基础003_V7-Memory Resources

一、综述

参考ug473.pdf。

常用Memory 资源：

在IP核中，Block memory（distributed memory为CLB中的资源）：

通常选用Native，而不用AXI接口：

Block RAM可配置单端口RAM、伪双端口RAM、双端口RAM、单端口ROM、双端口ROM、FIFO。

各个模式调用时可承受的最高频率，参考pg058.pdf：

二、主要功能

学习资源： http://www.asic-world.com/examples/verilog/memories.html

　　A-RAM/ROM

主要功能：

每一个Block RAM都可配置为1个36Kb的BRAM或1个36Kb的FIFO；同时也可以将其配置为2个单独的18Kb的BRAM或1个18KbBRAM + 1个18Kb的BRAM。

为什么是18k而不是16k(2的整次幂)？因为每8bit一个校验位。2*8 + 2 =18bit。

结论：无论是单端口RAM、伪双端口RAM还是双端口RAM，他们都只有一块Memory。

Single-port RAM:

同步示例：

module ram_sp_sr_sw (
clk         , // Clock Input
address     , // Address Input
data        , // Data bi-directional
cs          , // Chip Select
we          , // Write Enable/Read Enable
oe            // Output Enable
); 

parameter DATA_WIDTH = 8 ;
parameter ADDR_WIDTH = 8 ;
parameter RAM_DEPTH = 1 << ADDR_WIDTH;

//--------------Input Ports----------------------- 
input                  clk         ;
input [ADDR_WIDTH-1:0] address     ;
input                  cs          ;
input                  we          ;
input                  oe          ; 

//--------------Inout Ports----------------------- 
inout [DATA_WIDTH-1:0]  data       ;

//--------------Internal variables---------------- 
reg [DATA_WIDTH-1:0] data_out ;
reg [DATA_WIDTH-1:0] mem [0:RAM_DEPTH-1];
reg                  oe_r;

//--------------Code Starts Here------------------ 

// Tri-State Buffer control 
// output : When we = 0, oe = 1, cs = 1
assign data = (cs && oe && !we) ? data_out : 8'bz; 

// Memory Write Block 
// Write Operation : When we = 1, cs = 1
always @ (posedge clk)
begin : MEM_WRITE
   if ( cs && we ) begin
       mem[address] = data;
   end
end

// Memory Read Block 
// Read Operation : When we = 0, oe = 1, cs = 1
always @ (posedge clk)
begin : MEM_READ
  if (cs && !we && oe) begin
    data_out = mem[address];
    oe_r = 1;
  end else begin
    oe_r = 0;
  end
end

endmodule // End of Module ram_sp_sr_sw

异步（异步读、同步写）示例：

module ram_sp_ar_sw (
clk         , // Clock Input
address     , // Address Input
data        , // Data bi-directional
cs          , // Chip Select
we          , // Write Enable/Read Enable
oe            // Output Enable
); 

parameter DATA_WIDTH = 8 ;
parameter ADDR_WIDTH = 8 ;
parameter RAM_DEPTH = 1 << ADDR_WIDTH;

//--------------Input Ports----------------------- 
input                                     clk          ;
input [ADDR_WIDTH-1:0] address ;
input                                     cs           ;
input                                     we          ;
input                                     oe           ; 

//--------------Inout Ports----------------------- 
inout [DATA_WIDTH-1:0]  data       ;

//--------------Internal variables---------------- 
reg [DATA_WIDTH-1:0]   data_out ;
reg [DATA_WIDTH-1:0] mem [0:RAM_DEPTH-1];

//--------------Code Starts Here------------------ 

// Tri-State Buffer control 
// output : When we = 0, oe = 1, cs = 1
assign data = (cs && oe && !we) ? data_out : 8'bz; 

// Memory Write Block 
// Write Operation : When we = 1, cs = 1
always @ (posedge clk)
begin : MEM_WRITE
   if ( cs && we ) begin
       mem[address] = data;
   end
end

// Memory Read Block 
// Read Operation : When we = 0, oe = 1, cs = 1
always @ (address or cs or we or oe)
begin : MEM_READ
    if (cs && !we && oe) begin
         data_out = mem[address];
    end
end

endmodule // End of Module ram_sp_ar_sw

　　对应电路：

可以看出2^8 = 256由4个64拼接拼接而成：

这里其实调用的是CLB中SliceM下的Distributed RAM资源：

　　B-FIFO

FIFO的能力：

FIFO的IP核使用，具体可参考：pg057.pdf。

FIFO可调用shift reg、distributed RAM、Block RAM、BulitIn FIFO，关于使用，xilinx论坛有相关说法：

Q：

From PG057 (Fifo generator) I understand FIFO's can be implemented in 4 ways, using :

block RAM

distributed RAM

shift register

built-in FIFO (using FIFO18 / FIFO36)

is there any simple document / app note / overview describing on what basis you typically decide between the 4 implementations. What are the main tradeoffs, advantages, ... of each underlying memory type used?

I can imagine a few, but not sure if these are correct and complete :

block RAM is interesting for large, deep fifo's

distributed RAM is interesting for smaller fifo's

shift register is interesting for smaller fifo's, with short word width

built-in FIFO allow for the fastest fifo's

but that's just intuition ... so any corrections or further insights are welcome here!

A:

Its based your application , requirement and available resources in your target FPGA. The most of the points you mention correct. I would recommend you to refer target FPGA resource guide LUTs have lowest access time, FIFO18/FIFO36 good timing performance but require effort in design migration, BRAM very good for scalable memory requirement . You can also check about URAM/ultraram available in ultrascale devices