Megafunction

lpm_clshift (Shifter) Megafunction

Parameterized combinatorial logic shifter or barrel shifter megafunction. Altera® recommends using the lpm_clshift function instead of any other type of shifter.

Altera also recommends instantiating this function as described in Using the MegaWizard® Plug-In Manager.

This topic contains the following information:

 

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

 
FUNCTION lpm_clshift (data[LPM_WIDTH-1..0], distance[LPM_WIDTHDIST-1..0], direction)
   WITH (LPM_WIDTH, LPM_WIDTHDIST, LPM_SHIFTTYPE)
   RETURNS (result[LPM_WIDTH-1..0], underflow, overflow);

 

VHDL Component Declaration:

 
COMPONENT lpm_clshift
   GENERIC (LPM_WIDTH: POSITIVE;
      LPM_WIDTHDIST: POSITIVE;
      LPM_SHIFTTYPE: STRING := "LOGICAL";
      LPM_TYPE: STRING := "LPM_CLSHIFT";
      LPM_HINT: STRING := "UNUSED");
   PORT (data: IN STD_LOGIC_VECTOR(LPM_WIDTH-1 DOWNTO 0);
      distance: IN STD_LOGIC_VECTOR(LPM_WIDTHDIST-1 DOWNTO 0);
      direction: IN STD_LOGIC := '0';
      result: OUT STD_LOGIC_VECTOR(LPM_WIDTH-1 DOWNTO 0);
      underflow, overflow: OUT STD_LOGIC);
END COMPONENT;

 

VHDL LIBRARY-USE Declaration

 
LIBRARY lpm;
USE lpm.lpm_components.all;

 

Port Descriptions:

INPUT PORTS

Port Name Required Description Comments
data[] Yes Data to be shifted. Input port LPM_WIDTH wide.
distance[] Yes Number of positions to shift data[] in the direction specified by the direction port. Input port LPM_WIDTHDIST wide.
direction No Direction of shift. Low = left (toward the MSB), high = right (toward the LSB). Default value is 0 (low) = left (toward the MSB).

OUTPUT PORTS

Port Name Required Description Comments
result[] Yes Shifted data. Output port LPM_WIDTH wide.
underflow No Logical or arithmetic underflow. If "ROTATE" is specified as the LPM_SHIFTTYPE parameter value and overflow and underflow are connected, the output of overflow and underflow will be undefined (X) logic levels.
overflow No Logical or arithmetic overflow. If "ROTATE" is specified as the LPM_SHIFTTYPE parameter value and overflow and underflow are connected, the output of overflow and underflow will be undefined (X) logic levels.

 

Parameter Descriptions:
Parameter Type Required Description
LPM_WIDTH Integer Yes Width of the data[] and result[] ports. Any positive integer > 1.
LPM_WIDTHDIST Integer Yes Width of the distance[] input port. The LPM_WIDTHDIST parameter specifies the width of the distance port. The values on the distance port normally range from 0, which is "no shift", to (LPM_WIDTH-1) which is the maximum meaningful shift. The typical value assigned to LPM_WIDTHDIST is "the smallest integer not less than log-base-2(LPM_WIDTH)" or [log-base-2(LPM_WIDTH)]. Any value on the distance port greater than LPM_WIDTH-1 results in an undefined (X) logic level.
LPM_SHIFTTYPE String No Type of shifter: "LOGICAL", "ROTATE", "ARITHMETIC", or "UNUSED". If omitted, the default is "LOGICAL". The sign bit is extended for "ARITHMETIC". For a "LOGICAL" right shift, 0s are always shifted into the MSB.
LPM_HINT String No Allows you to specify Altera-specific parameters in VHDL Design Files (.vhd). The default is "UNUSED".
LPM_TYPE String No Identifies the library of parameterized modules (LPM) entity name in VHDL Design Files.

 

Truth Table/Functionality:

The lpm_clshift megafunction acts like a barrel shifter. It contains entirely combinatorial logic.

Overflow occurs when the shifted result exceeds the precision of the result bus. For "LOGICAL" values, overflow occurs when any 1 has been shifted out. For "ARITHMETIC" values, overflow occurs when a significant digit is shifted into or past the sign bit. Underflow occurs when the shifted result contains no significant digits.

Operation of the LOGICAL shifter is illustrated in the following example, in which LPM_WIDTH = 4:

Inputs Outputs
direction distance[1..0] data[3..0] overflow result[3..0] underflow
X 0 dcba 0 dcba 0
0 1 dcba d cba0 0
0 2 dcba d # c ba00 0
0 3 dcba d # c # b a000 0
1 1 dcba 0 0dcb !d & !c & !b & a
1 2 dcba 0 00dc !d & !c & (b # a)
1 3 dcba 0 000d !d & (c # b # a)

Operation of the ARITHMETIC shifter is illustrated in the following example, in which LPM_WIDTH = 4:

Inputs Outputs
direction distance[1..0] data[3..0] overflow result[3..0] underflow
X 0 dcba 0 dcba 0
0 1 0cba c cba0 0
0 1 1cba !c cba0 0
0 2 0cba c # b ba00 0
0 2 1cba c! # b! ba00 0
0 3 0cba c # b # a a000 0
0 3 1cba !c # !b # !a a000 0
1 1 0cba 0 00cb !c & !b & a
1 1 1cba 0 11cb c & b & !a
1 2 0cba 0 000c !c & (b # a)
1 2 1cba 0 111c c & (!b # !a)
1 3 0cba 0 0000 (c # b # a)
1 3 1cba 0 1111 (!c # !b # !a)

Operation of the ROTATE shifter is illustrated in the following example, in which LPM_WIDTH = 4:

Inputs Outputs
direction distance[1..0] data[3..0] result[3..0]
X 0 dcba dcba
0 1 dcba cbad
0 2 dcba badc
0 3 dcba adcb
1 1 dcba adcb
1 2 dcba badc
1 3 dcba cbad

 

Resource Usage:

The following table summarizes the resource usage for an lpm_clshift function used to implement a 16-bit bidirectional arithmetic shifter. Logic cell usage scales slightly faster than linearly in proportion to shifter width.

Design Goals Design Results
Device Family Optimization
APEX™ 20K Routability
  Speed
Width Logic Cells
16 142
16 141

Numbers of shared expanders used are shown in parentheses ( ).

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