Yes no question, different voltage level in a bank.

I am sorry since I found a thread and an application note.

http://www.alteraforum.com/forum/showthread.php?t=2144

http://www.altera.com/literature/an/an447.pdf

The application note says

Disable diode and apply series termination or use driver selection table

Is there a quick way to find the value of the series termination?

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Is it possible to drive a bank that works at 2.5 Volt with 3v3V ttl/cmos voltage levels without any modification.

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Which device?

For example, this works fine with Stratix II devices; I have a board with 2.5V devices communicating with 3.3V devices.

If you look in the device handbook, they typically have a table of I/O voltage compatibility.

Cheers,

Dave

the device is cyclone4 gx150

when I check the datasheet I see that I need to put a resistor

My question is the value of the resistor

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the device is cyclone4 gx150

when I check the datasheet I see that I need to put a resistor

My question is the value of the resistor

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Did you read this app note?

http://www.altera.com/literature/an/an447.pdf

The Cyclone IV handbook refers to it on p121 of the PDF.

If the device you are driving the Cyclone device with has the option to implement a series termination (eg., its another FPGA), then you just need to enable that feature.

Otherwise, you need to use an output resistor with a value such that the output driver impedance plus the resistor matches the impedance of the transmission line, eg. if your driver output impedance is about 10-ohms, and your transmission line is 65-ohms, then you need a 55-ohm resistor. If you have no idea what the output impedance is, then just put a resistor on the design, test a few values, and then load the best value.

Alternatively, you can simulate the board design; get the IBIS models of the driver, export the PCB layout to the simulator, and check the waveforms. If your layout is compact (no transmission lines really exist), then there will be no need for the series termination resistors.

Cheers,

Dave