Generating fast logic circuits for m-select n-port round Robin arbitration

Abstract

This paper generalizes the problem of Round Robin Arbitration (RRA) from 1-select to m-select (mRRA) and offers new circuit architectures for it. RRAs are found in networking equipment and computer systems with high throughput buses. We first propose fast/novel circuits for the fundamental problem of finding the first m 1's in an n-bit vector (from the left or right), i.e., generalized select Priority Encoder (mPE). The obvious solution to mPE is cascading m regular (1-select) PEs. Our solutions, however, are based on parallel prefix networks, where the nodes are replaced by "saturated adder"s. We use mPE as a building block to construct an mRRA, which has single cycle latency and can arbitrate up to m requests per clock cycle. We took two arbiters from the liter rature, TC-PPA (1-select) and 3DP2S (2-select), and generalized them into mRRAs, which we call mTC-PPA and 3DPmS-RRA. We wrote fully parameterized HDL code generators. Logic synthesis results show that mTC-PPA and 3DPmS-RRA are up to 100% faster than the cascade solution and have up to 65% smaller Area-Timing Products (ATP). Comparing mTC-PPA and 3DPmS-RRA, 3DPmS-RRA circuits are slightly faster than mTC-PPA on the average. In terms of ATP, mTC-PPA is superior by far and can be as small as 30% of 3DPmS-RRA.