Connecting 3.3V IO to 1.2V for pin compatibility

[Altera_Forum]

Hi, 

 I am designing a board containing a CycloneII FPGA. I am not sure which part numer i should to use, 35 or 50.  These two parts are pin compatible except that some pins are used as VCCINT(1.2) in cycloneII-50 but they are general purpose IO in cycloneII-35. My question is that if it is possible (or safe) to connect general-purpose io pins to 1.2V supply, so that i can use both 35 and 50 on the same board?  Regards, Kamran

[Altera_Forum]

If you want to be able to use two different devices in the same place on a board, list the second device at "Assignments --> Device --> Migration Devices". Then use the .pin file to see how to connect the pins. The .pin file will give the allowed/required connection for the pins that are different between the two devices.

[Altera_Forum]

ON the Same Assignments Device page, there is also a box called "Device and Pin Options". 

On the Unused Pin Tab - You may need to change the selection from the default of "As output driving Ground" to something like "As input tri-state with weak pull up" so that in the package where the pins are connected to Vccint (1.2v) you do not have an un-used I/O not driving this to ground.

[Altera_Forum]

You can still use a device wide assignment for your unused outputs to behave as "outputs driving ground". When you turn on Migration in Quartus II, the programming file will automatically set the I/Os as inputs - tristate for the pads which are VCC or GND on the larger devices.  

 Anytime you want to create a footprint to support multiple device densities, use Device Migration in Quartus II. The tools will take care of the pins which are I/Os in some densities, and VCC / GND in other densities. Then as Brad said, the .pin file will be your guide.

[Altera_Forum]

Thanks Desert Rat. I am now a little smarter in the Device Migration area of knowledge. 

As my old friend, O-bee-wan used to say, "The .pin file, trust the .pin file".