VHDL: signals and ports on which side of the "arrow" =>

vhdl fpga xilinx
chwi picture chwi · Apr 18, 2012 · Viewed 8.1k times · Source

very basic question:

How do I know where the port/signal/value should be placed on which side of the arrows? I noticed that by switching port_a => x to x <= port_a which seems very equal, I got an error. Also, x => port_a dont work

I even do not know which way the arrows should point.

Answers are really appreciated!

Answer

Martin Thompson picture Martin Thompson · Apr 18, 2012

<= is an assignment - specifically a signal assignment, driving a signal with a value from somewhere else. For a physical analogy, the thing on the right hand side drives a value onto the left hand side.

=> is a port mapping from a pin to a signal. This is not an assignment - the physical analogy might be soldering a pin to a wire.

You can only do "soldering" to instantiations, so => mapping only happens inside a port map. And there, "pins" always go on the left (because that's what the language rules say), which is why you can't do x <= port_a in a port map.

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