See Also

Parameterized single-clock FIFO megafunction. The scfifo function uses mega RAMs or M4K or M512 memory blocks in Stratix^™ and Stratix^™ GX devices, M4K memory blocks in Cyclone^™ devices, Embedded System Blocks (ESB) in APEX^™ 20K, APEX^™ II, ARM^®-based Excalibur^™, and Mercury^™ devices, Embedded Array Blocks (EAB) in ACEX^® 1K and FLEX 10KE devices, or DFFE primitives or latch arrays in FLEX^® 6000, MAX^® 3000, and MAX 7000 devices or if the USE_EAB parameter is set to "OFF". Altera^® strongly recommends using synchronous rather than asynchronous RAM functions.

The Quartus® II Compiler automatically implements suitable portions of this function in mega RAMs or M4K or M512 memory blocks in Stratix and Stratix GX devices, in M4K memory blocks in Cyclone devices, in ESBs in APEX 20K, APEX II, ARM-based Excalibur, and Mercury devices, and in EABs in ACEX 1K and FLEX 10KE devices. The Compiler automatically implements this function in logic cells in FLEX 6000, MAX 3000, and MAX 7000 devices.

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

The scfifo function must contain at least one word before it is able to read a word and must have at least one unused word before it is able to write a word.

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
• Truth Table/Functionality
• Resource Usage

FUNCTION scfifo (data[(LPM_WIDTH)-1..0], wrreq, rdreq, clock, aclr,
      sclr)
   WITH (LPM_WIDTH, LPM_NUMWORDS, LPM_SHOWAHEAD,
      UNDERFLOW_CHECKING, OVERFLOW_CHECKING, ALMOST_FULL_VALUE,
      ALMOST_EMPTY_VALUE, ALLOW_RWCYCLE_WHEN_FULL, USE_EAB,
      MAXIMIZE_SPEED)
   RETURNS (q[(LPM_WIDTH)-1..0], empty, full, almost_full,
      almost_empty,
      usedw[(LPM_NUMWORDS>=2 ? CEIL(LOG2(LPM_NUMWORDS)):1)-1..0]

COMPONENT scfifo
   GENERIC (LPM_WIDTH: POSITIVE;
      LPM_WIDTHU: POSITIVE;
      LPM_NUMWORDS: POSITIVE;
      LPM_SHOWAHEAD: STRING := "OFF";
      ALMOST_FULL_VALUE: POSITIVE:= 0;
      ALMOST_EMPTY_VALUE: POSITIVE:= 0;
      ALLOW_RWCYCLE_WHEN_FULL: STRING := "OFF";
      MAXIMIZE_SPEED: POSITIVE:= 5;
      OVERFLOW_CHECKING: STRING:= "ON";
      UNDERFLOW_CHECKING: STRING:= "ON");
   PORT (data: IN STD_LOGIC_VECTOR(LPM_WIDTH-1 DOWNTO 0);
      clock, wrreq, rdreq, aclr, sclr: IN STD_LOGIC;
      full, empty, almost_full, almost_empty: OUT STD_LOGIC;
      q: OUT STD_LOGIC_VECTOR(LPM_WIDTH-1 DOWNTO 0)
      usedw: OUT STD_LOGIC_VECTOR(LPM_WIDTHU-1 DOWNTO 0));
END COMPONENT;

LIBRARY altera_mf
USE altera_mf.altera_mf_components.all;

INPUT PORTS

OUTPUT PORTS

Parameter	Type	Required	Description
LPM_WIDTH	Integer	Yes	Width of data[] and q[] ports.
LPM_NUMWORDS	Integer	Yes	Depth of the scfifo. Number of words stored in memory, which is usually a power of 2.
LPM_WIDTHU	Integer	Yes	(VHDL only) Width of usedw[] port.
ALMOST_FULL_VALUE	Integer	No	The threshold value for the almost_full port.
ALMOST_EMPTY_VALUE	Integer	No	The threshold value for the almost_empty port.
LPM_SHOWAHEAD	String	No	Allows the data to appear on q[] before rdreq is asserted when set to "ON". Values are "ON" or "OFF" (the default is "OFF").
ALLOW_RWCYCLE_WHEN_FULL	String	No	Allows combined read/write cycles to an already full scfifo, so that it remains full. Values are "ON" or "OFF" (the default is "OFF"). This parameter is used only when the OVERFLOW_CHECKING parameter is set to "ON".
USE_EAB	String	No	Altera-specific parameter. Values are "ON", "OFF", and "UNUSED". Setting the USE_EAB parameter to "OFF" prevents the Quartus II software from using ESBs to implement the logic in APEX 20K, APEX II, ARM-based Excalibur, and Mercury devices, or EABs in ACEX 1K and FLEX 10KE devices; it can use only flipflops or latches. (The "ON" setting is not useful in memory functions: the Quartus II software automatically implements memory functions in ESBs or EABs by default.) This parameter is not available for simulation with other EDA simulators and for FLEX 6000, MAX 3000, MAX 7000, Stratix, and Stratix GX devices.
MAXIMIZE_SPEED	Integer	No	Altera-specific parameter. You can specify a value between 0 and 10. If used, the Quartus II software attempts to optimize a specific instance of the scfifo function for speed rather than area, and overrides the setting of the Optimization Technique option in the Assignment Organizer dialog box (Assignment menu). If MAXIMIZE_SPEED is unused, the value of the Optimization Technique option is used instead. If the setting for MAXIMIZE_SPEED is 6 or higher, the Compiler will optimize scfifo megafunctions for higher speed; if the setting is 5 or less, the Compiler will optimize for smaller area.
OVERFLOW_CHECKING	String	No	Disables the overflow-checking logic when set to "OFF", so that full is no longer checked for wrreq. Values are "ON" or "OFF" (the default is "ON"). Writing to a full scfifo gives unpredictable results.
UNDERFLOW_CHECKING	String	No	Disables the underflow-checking logic when set to "OFF", so that empty is no longer checked for rdreq. Values are "ON" or "OFF" (the default is "ON"). Reading an empty scfifo gives unpredictable results.

The scfifo function represents memory with synchronous inputs and/or outputs.

Synchronous Memory Operations
aclr sclr Clock rdreq wrreq Function L L X L L No change. L L not X X No change (requires positive clock edge). L L L H Write data to memory. L L H L Read memory and update q[]. L L H H Write data to memory and read memory to q[]. H L X X X Resets the scfifo to empty. The q[] port becomes undefined. L H X X Resets the scfifo to empty. The q[] port becomes undefined.

The following table summarizes the resource usage for an scfifo megafunction used to implement an 8-bit-wide FIFO buffer. A depth of 8 words is used for the FLEX 6000, MAX 3000, and MAX 7000 device families. A depth of 256 words is used for all other device families.

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