Friday 28 July 2017

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Synthesizing a Black Box

First some background: what is a Black Box? In synthesis, it is part of your design which is empty (contains no code). It might be an empty Verilog module instance, or an empty VHDL component instance.

A missing piece of code occurs when you use a pre-written piece of design, typically a piece of IP. All the main FPGA vendors provide a way of generating a design as a kind of macro - a piece of design that can be put into your final chip during place and route (or fitting). Tools like this include

  • Altera MegaWizard
  • Actel SmartGen
  • Lattice IPExpress
  • Xilinx CoreGen

(By the way, you'll get to try one of these tools if you come on Comprehensive VHDL)

Let's look at what happens in a bit more detail.

Running an IP Generation Tool

Running an IP Generation Tool (perhaps one of the four mentioned above) gives you a friendly GUI to type in your requirements for your piece of IP. Once the tool has finished, it will generate code. Typically you will get:

  1. A behavioral simulation model not synthesizable!
  2. An instantiation template to cut and paste into your code
  3. A so called "hard macro", that is the design in a format suitable for place and route; or a VHDL or Verilog file containing an instance of a known hardware component.

Synthesis of a piece of IP

The IP may be created as a hard macro, in a format understood by the place and route tool. This means that for synthesis, there is no implementation of the component - it is empty, a black box. This normally results in a warning during synthesis, perhaps something like

WARNING:Xst:2211 - "C:/users/training/vhdlfpga/ex09/source/ram1k8_xilinx.vhd"
         line 28: Instantiating black box module .

Note that some tools may even generate an error at this point.

You can normally disable the warning by adding an attribute or meta-comment to your code to tell the synthesis tool that the instance is actually a black-box

Place and Route

At place and route, there really must be some matching implementation, or the component instance name must be built-in to the place and route tool. If there is no matching implementation and the name is not a built-in name, then place and route will fail.

The format of a hard macro is vendor specific. Most tools support a number of formats:

  1. Actel: .edif, .edf, .adl
  2. Altera: .vqm
  3. Lattice: .edif, .ngo
  4. Xilinx: .ngc, .edif

If place and route does fail, you will get a message similar to this:

ERROR:NgdBuild:604 - logical block 'ram1k8cgen_inst' with type 'ram1k8cgen'
   could not be resolved. A pin name misspelling can cause this, a missing edif
   or ngc file, or the misspelling of a type name. Symbol 'ram1k8cgen' is not
   supported in target 'spartan3e'.

A simple solution is to copy the file into the appropriate location, typically inside a project directory. In some tools you can simply add the file to the project.

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