# Trying to find out the voltage threshold for input of a GPIO pin in an FPGA development board

I need to use a camera module along with De1-SoC development board. Camera requires 3.3V to work and it provides outputs with a 2.5V output pin.

I am trying to find what is the range of voltages that the FPGA will detect. I can not find that information easily anywhere in the specifications or any datasheets. This is the only official help I have found in the DE1-SoC User Manual. If the detection range for '0' is 0V-1.5V, and for '1' is 2.2V-5V I should be able to just plug the camera straight to the FPGA. Otherwise I would need to add additional voltage shifters.

If someone could point me to where I can find information regarding the official input detection characteristics I would be very grateful.

This is the link to the De1-Soc manual, and this is cyclone V manual from which I am too trying to fish out that information.

• How about a link to the data sheet? – Andy aka Feb 27 '17 at 11:47
• You'll find the Altera FPGA part number in the DE1-SoC board manual. Then head to the Altera website for the FPGA data sheet and you'll be able to find the electrical characteristics in there, including your threshold. – TonyM Feb 27 '17 at 11:50

Cyclone V pins are configurable for numerous different standards. Look into Cyclone V Device Handbook Vol.1 in chapter 5. Specifially, JEDEC Standard JESD8-5 (that is "2.5V low voltage CMOS") is supported.

From the De1-SOC-Schematics (download CD.zip from Terasic) you can see, that all banks have $VCC_{IO}=3.3V$ (except the HPS-banks which use $1.5V$). Pins on these banks will correctly detect 2.5V CMOS input (see "I/O Standards Voltage Levels"), but they will not be able to generate 2.5V output.

In general, Alteras Device Handbook (at least Vol.1) is a recommended read, if you want to put your board to use.

Also, the final answer as to the input and output capabilities for each pin will be found in the project's .qsf file. Here you will find entries like:

set_instance_assignment -name IO_STANDARD "3.3-V LVTTL" -to i_serIn

set_instance_assignment -name CURRENT_STRENGTH_NEW 8MA -to o_LED[0]

and so forth to let you know what that FPGA program intends for the pin. As @Andraes mentions, though, you can't get a 2.5V output from a pin that is powered in a 3.3V bank. The input will be OK if the camera drives hard to VCC.