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Created on: 12 March 2013
A single tri-state buffer with active low enable and a 4-bit wide tri-state buffer with single active low enable are written in VHDL code and implemented on a CPLD.
Both tri-state buffers are implemented in the same VHDL code of a single project.
This video shows the two tri-state buffers in operation on the home made Xilinx CPLD board.
Can't see the video? View on YouTube →
A single tri-state buffer with active low enable pin is shown below. A group of four tri-state buffers with a single enable pin is also shown. Both of these buffers are written in VHDL and implemented on a CPLD.
The VHDL code for the single and quad tri-state buffers is shown below.
library IEEE; use IEEE.STD_LOGIC_1164.ALL; entity tri_state_buffer_top is Port ( A : in STD_LOGIC; -- single buffer input EN : in STD_LOGIC; -- single buffer enable Y : out STD_LOGIC; -- single buffer output -- 4 input / output buffer with one enable IN4 : in STD_LOGIC_VECTOR (3 downto 0); EN4 : in STD_LOGIC; OUT4 : out STD_LOGIC_VECTOR (3 downto 0)); end tri_state_buffer_top; architecture Behavioral of tri_state_buffer_top is begin -- single active low enabled tri-state buffer Y <= A when (EN = '0') else 'Z'; -- 4 input/output active low enabled tri-state buffer OUT4 <= IN4 when (EN4 = '0') else "ZZZZ"; end Behavioral;
The single tri-state buffer is created in VHDL using the following line of code:
Y <= A when (EN = '0') else 'Z';
When the EN pin is low, then the logic level on the A input will appear on the Y output. If a logic 1 is on the EN pin, the output Y will be tri-stated (made high impedance indicated by Z in VHDL).
The quad input tri-state buffer with single enable is created with the following VHDL code:
OUT4 <= IN4 when (EN4 = '0') else "ZZZZ";
When the enable pin EN4 is low, then each of the logic states on the inputs of the buffers will appear on their corresponding outputs. If the EN pin has a logic high applied to it, then all buffers will be tri-stated ("ZZZZ" is used to tri-state all four buffers in the VHDL code).