The
process of data transmission on an on-chip bus leads to switching activity on
the bus wires, which charges and discharges the capacitance associated with the
wires and consequently leads to dynamic power dissipation.
Bus
encoding is widely used technique to reduce dynamic switching power. For any
encoding scheme the sender encoder encodes the signal, while receiver decoder
decodes the signal with inverse function. The power reduction encoding
techniques can be divided into 2 categories: a) self-switching power reduction
b) coupling power reduction.
Self-switching
is bit toggling between 0 and 1 level on a wire over time, causing this wire
capacitance charging and discharging with respect to its metal layer. Following
techniques are used to address this power dissipation:
1.
Address bus encoding. It exploits
the high regularity associated with address streams, which is characterized by
local and temporal locality.
a.
Gray code encoding. This scheme
guarantees only bit flip in case of sequential addresses access.
b.
T0 code. It uses an extra signal on
bus which indicates whether the currently accessed address is the sequential of
the previously accessed. If yes, the address bus isn’t toggled, and the
receiver is responsible to calculate the address based on the previous.
c.
T0-C code. Here the extra signal is
eliminated and instead a new address is sent to indicate the address regularity
finished.
2.
Data bus encoding. Data bus on the
contrary to address bus doesn’t possess any regularity but rather can be
considered random. Therefore, no local and temporal locality can be effectively
exploited.
a.
Bus-invert code. It uses Hamming
distance (the number of changed bits) computation between the current value and
the next value on the bus and inverts the value if the distance is greater than
half of the bit width. An additional indication signal is used to indicate the
value is inverted.
b.
Transition signaling. In this scheme
logical 1 is indicated by level transition from 0 to 1 or from 1 to 0, while
logical 0 doesn’t cause transition. This scheme ensures the number of
transitions on bus is equal to the number of 1s and is effective with data
where the number of 1s is less than the number of 0s.
Coupling
power is dissipated when crosstalk between different wires of the bus happens.
Following techniques are used to address this power dissipation:
1.
Address bus encoding.
a.
Permutation of address bus lines is
done at physical design stage to reduce coupling. It can be achieved by
orthogonal layout of the wires or passing them through different metal layers.
2.
Data bus encoding.
a.
CBI (coupling bus-invert). Is very
similar to previously explained bus-invert code scheme but inverts the data to
achieve better cross-coupling effect.
b.
Transition pattern coding scheme
(TPC). It adds signal to the bus to encode codeword patterns in which
neighboring lines change in phase.
