communication with 3.3V devices with cyclone 3

[Altera_Forum]

Hi! 

 Currently I have design board with cyclone 3. Cyclone 3 VCCIO supplies are connected to 2.5V. Other devices have 3.3V voltage supply.  Regards to Cyclone 3 manual, it can be directly connected to 3.3V devices.   Does anybody have any experience with connecting 2.5V cyclone 3 to 3.3V logic?  Thanx, Pita

[Altera_Forum]

Hello, 

 following the Cyclone III manual and particularily table 7-8 acceptable input and output levels, 3.3V logic should be driven by a VCCIO of 3.3V. Driving inputs in a 2.5V bank from 3.3V is possible, but Altera requires proper termination to prevent overshoot and possible IO buffer damage at the input. The topic isn't new, but hasn't been presented that detailed before Cyclone III, see e. g. AN447.  The recommendation of 3.3V VCCIO for driving 3.3V logic allows to meet the minimum VOH level of the voltage standard (0.7 x 3.3 = 2.1 V) also with a loaded output and supply voltage tolerances as required by specification. You may however decide, that no additional load must be taken into account and use 2.5V VCCIO. As the typical VTH is 1.65 V, you get sufficicient noise margin in normal operation. Transition times may be however sligthly unsymmetric. Inputs with LVT characteristics (as Altera CPLD and FPGA itself) can be driven without restrictions anyway.  Regards, Frank

[Altera_Forum]

Probably the optimal solution is used 3.0V for IO? Since no external termination is required. Also 3.3V logic can be driven without external components with 3.0V FPGA output. 

 Thanx, Amir

[Altera_Forum]

Regarding voltage margin, 3.0 V may be a compromise. Also most 3.3V logic devices could operate at this supply level. Series termination of fast outputs should be considered with trace lengths above a few inches to my opinion. Devices as AD converters have typical 50 ohms series termination, most programmable devices have the option anyway. You may determine the necessity for termination from measurements, a low capacity active probe and an above 500 MHz oscilloscope are required however for a realistic display.

[Altera_Forum]

I have one question more regarding cyclone 3 configuration mode and voltage applied to VCCIO. 

 When VCCIOs are connected to 3.0V is it possible to configure cyclone 3 in FPP mode?   Namely, in manual(Configuring Cyclone 3 Devices) are written how to connect Cyclone 3 in FPP mode for configuration voltage standards: 3.3V/2.5V and 1.8/1.5V.  On my board probably up to 4 Cyclone 3 devices will be connected in FPP configuration.   Regards, Amir

[Altera_Forum]

--- Quote Start ---  When VCCIOs are connected to 3.0V is it possible to configure cyclone 3 in FPP mode?  --- Quote End ---   I don't know. I suggest to setup a design with FPP and try to compile it. Technically, it should work, but I don't know, if Quartus software would accept it. (Cause Cyclone III can't measure the real IO voltage, there are some options left...) Basically, I didn't yet understand the purpose of selecting different voltages with MSEL pinstrapping. May be, someone can clarify the point?