Systemverilog interfaces over hierarchical boundaries

Question

I have experienced some back-end issues using systemverilog interfaces when and interface is traversing over hierarchical boundaries. I've tried to sketch the situation in the attached drawing.

The top picture shows the "regular" method of using interfaces. the interface and connected module are all instantiated at the same level of hierarchy. This works for simulation and back-end.

The middle picture shows my situation. At the toplevel I have a module and interface instantiation. The interface is connected to the purple module and then connected to 2 sub-modules. In simulation this works.

Then the synthesis tool complains that the interface at the purple level should be an modport. So I added that. However the synthesis tool is interpreting the wires in the as bidirectional and adds logic to facilitate this. In my design all wires are unidirectional.

The only workaround I could find to fix this issue is depicted in the lower picture. I connect via a modport the original interface (labeled A). Then I instantiate a new interface (labeled B) which has the same parent as interface A. Both interfaces A and B are connected to a connect module which contains a lot of statements like:

assign interfaceB.rx1 = interfaceA.rx1;

assign interfaceB.rx2 = interfaceA.rx2;

assign interfaceA.statusX = interfaceB.statusX;

etc

so it is just a "dumb" connection of interface A and B.

This way of work feels very wrong as this connect module is creating a lot of overhead. Is there a good / easier way of using the interface over hierarchical boundaries that is not only working in simulations but also works for synthesis?

Thanks

dave_59 dave_59 · Accepted Answer · 2018-12-13T16:22:54

Hierarchical composition is definitely a shortcoming of SystemVerilog interfaces.

You can simplify your solution by creating InterfaceB with a port list and making connections to the individual signals from the InterfaceA port. That eliminates the connect module.

interface InterfaceB( input rx1, rx2, output ...)'
 modport .../ same as what you have
endinterface

Systemverilog interfaces over hierarchical boundaries

1 Answers