Xilinx Virtex-6 Manual page 232

Chapter 4: About Design Elements
LUT6
Primitive: 6-Input Lookup Table with General 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 will be 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 LUTs logical function.
The INIT value is calculated by assigning a 1 to corresponding INIT bit value when the associated inputs are
applied. For instance, a Verilog INIT value of 64'h8000000000000000 (X"8000000000000000" for VHDL) makes
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) makes 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 logic 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 than 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
0 0
232
I3 I2
0 0
0 0
0 0
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I1 I0
0 0
0 1
1 0
Virtex-6 Libraries Guide for HDL Designs
UG623 (v 14.5) March 20, 2013
Outputs
O
INIT[0]
INIT[1]
INIT[2]