Mixed voltage in JTAG chain.

You can have mixed voltages provided you make sure you don't exceed the higher one.

Take a simple two device chain where one device is 3.3V and one is 2.5V (View in fixed font)

3.3V 2.5V

TDI ------->[ ] TDO ----[ 1k ]-------> TDI [ ] TDO --------------->

|| || 2.2k -- Ground

Where each device must also have TMS and TCK (programmable devices rarely have TRST).

You could pull each of TMS and TCK from 2.5V (a valid high to a 3.3V device) and provide programmer power reference from the 2.5, provided the *last* object in the chain is 2.5V (because the output will drive to it's VCCIO voltage)

You will also need to convert the TDO -> TDI line when going *from* a

3.3V device *to* a 2.5V device. A simple resistor divider works fine (a series 1k resistor on the output of TDO and a 2.2k resistor to ground after that (on the lower voltage device TDI pin) would do it.

Don't attempt to drive the TDI pin directly with the 3.3V as you will very probably damage both the input and the previous output (much depends on the tolerance of the device - see the datasheet for details).

Cheers

PeteS

If you are looking at something like a Spartan-3 at 2.5V with say Platform Flash devices then you really want to put series resistors in line to the inputs of the Spartan-3.

Alternatively if you have control of your 3.3V supply and can make it, and live with it, at 3.0V then providing your 2.5V is than, or greater, then the protection diodes should not operate and you can get away without the resistors.

As as stated elsewhere, and if you have the option, you can run the whole chain at 2.5V. Just be careful that your programming cable is happy with this. Some are not happy at all at 2.5V and some become intermittant at 2.5V as we have found testing competitor cables with some of our products. I will state for the record that this issue does not occur with our own cables that we now supply with all of our development board products. We have also moved all our FPGA JTAG chains to 3.0V-3.3V, with resistors, to allow better reliability with non-Enterpoint JTAG cables.

John Adair Enterpoint Ltd. - Soon to be home of Broaddown4. The Ultimate Virtex4 Development Board.