altshift_taps Megafunction

altshift_taps (Shift Register with Taps) Megafunction

Parameterized shift register with taps megafunction. Implements a RAM-based shift register for efficient creation of very large shift registers. Outputs are visible at regularly spaced points along the shift register ("taps").

This topic contains the following information:

AHDL Function Prototype (also applies to Verilog HDL)
VHDL Component Declaration
VHDL LIBRARY-USE Declaration (not required if the VHDL Component Declaration is used)
Port Descriptions
Parameter Descriptions
Resource Usage

AHDL Function Prototype (port name and order also apply to Verilog HDL):

FUNCTION altshift_taps (shiftin[WIDTH-1..0], clock, clken)
   WITH (NUMBER_OF_TAPS, TAP_DISTANCE, WIDTH)
   RETURNS (shiftout[WIDTH-1..0], taps[(WIDTH * NUMBER_OF_TAPS)-1..0]);

VHDL Component Declaration:

COMPONENT altshift_taps
   GENERIC
      (NUMBER_OF_TAPS   :INTEGER := 4;
      TAP_DISTANCE      :INTEGER := 3;
      WIDTH             :INTEGER := 8);

   PORT (shiftin   : IN STD_LOGIC_VECTOR (WIDTH-1 DOWNTO 0);
         clock     : IN STD_LOGIC;
         clken     : IN STD_LOGIC := '1';
         shiftout  : OUT STD_LOGIC_VECTOR (WIDTH-1 DOWNTO 0):=(OTHERS => '0');
         taps      : OUT STD_LOGIC_VECTOR (WIDTH*NUMBER_OF_TAPS)-1 DOWNTO 0):= (OTHERS => '0'));

END COMPONENT;

VHDL LIBRARY-USE Declaration

LIBRARY altera_mf;
USE altera_mf.altera_mf_components.all;

Port Descriptions:

INPUT PORTS

Port Name	Required	Description	Comments
`shiftin[]`	Yes	Data input to the shifter.	Input port `WIDTH` wide.
`clock`	Yes	Positive-edge triggered clock.
`clken`	No	Clock enable for the `clock` port.

OUTPUT PORTS

Port Name	Required	Description	Comments
`shiftout[]`	No	Output from the end of the shift register.	Output port `WIDTH` wide.
`taps[]`	No	Output from the regularly spaced taps along the shift register.	Output port `WIDTH * NUMBER_OF_TAPS` wide. This port is an aggregate of all the regularly spaced taps (each `WIDTH` bits) along the shift register.

Parameter Descriptions:

Parameter	Type	Required	Comments
`NUMBER_OF_TAPS`	Integer	No	Specifies the number of regularly spaced taps along the shift register.
`TAP_DISTANCE`	Integer	No	Specifies the distance between the regularly spaced taps in clock cycles. This number translates to the number of RAM words that will be used.
`WIDTH`	Integer	No	Specifies the width of the input pattern.

Resource Usage:

The RAM block used to implement the shift register must have TAP_DISTANCE words and NUMBER_OF_TAPS * WIDTH bits per word.

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