Convert 5V signals to 3.3V

[jrz126]

I'm going to be working on a new design to replace a current card with an obsolete processor.

It will probably be a few months before the prototypes are built, and I'd like to get a jump on some of the FPGA programming. Right now it looks like most of the components are going to stay the same. So I want to connect my FPGA development board to some of the signals on a current card. The problem is the new FPGA is 3.3V and the old FPGA is 5V. I'm pretty sure the new fpga cant handle 5V inputs.

So whats the best way to convert these signals? Most of them are inputs to the FPGA so would I be able to get away with a simple voltage divider? They are somewhat slow too, (around 4MHz).

 

[dknguyen]

Yeah, the voltage divider would probably work. Safest bet of the simple methods.

 

[mbird]

Hi --

I ran into same thing so if you are using same FPGA as me (Spartan 3) check this out:

Spartan-3/-3E I/O can be made 5V-tolerant by using an external series current limiting resistor to limit the current into the upper clamp diode to 10 mA. This makes the input 5V-tolerant, but an I/O configured as an output still cannot drive 5 Volts and the resulting VOH does not meet the input specifications of the 5V device.

See here for the Xilinx article on this topic:
**broken link removed**

 

[mbird]

If your FPGA is a Xilinx Spartan 3 then check this out:
**broken link removed**