How do languages related to FPGAs?

2
votes

I believe at university I wrote a program for an FPGA which was in a language derived from C. I am aware about languages such as VHDL and verilog. However, what I dont understand is the amount of choice a programmer has regarding which to use? Is it dependent on the FPGA? I am going to be using a Xilinx FPGA.

I am confused because the C-variant language was, unsurprisingly, similar to C- however I know things like VHDL are nothing like C. Therefore if I have a choice I would prefer to programme an FPGA using a C-variant language. The Xilinx website had a million documents and it wasn't overly clear.

3
fpga

3 Answers

6
votes

It was probably Verilog that you used. It's rather C-like in a lot of it's constructs. I wouldn't say it's "like C", but some syntax is similar.

VHDL is based on ADA, so yes, it's rather different.

There are some small FPGA specific languages around, but VHDL and Verilog are the big two. I think most others have died now.

Remember that writing hardware and writing software are two rather different things. You can't really describe hardware constructs in a language like C (*). The language needs to have special features to allow you to describe exactly what you want. The code needs to be structured in a way that will make the hardware efficient. Don't fool yourself into thinking that you can take a piece of software and magically run it on an FPGA just by changing the language/compiler. (This is targeted more at your follow up question to Marty).

Trying to use C to write a circuit description, is like trying to program a computer in English. You could do it, but it's really the wrong language for the job.

(*) Yes, I know there's SystemC (a C++ class library that is meant to make code synthesisable), but I've yet to see anyone get good results from it, and certainly not on FPGAs. Even then the code has to be structured in a similar way as for an HDL.

3
votes

Clearly HDL are still preferable when programming FPGA (Xilinx, Altera etc : all accept VHDL or Verilog).

However, things are changing (slowly) : there are now excellent so-called behavioral synthesizers that allow you to code in C and generate hardware, expressed for you in VHDL or Verilog at the register-transfer level. They are sometimes refered as HLS : high-level synthesis.

The problem is that they are quite expensive.

  • Synfony from Synopsys
  • Cynthesizer from Forte Design Systems
  • CatapultC from Calypto (was from Mentor)
  • ImpulseC

At the academic level :

Basically, these tools work by extracting a dependency graph from the C program : nodes represents computations and edges represent variables : that is all what you do when you program, in either C or other programming language. Using this internal representation, the compiler can do hardware-relevant transformations like : register allocation (mapping variables to register, or keeping them combinatorial i.e on wires), operations scheduling (deciding if operation execute in same clock cycle) etc, and finally generate HDL automatically.

Hope this helps

JCLL

1
votes

Usually FPGA vendors will have toolchains that support both Verilog and VHDL - it's up to you to choose which language you'd like. It's generally just these two languages that are supported.

For more C-like languages, a long-shot option is to use the synthesisable subset of SystemC. This is C++ with circuit-friendly stuff added. I'm not sure if the FGPA tools support this though.