Megafunction

lpm_abs (Absolute Value) Megafunction

Parameterized absolute value megafunction. Altera® recommends using the lpm_abs function to replace all other types of absolute value functions.

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_abs (data[LPM_WIDTH-1..0])
   WITH (LPM_WIDTH, ADDERTYPE)
   RETURNS (result[LPM_WIDTH-1..0], overflow);

 

VHDL Component Declaration:

 
COMPONENT lpm_abs
   GENERIC (LPM_WIDTH : POSITIVE;
      LPM_TYPE: STRING := "LPM_ABS";
      LPM_HINT: STRING := "UNUSED");
   PORT (data : IN STD_LOGIC_VECTOR(LPM_WIDTH-1 DOWNTO 0);
      result : OUT STD_LOGIC_VECTOR(LPM_WIDTH-1 DOWNTO 0);
      overflow: OUT STD_LOGIC);
END COMPONENT;
NOTE The ADDERTYPE parameter is supported only for backward compatibility, but should not be used in newer projects.

 

VHDL LIBRARY-USE Declaration

 
LIBRARY lpm;
USE lpm.lpm_components.all;

 

Port Descriptions:

INPUT PORTS

Port Name Required Description Comments
data[] Yes Signed number. Input port LPM_WIDTH wide.

OUTPUT PORTS

Port Name Required Description Comments
result[] Yes Absolute value of data[]. Output port LPM_WIDTH wide.
overflow No High if data = -2 ^ (LPM_WIDTH-1). Two's complement allows one more negative number than positive. The overflow port detects that singular instance and goes high to indicate that no positive equivalent exists.

 

Parameter Descriptions:
Parameter Type Required Description
LPM_WIDTH Integer Yes Width of the data[] and result[] ports.
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:
Inputs Outputs
data[LPM_WIDTH-1..0] result[LPM_WIDTH-1..0] overflow
>= 0 data[LPM_WIDTH-1..0] 0
< 0 data[LPM_WIDTH-1..0] 0
100... 100... 1

 

Resource Usage:

The following table summarizes the resource usage for an lpm_abs function used to implement a 16-bit absolute value converter with an up/down control and an overflow output.

Design Goals Design Results
Device Family Optimization
APEX™ 20K Routability
  Speed
Width Logic Cells
16 23
16 17

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

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