Priority Encoder

A priority encoder is used to find the trailing 1 in a Logic (or, equivalently, count the number of trailing 0s). In ROHD HCL, our PriorityEncoder abstract class searches from the LSB to find the leading-1, returning the index of its position from the LSB. If there is no 1 in the Logic, it returns 1 past the width of the Logic. Additionally, if outputValid is set to true, then a valid output is produced and will be set to 1 if a trailing-1 is found, otherwise 0.

Parallel-Prefix Priority Encoder

We provide one implementation based on ParallelPrefix trees, which finds the trailing-1 position using a prefix-tree and then encodes that position. This ParallelPrefixPriorityEncoder allows you to select the prefix-tree of your choice to use in the find position portion, which is followed by a OneHotToBinary module to encode that position.

    final bitVector = Logic(width: 5);
    // ignore: cascade_invocations
    bitVector.put(8);
    final encoder = ParallelPrefixPriorityEncoder(bitVector,
        ppGen: BrentKung.new, outputValid: true);
    final valid = encoder.valid!;
    // encoder.out.value.toInt() will be 3 and valid.value.toBool() will be true

Recursive Priority Encoder

We provide a more direct encoding implementation that builds up the trailing-1 position using a binary recursion tree. There are two flavors of this implementation: RecursivePriorityEncoder is a straight recursion that builds up the binary tree construction using straightforward logic, whereas RecursiveModulePriorityEncoder uses the same algorithm, but produces a Module for each node of the tree (this results in more readable output Verilog).

    final bitVector = Logic(width: 5);
    // ignore: cascade_invocations
    bitVector.put(8);
    final encoder = RecursiveModulePriorityEncoder(bitVector, outputValid: true);
    final valid = encoder.valid!;
    // encoder.out.value.toInt() will be 3 and valid.value.toBool() will be true