VHDL Tutorial

Signal Parameters in VHDL
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Signal Parameters

 

The  third  class  of  object  that  we  can  specify  for  formal  parameters  is  signal,  which indicates that the algorithm performed by the procedure involves a signal passed by the caller.   A signal parameter can be of any of the modes in, out or inout.

The  way  that  signal  parameters  work  is  somewhat  different  from  constant  and variable parameters.  When a caller passes a signal as a parameter of mode in, instead

of passing the value of the signal, it passes the signal object itself.   Any reference to the formal parameter within the procedure is exactly like a reference to the actual sig- nal itself.

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EXAMPLE

 

Suppose  we  wish  to  model  the  receiver  part  of  a  network  interface.   It  re- ceives fixed-length packets of data on the signal rx_data.  The data is synchronized with changes, from 0 to 1, of the clock signal rx_clock.  Figure 5-4 is an outline

of part of the model.

FIGURE 5-4

 

architecture behavioral of receiver is

     type declarations, etc

signal recovered_data : bit;

signal recovered_clock : bit;

procedure receive_packet ( signal rx_data : in bit;

signal rx_clock : in bit;

data_buffer : out packet_array ) is

begin

for index in packet_index_range loop wait until rx_clock = '1'; data_buffer(index) := rx_data;

end loop;

end procedure receive_packet;

begin

packet_assembler : process is variable packet : packet_array;

begin

receive_packet ( recovered_data, recovered_clock, packet );

end process packet_assembler;

end architecture behavioral;

 

An outline of a model of a network receiver, including a procedure with signal parameters of mode in.

 

During execution of the model, the process packet_assembler calls the proce- dure receive_packet, passing the signals recovered_data and recovered_clock as ac- tual parameters.   The wait statement mentions rx_clock, and since this stands for recovered_clock, the process is sensitive to changes on recovered_clock while it is

suspended.

 

 

 

Now  lets  look  at  signal  parameters  of  mode  out In  this  case,  the  caller  must name a signal as the actual parameter, and the procedure is passed a reference to the driver for the signal.  When the procedure performs a signal assignment statement on the formal parameter, the transactions are scheduled on the driver for the actual signal parameter.

 

EXAMPLE

 

Figure 5-5  is  an  outline  of  an  architecture  body  for  a  signal  generator.  The procedure generate_pulse_train has in mode constant parameters that specify the characteristics of a pulse train and an out mode signal parameter on which it gen- erates the required pulse train.  The process raw_signal_generator calls the procedure, supplying raw_signal as the actual signal parameter for s.  A reference to the driver  for  raw_signal  is  passed  to  the  procedure,  and  transactions  are  generated on it.

FIGURE 5-5

 

library ieee;  use ieee.std_logic_1164.all;

architecture top_level of signal_generator is signal raw_signal : std_ulogic;

procedure generate_pulse_train ( width, separation : in delay_length;

number : in natural;

signal s : out std_ulogic ) is

begin

for count in 1 to number loop

s <= '1', '0' after width;

wait for width + separation;

end loop;

end procedure generate_pulse_train;

begin

raw_signal_generator : process is begin

generate_pulse_train ( width => period / 2,

separation => period period / 2, number => pulse_count,

s => raw_signal );

end process raw_signal_generator;

end architecture top_level;

 

An outline of a model for a signal generator, including a pulse generator procedure with an out mode

signal parameter.

 

 

 

As  with  variable-class  parameters,  we  can  also  have  a  signal-class  parameter  of mode inout.  When the procedure is called, both the signal and a reference to its driver are passed to the procedure.   The statements within it can read the signal value, in- clude it in sensitivity lists in wait statements, query its attributes and schedule trans- actions using signal assignment statements.

 

 

 

 

 

 

 

 

 

 

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