CODEWITHSUNDEEP
vhdl
chwi
chwi

Reputation: 2802

Signals and ports on which side of the "arrow" =>

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 doesn't work.

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

Upvotes: 3

Views: 10364

Answers (3)

newb7777
newb7777

Reputation: 563

If => is for a ports list then how is this possible? Please note that clk_40Mhz_i is a pin.

COMPONENT clk_wiz_v3_5 is 

PORT  
 (-- Clock in ports
  CLK_IN1           : in     std_logic;
  -- Clock out ports
  CLK_OUT1          : out    std_logic;
  -- Status and control signals
  RESET             : in     std_logic;
  LOCKED            : out    std_logic
);

END COMPONENT;

xclk_wiz_v3_5: clk_wiz_v3_5
   PORT MAP (
     CLK_IN1          => clk_40Mhz_i,
     -- Clock out ports
     CLK_OUT1         => clk_40Mhz,
     -- Status and control signals
     RESET            => pic_fpga_reset,
     LOCKED           => clk_locked   
   );

   pic_fpga_reset          <= not(processor_fpga_resetn_i);
   reset                   <= not(clk_locked);

Upvotes: 0

Israel Unterman
Israel Unterman

Reputation: 13510

Signal assignments go from right to left using <=. The right side must be an input signal from the entity or a signal declared in a process. The left side can be an output signal (or input/buffer) from the entity, a signal declared in the process or a variable declared in the process.

Beside port mapping mentioned in other answers, the => arrow is also used for a totally different thing - to construct vectors. For example, if v is a 4 bit vector, then v <= (others => '0') will assign "0000" to v. The => within the parentheses is a shortcut for assigning different values in different places inside the vector.

Upvotes: 2

Martin Thompson
Martin Thompson

Reputation: 16792

<= 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.

Upvotes: 6

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