Level 4 - Technology-Specific Module Generators

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Level 4 - Technology-Specific Module Generators

Level 4 includes a few technology dependent module generators.

Pros: takes advantage of technology specific features
Cons: there are not a lot of modules here, non portable

Details

The fact that the Logic.Modules classes described in the previous Users Manual section are technology-independent has advantages and disadvantages. The main advantage is that they will work with any FPGA technology that JHDL supports (that is any technology for which JHDL has a TechMapper). The main disadvantage is that they don't take advantage of technology-specific features.

An example of this would be a module generator for delay lines. The only way to build this in a technology-independent way is to string together the proper number of flip flops in a serial chain. This may be very large. However, if the technology were XC4000, the best way to build delays would be with a counter and a dual-ported memory. In Virtex the SRL cell would be an even more efficient solution. However, the Logic.Modules version will be required to use flip flops.

To alleviate this, a small set of technology-optimized module generators have been created. These include delays, array multipliers, and a few other items. Technology-optimized modules may often operate slightly differently than the technology-independent ones in Logic.Modules. This is to be expected since slight changes in a module's functionality can often make it possible to take advantage of some technology feature. Thus, it is incumbent on designers to carefully read the documentation for these modules to determine if they will serve their needs.

Importing A Technology-Specific Module Generator

Packages for module generators are imported like any other library. In the case of module generators for Virtex the line would be:

     import byucc.jhdl.Xilinx.Virtex.Modules.*;

Similar packages will exist for other technologies.

Circuit Modules in Technology-Specific Module Generator Packages

As far as possible, the structure of each technology-specific module generator package mirrors that of Logic.Modules. That is, there will be a set of module generator classes like upcnt above which can be directly instantiated using new. In addition, there may be a class whose name is all uppercase letters and which contains static methods to make calling the module generator easier.

An simple example might be an array multiplier for the Virtex technology. The class to directly instance it might have a constructor like this:

     public arraymultiplier(Node parent,
                            Wire a,
	 		    Wire b,
		 	    Wire q,
			    String instanceName,
			    boolean signed,
			    int pipelineStyle) {

The corresponding MULTIPLIERS class might support the following static method calls:

     MULTIPLIERS.Unsigned_ArrayMultiplier(Node parent,
                                          Wire a,
                                          Wire b,
                                          Wire q,
                                          String instanceName,
                                          int pipelineStyle);

     MULTIPLIERS.Signed_ArrayMultiplier(Node parent,
                                        Wire a,
                                        Wire b,
                                        Wire q,
                                        String instanceName,
                                        int pipelineStyle);

As always, examine the API documentation for the package in question to understand what is available.

Where To Go Next?

If you still cannot find the a pre-written circuit module that you need, the last place to look is in the Contrib.Logic package in the JHDL distribution. The circuit modules there have been contributed by other JHDL users. Their inclusion in Contrib.Logic should not be construed as indicating that the JHDL Development group has done any kind of testing on them. We simply package them with the distribution as a service.

We actively solicit the contribution of designs and module generators for inclusion in this package for future releases. No specific requirements have been set for materials to be included in this package. If you have something to contribute, contact the JHDL development team.

This is the end of the series of sections in the Users Manual on the levels of design in JHDL. The remainder of the Users Manual sections deal with other specialized topics such as clocking, how to run the JHDL tool suite for simulation, and netlisting JHDL designs. Read on!

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