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AR# 16526

8.2i Schematic - How do I use constants in an ISE 8.2i schematic design?


Keywords: ECS, constant, GND, ground, VCC, bus

In ECS, how do I place a constant value as a bus input to a schematic symbol?

This Answer Record is for ISE 8.2i and earlier. In ISE 9.1i, a Constant symbol has been added to the schematic editor for adding constant values to a bus.


ISE Schematic Editor does not currently have a constant value parameter that can be placed on a bus input to a schematic symbol. However, there are two simple ways to work around this problem, as described below:

1. You can name a bus input to apply VCC and GND to each bit as needed, as described in the following example:

a. Connect a net named "one" to a VCC symbol and a net named "zero" to a GND symbol.
b. Name an eight-bit bus "one,one,zero,zero,zero,one,one,zero" to give the bus a constant value of C6 (11000110).
NOTE: Be sure not to leave spaces between a comma and the next element of the compound bus name.

2. Generate your own constant schematic symbol using HDL code, as shown in the following example:

entity constant_symbol is
Port ( constant_out : out std_logic_vector(7 downto 0));
end constant_symbol;

architecture Behavioral of constant_symbol is

constant_out <= "11110000";

end Behavioral;
AR# 16526
Date Created 09/03/2007
Last Updated 04/13/2009
Status Archive
Type General Article