratings of its output driver.

Simon Richter
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The application note seems to explain the situation pretty clearly. Emphasis mine:

The reverse current does not interfere with FPGA operation. Still, care must be taken to ensure proper regulation of the VCCAUX voltage. There are two ways to make sure that the VCCAUX regulator works correctly: [...]

You seem to understand how "reverse" current through the clamp diodes arises. So you'd need to check two things: that your power supply is still able to regulate the 2.5 V supply (as described by the application note), and that your JTAG controller's logic thresholds are compatible with 2.5 V inputs (they probably are). (Also consider any other devices on the chain.) You didn't include any schematics, so you'll have to check this out yourself.

Subject to those conditions, the answer to your question is yes: it could work.

TypeIA
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