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when i consult the cyclone 3 datasheet,the VCCIO for c-3 is 3.3V,but when set the pin planner in QUARTUSII or the menu of assignment->device/ device and pin options/voltage tab:default I/O standard is 2.5V ,of course it could change.why for a VCCIO 3.3V chip the default I/O standard in system is 2.5V,a smaller voltage??
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Cyclone 3 provides several settings for VCCIO for each of the IO banks.
The VCCIO that you should use should be the VCCIO as implemented on your FPGA board for that bank and this depends on the I/O standard that you use to communicate with the peripherals connected to the IOs of that bank.
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