Download XST User Guide

R
Decoders
Decoders
A decoder is a multiplexer whose inputs are all constant with distinct one-hot (or one-cold)
coded values. Please refer to “Multiplexers” in this chapter for more details. This section
shows two examples of 1-of-8 decoders using One-Hot and One-Cold coded values.
Log File
The XST log file reports the type and size of recognized decoders during the Macro
Recognition step.
Synthesizing Unit <dec>.
Related source file is decoders_1.vhd.
Found 1-of-8 decoder for signal <res>.
Summary:
inferred
1 Decoder(s).
Unit <dec> synthesized.
==============================
HDL Synthesis Report
Macro Statistics
# Decoders
1-of-8 decoder
==============================
...
: 1
: 1
The following table shows pin definitions for a 1-of-8 decoder.
IO pins
Description
s[2:0]
Selector
res
Data Output
Related Constraints
A related constraint is DECODER_EXTRACT.
VHDL (One-Hot)
Following is the VHDL code for a 1-of-8 decoder.
library ieee;
use ieee.std_logic_1164.all;
entity dec is
port (
sel: in std_logic_vector (2 downto 0);
res: out std_logic_vector (7 downto 0)
);
end dec;
XST User Guide
www.xilinx.com
1-800-255-7778
91