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Hi, if your frequencies are low enough (let's say under 50MHz) you can go on without series resistors also with 25mm, with higher frequencies a resistor array of 22ohm is recommendable.
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My ARM works with 72 MHz, but the external data bus is slower. So, I think this could work
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Signal integrity is a function of the frequency content of the signal edge rate for each separate signal transition regardless of the clock frequency or how often the data signals toggle. Even if a signal toggles just once, the noise will depend on the edge rate and the squareness of the corners at the start and end of the transition. Clock frequency is relevant to the extent that a longer clock period allows more time for reflections to settle out before the data is registered at the destination, but even with a slow clock frequency avoid excessive voltages from noise that exceed the device specifications. Avoid noise on clocks of any frequency so that the receiving device won't interpret ringing as false clock edges.
There are more termination methods than just series resistors. Some FPGA device families offer more than one method, and you could implement any method external to the FPGA if there is room on the board. You can find documentation on the web beyond the Mentor article already provided by searching for things like "parallel termination", "Thevenin termination", and "diode termination" in addition to "series termination". You should be able to find recommendations for bidirectional signals.
There are additional techniques that can help signal integrity like routing a signal near a ground plane or adjacent to a ground trace on the same layer to provide a good return current path for the signal with minimal area for the entire current loop.
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On chip series termination respectively selecting a low current strength (another name for the same thing) with FPGA can save external resistors in most cases.
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The assignments you have available for this depend on the device family. Any of these assignments can be set in the Assignment Editor. Most or all of them are also available in the Pin Planner, but you might have to use "Customize Columns" on the "All Pins List" right-click menu to enable them to be displayed. The assignments are called "Current Strength", "Termination" for families like Cyclone II and "Input Termination" and "Output Termination" for some families, "Slew Rate" or "Slow Slew Rate" (not for Cyclone II), and possibly others.
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