A VLSI High-Performance Encoder with Priority Lookahead

In this paper we introduce a VLSI priority encoder that uses a novel priority lookahead scheme to reduce the delay for the worse case operation of the circuit, while maintaining a very low transistor count. The encoder's topmost input request has the highest priority; this priority descends linearly. Two design approaches for the priority encoder are presented, one without a priority lookahead scheme and one with a priority lookahead scheme. For an N-bit encoder, the circuit with the priority lookahead scheme requires only 1.094 times the number of transistors the circuit without the priority lookahead scheme. Having a 32-bit encoder as an example, the circuit with the priority lookahead scheme is 2.59 times faster than the circuit without the priority lookahead. The worst case operation delay is 4.4 ns for this lookahead encoder, using a 1m scalable CMOS technology. The proposed lookahead scheme can be extended to larger encoders.