The altera DE2-115 board user manual section 4.8 page 47 describes the GPIO expansion header. It is clear that it has 3.3V and 5V power supply.
However, it later says that "The voltage level of the I/O pins on the expansion headers can be adjusted to 3.3V, 2.5V, 1.8V, or 1.5V using JP6 (The default value is 3.3V, see Figure 4-17)."
(1) I am confused, why is 5V power supply provided if no IO standard goes upto that? Also, why have IO standard down to 1.5V when power supply provided only goes downto 3.3V? Certainly an IC that takes 3.3V won't work with IO of 1.5V. Am I missing something here?
(2) Even though there are clamp diodes on these pins of the expansion header, is it ok to use a 5V IC that shall have VOH(max) of close of 5V?
Best Answer
The FPGA doesn't use 5V, but some of the other parts on the board might. (The HD44780 LCD they're using often requires 5V on VCC, for instance.) Since it's there, they might as well provide it on the expansion connector in case you find it useful.
Absolutely not. The clamp diodes are a last resort, and are only intended to protect the FPGA from brief overvoltage. Connecting a 5V output directly to a FPGA pin will cause a large current to flow through the protection diode, destroying it.
Some development boards I've seen use small (e.g, 100Ω) series resistors on FPGA I/Os to limit the current through protection diodes. This limits the speed of these I/Os, though, and I would not recommend that you rely on this for large level shifts, like 5V to 3.3V.
Bottom line: if you need to interface a 5V-only part to an FPGA, use a real level shifter. A 74LVC245, for example.