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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
Honored Contributor
