Adaptive Logic Modules
For the packing of two five-input functions into one ALM, the functions
must have at least two common inputs. The common inputs are dataa
and datab. The combination of a four-input function with a five-input
function requires one common input (either dataaor datab).
In the case of implementing two six-input functions in one ALM, four
inputs must be shared and the combinational function must be the same.
For example, a 4 × 2 crossbar switch (two 4-to-1 multiplexers with
common inputs and unique select lines) can be implemented in one ALM,
as shown in Figure 2–8. The shared inputs are dataa, datab, datac, and
datad, while the unique select lines are datae0 and dataf0for
function0, and datae1and dataf1for function1. This crossbar
switch consumes four LUTs in a four-input LUT-based architecture.
Figure 2–8. 4 × 2 Crossbar Switch Example
4 × 2 Crossbar Switch
Implementation in 1 ALM
sel0[1..0]
inputa
inputb
dataf0
datae0
dataa
datab
datac
datad
Six-Input
LUT
(Function0)
out0
combout0
combout1
inputc
inputd
out1
sel1[1..0]
Six-Input
LUT
(Function1)
datae1
dataf1
In a sparsely used device, functions that could be placed into one ALM
may be implemented in separate ALMs. The Quartus II Compiler spreads
a design out to achieve the best possible performance. As a device begins
to fill up, the Quartus II software automatically utilizes the full potential
of the Stratix II ALM. The Quartus II Compiler automatically searches for
functions of common inputs or completely independent functions to be
placed into one ALM and to make efficient use of the device resources. In
addition, you can manually control resource usage by setting location
assignments.
Any six-input function can be implemented utilizing inputs dataa,
datab, datac, datad, and either datae0and dataf0or datae1and
dataf1. If datae0and dataf0are utilized, the output is driven to
register0, and/or register0is bypassed and the data drives out to
the interconnect using the top set of output drivers (see Figure 2–9). If
2–12
Stratix II Device Handbook, Volume 1
Altera Corporation
May 2007
ALTERA [ ALTERA CORPORATION ]
