A channel for dynamic communication between a block and its environment.
port ( port_declaration, port_declaration, &ldots;);
-- port declarations:
port_signal_name : in port_signal_type := initial_value
port_signal_name : out port_signal_type := initial_value
port_signal_name : inout port_signal_type := initial_value
port_signal_name : buffer port_signal_type := initial_value
port_signal_name : linkage port_signal_type := initial_value
Ports are a part of the block interface: external - if defined by a design entity, or internal - if defined by a block statement. Each element listed in a port interface list declares a formal port, which provides a channel for dynamic communication between a block and its environment.
In practice, ports are most often used in entities and components, where they serve for declaring interface signals of a design entity (system design) or component, respectively.
In both cases, each interface element is a signal. It can be preceded by a keyword signal. After the signal's name, a mode is specified. The mode declares the direction of data flow through the port. There are five modes available in VHDL for ports:
· in input port. A variable or a signal can read a value from a port of mode in, but is not allowed to assign a value to it.
· out output port. It is allowed to make signal assignments to a port of the mode out, but it is not legal to read from it.
· inout bi-directional port. Both assignments to such a port and reading from it are allowed.
· buffer output port with read capability. It differs from inout in that it can be updated by at most one source, whereas inout can be updated by zero or more sources.
· linkage . The value of the port may be read or updated, but only by appearing as an actual corresponding to an interface object of mode linkage.
If a port is declared with a reserved word bus, then the signal declared by that port is a guarded signal of signal kind bus.
A port can be assigned a default value, which is specified by an expression evaluating to the same type as the port itself.
entity Mux8to1 is
Inputs : in Std_Logic_Vector(7 downto 0);
Select_s : in Std_Logic_Vector(2 downto 0);
Output : out Std_Logic
Entity of a multiplexor 8-to-1 contains three ports: eight data inputs (specified as a vector), address inputs and one output.
component MemDev is
Data : inout Std_Logic_Vector(7 downto 0);
Addr : in Std_Logic_Vector(9 downto 0);
NotCS : in Std_Logic;
RdNotWr : in Bit
end component MemDev;
Memory device is specified here as a component with four signals: data is a bi-directional data bus, address is a ten-bit input, and NotCS and RdNotWr are single inputs signals. Note that the keyword is in the header can be used in VHDL 93 only (in VHDL 87 it must be omitted).
· Ports declarations are signal declarations and port signals need not to be re-declared.