RAM256X1S - 2021.2 English

UltraScale Architecture Libraries Guide (UG974)

Document ID
Release Date
2021.2 English

Primitive: 256-Deep by 1-Wide Random Access Memory (Select RAM)

  • Families: UltraScale, UltraScale+


This design element is a 256-bit deep by 1-bit wide random access memory with synchronous write and asynchronous read capability. This RAM is implemented using the LUT resources of the device (also known as Select RAM), and does not consume any of the block RAM resources of the device. If a synchronous read capability is preferred, a register can be attached to the output and placed in the same CLB.

The RAM256X1S has an active-High write enable (WE) so that when that signal is High, and a rising edge occurs on the WCLK pin, a write is performed recording the value of the D input data pin into the memory array. The output O displays the contents of the memory cell addressed by A, regardless of the WE value. When a write is performed, the output is updated to the new value shortly after the write completes.

Port Descriptions

Port Direction Width Function
O Output 1 Read/Write port data output addressed by A
D Input 1 Write data input addressed by A
A Input 8 Read/Write port address bus
WE Input 1 Write Enable
WCLK Input 1 Write clock (reads are asynchronous)

Design Entry Method

Instantiation Yes
Inference Recommended
IP Catalog No

If instantiated, the following connections should be made to this component:

  • Tie the WCLK input to the desired clock source, the D input to the data source to be stored, and the O output to an FDCE D input or other appropriate data destination.

  • Connect the WE clock enable pin to the proper write enable source in the design.

  • Connect the 8-bit A bus to the source for the read/write.

You can use the INIT attribute to specify the initial contents of the RAM. If left unspecified, the initial contents default to all zeros.

Available Attributes

Attribute Type Allowed Values Default Description
INIT HEX Any 256-bit value All zeros Specifies the initial contents of the RAM.
IS_WCLK_INVERTED BINARY 1'b0 to 1'b1 1'b0 Specifies whether or not to use the optional inversion on the WCLK pin.

VHDL Instantiation Template

Unless they already exist, copy the following two statements and paste them before the entity declaration.
Library UNISIM;
use UNISIM.vcomponents.all;

-- RAM256X1S: 256-deep by 1-wide positive edge write, asynchronous read
--            single-port distributed LUT RAM
--            UltraScale
-- Xilinx HDL Language Template, version 2021.2

RAM256X1S_inst : RAM256X1S
generic map (
   INIT => X"0000000000000000000000000000000000000000000000000000000000000000", -- Initial contents of RAM
   IS_WCLK_INVERTED => '0') -- Specifies active high/low WCLK
port map (
   O => O,  -- Read/Write port 1-bit ouput
   A => A,  -- Read/Write port 8-bit address input
   D => D,  -- RAM data input
   WCLK => WCLK,  -- Write clock input
   WE => WE -- Write enable input

-- End of RAM256X1S_inst instantiation

Verilog Instantiation Template

// RAM256X1S: 256-deep by 1-wide positive edge write, asynchronous read  (Mapped to four LUT6s)
//            single-port distributed LUT RAM
//            UltraScale
// Xilinx HDL Language Template, version 2021.2

RAM256X1S #(
   .IS_WCLK_INVERTED(1'b0)  // Specifies active high/low WCLK
) RAM256X1S_inst (
   .O(O),       // Read/write port 1-bit output
   .A(A),       // Read/write port 8-bit address input
   .WE(WE),     // Write enable input
   .WCLK(WCLK), // Write clock input
   .D(D)        // RAM data input

// End of RAM256X1S_inst instantiation

Related Information

  • See the UltraScale Architecture Configurable Logic Block User Guide (UG574).