AR# 3030: Foundation XVHDL, XC9500: How to set an output to high impedance (Hi-Z)
AR# 3030
Foundation XVHDL, XC9500: How to set an output to high impedance (Hi-Z)
Description
Keywords: high, z, impedance, xvhdl, foundation, 9500, unused, tie
Urgency: Standard
General Description:
Sometimes it is necessary to tie unused pins to high impedance. This may be the case if changes were made to the logic of a device but the pin is on a bus externally. Tieing the pin to gnd or vcc may have drastic effects on the bus.
Solution
To tie an unused pin to High-Z in Foundation XVHDL, change the pin to an output std_logic vector, and declare a signal as follows: signal: bit :='0';
Outside of a process use the following equation:
outz <= 'Z' when controlz = '0' else '0';
Example:
library IEEE; use IEEE.std_logic_1164.all;
entity hiz is port( a:in bit; b:in bit; clk:in bit; d:out bit; oz:out std_logic ); end hiz;
architecture rewrite of hiz is signal controlz:bit:='0'; begin
process(clk) if clk'event and clk='1' then d<=a and b; end if; end process;
oz<='Z' when controlz='0' else '0';
--oz will always be 'Z' since controlz is tied to gnd --and never used.
end rewrite;
------------------------------- This will implement the following equations as seen in the fitting report