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AR# 4407

FPGA Express: leaving pins open on VHDL component instantiations (VSS-538, VSS-544)


Keywords: FPGA, Express, VHDL, instantiate

Urgency: Standard

General Description:
When a black box is instantiated in VHDL code, all declared pins must be accounted for. This
becomes a problem if a component is declared with all its pins but only some are needed, or if a
component is instantiated multiple times are different pins are used each time.

If pins are left unconnected, FPGA Express returns this error:

Error: C:/designs/top.vhd line 53
Not enough element associations - 3 expected. (VSS-538) (FPGA-dm-hdlc-unknown)

If the keyword "open" is used to leave declared pins unconnected, FPGA Express returns
this error:

Error: C:/designs/top.vhd line 55
OPEN is not a legal actual part of an element association here (check if the input ports are
properly mapped). (VSS-544) (FPGA-dm-hdlc-unknown)


When declaring the component, assign initial values to any pins that may be left unconnected
when instantiated. These values will not be used in synthesis, as these ports will either be
connected to valid signals or left unconnected entirely. Then, at the component instantiation,
use the "open" keyword to denote unconnected ports.

component STARTUP_VIRTEX port (
GTS: in STD_LOGIC := '1'; -- initial values defined
CLK: in STD_LOGIC := '1');
end component;


mystart : STARTUP_VIRTEX port map (
GTS => open, --open keyword used to leave pin unconnected
CLK => open);
AR# 4407
Date Created 08/05/1998
Last Updated 08/11/2003
Status Archive
Type General Article