Xilinx Virtex-6 Manual page 244

Chapter 4: About Design Elements
LUT6_L
Primitive: 6-Input Lookup Table with Local Output
Introduction
This design element is a 6-input, 1-output look-up table (LUT) that can either act as an asynchronous 64-bit ROM
(with 6-bit addressing) or implement any 6-input logic function. LUTs are the basic logic building blocks and are
used to implement most logic functions of the design. A LUT6 is mapped to one of the four look-up tables in
the slice. The functionality of the LUT6, LUT6_L and LUT6_D is the same. However, the LUT6_L and LUT6_D
allow the additional specification to connect the LUT6 output signal to an internal slice, or CLB connection, using
the LO output. The LUT6_L specifies that the only connections from the LUT6 are within a slice, or CLB, while
the LUT6_D allows the specification to connect the output of the LUT to both inter-slice/CLB logic and external
logic as well. The LUT6 does not state any specific output connections and should be used in all cases except
where internal slice or CLB signal connections must be implicitly specified.
An INIT attribute consisting of a 64-bit hexadecimal value must be specified to indicate the LUT's logical
function. The INIT value is calculated by assigning a 1 to the corresponding INIT bit value when the associated
inputs are applied. For instance, a Verilog INIT value of 64'h8000000000000000 (X"8000000000000000" for
VHDL) will make the output zero unless all of the inputs are one (a 6-input AND gate). A Verilog INIT value
of 64'hfffffffffffffffe (X"FFFFFFFFFFFFFFFE" for VHDL) will make the output one unless all zeros are on the
inputs (a 6-input OR gate).
The INIT parameter for the FPGA LUT primitive is what gives the LUT its logical value. By default, this value is
zero, thus driving the output to a zero regardless of the input values (acting as a ground). However, in most
cases a new INIT value must be determined in order to specify the logic function for the LUT primitive. There
are at least two methods by which the LUT value can be determined:
The Logic Table Method -A common method to determine the desired INIT value for a LUT is using a logic
table. To do so, simply create a binary truth table of all possible inputs, specify the desired logic value of the
output and then create the INIT string from those output values.
The Equation Method -Another method to determine the LUT value is to define parameters for each input to
the LUT that correspond to their listed truth value and use those to build the logic equation you are after. This
method is easier to understand once you have grasped the concept and is more self-documenting that the above
method. However, this method does require the code to first specify the appropriate parameters.
Logic Table
Inputs
I5 I4
0 0
0 0
244
I3 I2
0 0
0 0
www.xilinx.com
I1 I0
0 0
0 1
Virtex-6 Libraries Guide for HDL Designs
UG623 (v 14.5) March 20, 2013
Outputs
LO
INIT[0]
INIT[1]