Chisel: how to implement a one-hot mux that is efficient?

Question

I have a table, where each row of the table contains state (registers). There is logic that chooses one particular row. Only one row receives the "selected" signal. State from that chosen row is then accessed. Either a portion of the state is connected as an output to the IO of the module, or else a portion of the IO is used as input to update the state.

If I were implementing this with a circuit, I would use pass-gates. The selected signal would turn on one set of pass-gates, which would connect the row's registers to a bus. The bus would then be wired to the IO bundle. This is fast, small area, and low energy.

There is a straight forward way of implementing this in Chisel. It encodes the selected row as a binary number, and then applies that number to the select input of a traditional mux. Unfortunately, for a table with 20 to 50 rows, and state of hundreds of bits, this implementation can be quite slow, and wasteful in area and energy.

The question has two parts: 1) Is there a way to specify busses in Chisel, such that you have pass-gates or traditional tri-state drivers all hung off the bus?

2) Failing that, is there a fast, small area, low energy way of doing this in Chisel?

Thanks

Answer 1

1) Chisel does not fully support bidirectional wires, but via the experimental Analog type (see example), you can at least stitch a bus through your Chisel code between Verilog Black Boxes.

2) Have you tried Mux1H in chisel3.util? It emits essentially a sum of products of the inputs and their corresponding select bits. I'm not sure how this compares to your proposed implementation. I would love to see a QOR comparison. If this construct is not sufficient and you cannot express precisely what you want in chisel, you can use a parameterized BlackBox to implement your one-hot mux and instantiate it as you please.