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Multiply-accumulate (MAC) operations are common
in data processing and machine learning but costly in terms
of hardware usage. Stochastic Computing (SC) is a promising
approach for low-cost hardware design of complex arithmetic
operations such as multiplication. Computing with deterministic
unary bit-streams (defined as bit-streams with all 1s grouped
together at the beginning or end of a bit-stream) has been recently
suggested to improve the accuracy of SC. Conventionally, SC
designs use multiplexer (MUX) units or OR gates to accumulate
data in the stochastic domain. MUX-based addition suffers
from scaling of data and OR-based addition from inaccuracy.
This work proposes a novel technique for MAC operation
on unary bit-streamsthat allows exact, non-scaled addition of
multiplication results. By introducing a relative delay between
the products, we control correlation between bit-streams and
eliminate OR-based addition error. We evaluate the accuracy of
the proposed technique compared to the state-of-the-art MAC
designs. After quantization, the proposed technique demonstrates
at least 37% and up to 100% decrease of the mean absolute
error for uniformly distributed random input values, compared
to traditional OR-based MAC designs. Further, we demonstrate
that the proposed technique is practical and evaluate area, power
and energy of three possible implementations.
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