Selectable LVTTL Inputs - Cyclone II

Hi Chris:

Since they are inputs, with VCCIO connected to 3.3 V, The inputs VIH is usually compatible with LVTTL 3.3 and LVTTL 2.5. LVTTL 1.8 is typically not compatible with LVTTL 3.3 since the standard VIH is 2.0 V for LVTTL 3.3.

That being said however. It is possible for the Cyclone III to accept LVTTL 1.8 using a LVTTL 3.3 input, because they specify their VIH as 1.7 V instead of 2.0 V in the handbook for this IO standard.

So As long as your LVTTL 1.8 device can at least drive 1.7 V VOH (Plus some margin) you should be OK.

The IO spec are in the Cyclone III Device Handbook Volume 2 section 1-9 (Table 1-12)

Pete