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Chapter 3: XST Hardware Description Language (HDL) Coding Techniques 3-Bit 1-of-9 Priority Encoder Coding Examples For this example XST may infer a priority encoder. Use Priority Encoder Extraction (PRIORITY_EXTRACT) with a value force to force its inference. 3-Bit 1-of-9 Priority Encoder Pin Descriptions IO Pins Description sel Selector code Encoded Output Bus 3-Bit 1-of-9 Priority Encoder VHDL Coding Example --- 3-Bit 1-of-9 Priority Encoder -library ieee; use ieee.std_logic_1164.all; entity priority_encoder_1 is port ( sel : in std_logic_vector (7 downto 0); code :out std_logic_vector (2 downto 0)); attribute priority_extract: string; attribute priority_extract of priority_encoder_1: entity is "force"; end priority_encoder_1; architecture archi of priority_encoder_1 is begin code <= "000" when "001" when "010" when "011" when "100" when "101" when "110" when "111" when "---"; sel(0) sel(1) sel(2) sel(3) sel(4) sel(5) sel(6) sel(7) = = = = = = = = ’1’ ’1’ ’1’ ’1’ ’1’ ’1’ ’1’ ’1’ else else else else else else else else end archi; 80 www.xilinx.com XST User Guide UG627 (v 11.3) September 16, 2009
