Problem

In Null Convention Logic, all computation is done with oscillating DATA/NULL cycles. The NULL cycles clear the whole circuit, the DATA cycles perform the computation. This requires 2 transition sets per computation (Previous DATA => NULL => Next DATA). Sometimes the DATA output from a component will not be used (think the unselected inputs of a MUX).

Solution

When using steering, the DATA wavefront is only passed to the necessary components. In the case of a MUX, the outputs of these components are then combined as needed with TH1n gates:

The TH22 gates in this example do not allow the DATA wavefront to apss into the components unless they are selected by the DMUX1. During any particular DATA wavefront, only one of these components will be active at a time; the other component will remain NULL. This saves transitions, and thus power. This method also works if only some, or no inputs are shared.

Details

The DMUX1 component is a decoder, that only outputs the DATA1 lines. Technically, I think it matches the NCL network criteria, if you consider the outputs to be single rail outputs, which have only one DATA value. The DMUX1 is used to enable only the selected component.

We will use steering when our ALU has more than just the Adder with glue logic. At some point we’ll have a multiplier, and a shifter, only one of these will operate at a time.

For more on steering, see Karl Fant’s site. His blog posts have lots of good theory too.