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Altera_Forum's avatar
Altera_Forum
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11 years ago

Help setting Cyclone III pins to 3.3V

Hello, I am trying to set two pins on the Cyclone III I'm using (EP3C120F780C7N) to work at 3.3V. I'm developing a custom slave I2C controller and the I2C bus will be 3.3V. I changed the pin ...
Altera_Forum's avatar
Altera_Forum
Icon for Honored Contributor rankHonored Contributor
11 years ago

FPGA pins will not "magically" support an output of 3.3V just by changing the I/O standard to 3.3V :)

The I/O standard setting in the pin planner has to match the board design, i.e., if you select an I/O standard of "3.3V-LVTTL" then the board also has to have VCCIO = 3.3V in the banks where those pins exist.

In your case however, because I2C only needs the FPGA to drive low, and you have pull-ups to 3.3V, all you need to do is change your I2C logic. The fact that you can see 2.5V from the FPGA means that you are probably *driving* a logic high from a bank with VCCIO = 2.5V, rather than tri-stating when your I2C controller needs to output a logic high. The pins in the 2.5V bank will tolerate the 3.3V pull-up voltage.

Cheers,

Dave