Stratix Architecture
Adder/Subtractor/Accumulator
The adder/subtractor/accumulator is the first level of the adder/output
block and can be used as an accumulator or as an adder/subtractor.
Adder/Subtractor
Each adder/subtractor/accumulator block can perform addition or
subtraction using the addnsubindependent control signal for each first-
level adder in 18 × 18-bit mode. There are two addnsub[1..0]signals
available in a DSP block for any configuration. For 9 × 9-bit mode, one
addnsub[1..0]signal controls the top two one-level adders and
another addnsub[1..0]signal controls the bottom two one-level
adders. A high addnsubsignal indicates addition, and a low signal
indicates subtraction. The addnsubcontrol signal can be unregistered or
registered once or twice when feeding the adder blocks to match data
path pipelines.
The signaand signbsignals serve the same function as the multiplier
block signaand signbsignals. The only difference is that these signals
can be registered up to two times. These signals are tied to the same
signaand signbsignals from the multiplier and must be connected to
the same clocks and control signals.
Accumulator
When configured for accumulation, the adder/output block output feeds
back to the accumulator as shown in Figure 2–34. The
accum_sload[1..0]signal synchronously loads the multiplier result
to the accumulator output. This signal can be unregistered or registered
once or twice. Additionally, the overflowsignal indicates the
accumulator has overflowed or underflowed in accumulation mode. This
signal is always registered and must be externally latched in LEs if the
design requires a latched overflowsignal.
Summation
The output of the adder/subtractor/accumulator block feeds to an
optional summation block. This block sums the outputs of the DSP block
multipliers. In 9 × 9-bit mode, there are two summation blocks providing
the sums of two sets of four 9 × 9-bit multipliers. In 18 × 18-bit mode, there
is one summation providing the sum of one set of four 18 × 18-bit
multipliers.
Altera Corporation
July 2005
2–63
Stratix Device Handbook, Volume 1
ALTERA [ ALTERA CORPORATION ]
