Why does Altera DE2-115 board GPIO expansion header contain 5V & 3.3V power but the IO standard has no 5V?

Question

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?

Answer 1

1. 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.

2. 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.

Answer 2

Level shifters.

If you want to run a circuit at 5V, you can use the 5V line for power to your circuit. You then use the 3.3V and 5V to power level shifters to interface with the GPIO pins.

If you want to run your circuit at 1.5V or 1.8V (for example high speed memories typically have those levels), you can simply use a regulator to get the power supply for the circuit, and then set the I/O standard to 1.5V or 1.8V.

If you want to use a 3.3V device, then select the 3.3V logic levels! Just because you can select a lower voltage like 1.5V, doesn't mean you should. The choice is there to match the I/O voltage to whatever circuit you have attached.

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Finally, no, it is not OK to connect a 5V output from a circuit directly to the FPGA pin, you must use a level shifter. If you do not, you will fry the I/O pin clamp diode and then the I/O pin itself.

This information can be found in the Cyclone IV Datasheet on page 12. In Table 1-15 it clearly states the maximum input voltage (Vih max) is 3.6V for the 3.3V I/O standard.

Answer 3

Further answer to point (1) beyond Tom Carpenter's answer: to run other devices, not necessarily interfacing with the FPGA digital I/O. I have worked with a custom-design expansion board for the 2x 40-pin GPIO connectors from Terasic products using Altera FPGAs where all of the digital signals were 3.3V, but some of the peripherals and other devices used +5V. These included the ADC and DAC (3.3V digital I/O, +5V analog side), a voltage reference, and the op-amp circuits for signal processing.

If necessary, the +5V could even be used to create a "clean" 3.3V or other voltage supply for devices on the expansion board.

For (2), the answer is mostly no. However, it is still possible to interface to 5V logic, with the correct protection and additional circuitry. When not able to use, as preferred, a level shifting IC, consider these older Altera app notes:

• Application Note 107 (2001)
• Using Cyclone Devices in Multiple-Voltage Systems (C51011-1.3) (2008)

However, these app notes grow less applicable as I/O technology interfaces increase in speed and decrease in voltage compatibility. Whenever possible, try to use logic shifters.