Asynchronous comparator design Motivation Background: Sync and Async comparators Delay-insensitive carry-lookahead comparators Complexity Analysis Conclusions
Motivation operations in digital systems. Comparison is one of the most important operations in digital systems. Comparators are used in ALUs, cache memory, MMU and data hazard detection. Integer comparison: use an integer adder. Problem: addition time > comparison time. High speed comparison is needed.
Background: Binary Comparison Worst case: A=B A: 01101110 B: 01101110 Best case: A>B B: 10101110 Comparators can perform average case behavior
Background Ripple-Carry Comparator(Sync): Flow table specification:
Background Delay-Insensitive Ripple-Carry Comparator: Flow table:
Background Ripple-Carry Comparator: Logic complexity: O(n) Time complexity: O(n) Delay-Insensitive Ripple-Carry Comparator: Time complexity: O(1)
Carry-Lookahead Comparators RCC requires n stage-propagation delays. Use carry-lookahead comparators(CLC). CLCs: Logic complexity: O(n) Time complexity: O(log n)
8-bit carry-lookahead comparator
DI Carry-Lookahead Comparator Delay-Insensitive Carry-Lookahead Comparators may be implemented by using delay-insensitive code. 1. dual-rail signaling: input bits 2. one-hot code: internal signals, s, g, e (S, G, E). S: smaller (A<B) G: greater (A>B) E: equal (A=B)
CMOS Implementation DI P-module:
CMOS Implementation DI I-module:
CMOS Implementation DI SI-module: 3 2-input AND gates.
CMOS Implementation Speed-up circuits for S and G signals: Dynamic OR gates.
CMOS Implementation Speed-up circuits for E signals: Dynamic OR gate:
SPICE Simulation: SPICE Simulation contains two parts: Random number inputs: 10000 random generated input pairs Statistical data: running examples on a 32-bit ARM emulator
SPICE Simulation: Random number inputs: 10000 random generated input pairs a. RCC: 32.4ns b. CLC: 6.2ns
SPICE Simulation: Confidence Limits:
SPICE Simulation: Confidence Limits:
SPICE Simulation: Distribution of typical-case comparisons:
SPICE Simulation: SPICE simulation results: dynamic traces
Conclusions A new pratical design of DI comparator. Theoretically, Logic Complexity:((n)). Time Complexity:((1)). Reality: more than 2 times faster than its sync counterpart with 80% usable clock. Suitable for VLSI implementation.