FPGA input voltage ground device

[Antosha]

Hello,

Say i have a device that has:
output voltage (say 3.3)
ground
tx output
and rx input.

How would i connect it to a FPGA?
Precisely, how do i
1)input external ground (of the device) to a FPGA so that it (the FPGA) can measure the voltage on the pin that is connected to the tx pin of the device.
2)input external voltage to a FPGA so that it can output voltage on the pin that is connected to the rx pin of the device?

Do i have to use a buffer to implement this?
Thanks

 

[mrflibble]

Can you answer for yourself how you would connect it to say a microcontroller? For an fpga it's not going to be much different. The main difference is that with an fpga you have the luxury of multiple IO standards to choose from.

 

[Antosha]

Can you answer for yourself how you would connect it to say a microcontroller? For an fpga it's not going to be much different. The main difference is that with an fpga you have the luxury of multiple IO standards to choose from.

I'll connect the ground together. I wont touch the voltage lines.
I'll connect the rx and tx to the corresponding pins on the microcontroller.
In certain circumstances i will use a buffer.

Its the same with the FPGA? Does it matter that they have different power supplies? How much load will the fpga have on the rx, tx lines?

 

[mrflibble]

I'll connect the ground together. I wont touch the voltage lines.
I'll connect the rx and tx to the corresponding pins on the microcontroller.
In certain circumstances i will use a buffer.

Its the same with the FPGA? Does it matter that they have different power supplies? How much load will the fpga have on the rx, tx lines?

Sounds good, including the part of "sometimes use a buffer". Does it matter that they have different power supplies? Not any different than with an MCU. Do you directly connect a 5 Volt logic output to the input pin of an MCU running at 1.8 Volt? Probably not without reading the datasheet real good to see if it would tolerate that 5 Volt. Same story for the fpga.

But if your RX/TX lines are 3.3 Volt logic then you should have no problem interfacing that to the typical fpga of your choice. In the simplest scenario you would use a 3.3 Volt VCCIO for the IO bank where you connect that RX and TX.

The specific details on IOs are in the datasheets from xilinx/altera/therest. Were you looking at a specific fpga to use, or was this more of a general question?

In any event, hope this clarifies things somewhat.

 

[Antosha]

The specific details on IOs are in the datasheets from xilinx/altera/therest. Were you looking at a specific fpga to use, or was this more of a general question?

I am planning to use the FPGA as a coprocessor to the DSP.
I was thinking about connecting a dsp (like the blackfin bf533) to an FPGA (like cyclone iv) via two ppi interface and spi. One of the ppi would be used for input, the other for output. The spi would be used for commands.

Can i connect them directly? Or would i need resistors, buffers? What would be the best practice design?

Thanks!

 

[mrflibble]

If the DSP io's are compatible with the IO standards on the fpga (and they probably are) then you can connect them directly. The main thing to consider is speed, so you might want to use series termination resistors on those lines.

If the IO's are both for example 2.5 Volt cmos logic (LVCMOS25 IO standard) then all you'd need is 1 small resistor for each data line.