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各厂商综合工具，对HDL综合时都定义了一些综合属性这些属性可指定a declaration,a module item,a statement, or a port connection 不同的综合方式。 语法为： /* synthesis, <any_company_specific_attribute = value_or_optional_value */ 下面就是Altera的几个常用的Synthesis attributes Noprune A Verilog HDL synthesis attribute that prevents the Quartus II software from removing a register that does not directly or indirectly feed a top-level output or bidir pin. For example: reg reg1 /* synthesis noprune */; keep A Verilog HDL synthesis attribute that directs Analysis & Synthesis to not minimize or remove a particular net when optimizing combinational logic. For example: wire keep_wire /* synthesis keep */; preserve A Verilog HDL synthesis attribute that directs Analysis & Synthesis to not minimize or remove a particular register when eliminating redundant registers or registers with constant drivers. For example: reg reg1 /* synthesis preserve */; ram_init_file A Verilog HDL synthesis attribute that specifies initial contents of an inferred memory. For example: reg [7:0] mem[0:255] /* synthesis ram_init_file = " my_init_file.mif" */; ramstyle A Verilog HDL synthesis attribute that specifies the type of TriMatrix Memory block to use when implementing an inferred RAM. M512", "M4K", "M9K", "M144K", "MLAB", "M-RAM” For example: reg [0:7] my_ram[0:63] /* synthesis ramstyle = "M512" */; translate_off or translate_on Verilog HDL synthesis directives that direct Analysis & Synthesis to ignore portions of the design code that are specific to simulation and not relevant to logic synthesis. For example: parameter tpd = 2; // Generic delays // synthesis translate_off #tpd; // synthesis translate_on 关于状态机有下面三个综合属性: full_case
A Verilog HDL synthesis attribute that directs Analysis & Synthesis to treat unspecified state values in a Verilog Design File Case Statement as don't care values, and therefore to treat the Case Statement as "full". 仅用于Verilog ,与case 语句一起使用表明所有可能的状态都已经给出不需要其他逻辑保持信号的值. module full_case (a, sel, y);
   input [3:0] a;
   input [1:0] sel;
   output y;
   reg y;
   always @(a or sel)                case (sel)      // synthesis full_case
         2'b00: y="a"[0];
         2'b01: y="a"[1];
         2'b10: y="a"[2];
      endcase
endmodule parallel_case
A Verilog HDL synthesis attribute that directs Analysis & Synthesis to implement parallel logic rather than a priority scheme for all case item expressions in a Verilog Design File Case Statement. 仅用于Verilog ,与case 语句一起使用强制生成一个并行的多路选择结构而不是一个优
先译码结构.

module parallel_case (sel, a, b, c);
   input [2:0] sel;
   output a, b, c;
   reg a, b, c;
   always @(sel)                  begin
      {a, b, c} = 3'b0;
      casez (sel)                // synthesis parallel_case
         3'b1??: a = 1'b1;
         3'b?1?: b = 1'b1;
         3'b??1: c = 1'b1;
      endcase
   end
endmodule

syn_encoding
A Verilog HDL synthesis attribute that determines how the Quartus II software should encode the states of an inferred state machine.
强制重新状态机的状态编码方式.有default,one-hot,sequential,gray,johnson,compact,user几种编码方式 (* syn_encoding = "user" *) reg [1:0] state;
parameter init = 0, last = 3, next = 1, later = 2; always @ (state) begin
case (state)
init:
out = 2'b01;
next:
out = 2'b10;
later:
out = 2'b11;
last:
out = 2'b00;
endcase
end In the above example, the states will be encoded as follows: init   = "00"
last   = "11"
next   = "01"
later   = "10"


希望可以帮到你

/* synthesis preserve = 1 */  避免Reg被优化
/* synthesis keep = 1 */ 避免wire被优化

常见用法

Noprune  A Verilog HDL synthesis attribute that prevents the Quartus II software from removing a register that does not directly or indirectly feed a top-level output or bidir pin. For example: reg reg1 /* synthesis noprune */;   keep  A Verilog HDL synthesis attribute that directs Analysis & Synthesis to not minimize or remove a particular net when optimizing combinational logic. For example: wire keep_wire /* synthesis keep */;   preserve  A Verilog HDL synthesis attribute that directs Analysis & Synthesis to not minimize or remove a particular register when eliminating redundant registers or registers with constant drivers. For example: reg reg1 /* synthesis preserve */;   ram_init_file A Verilog HDL synthesis attribute that specifies initial contents of an inferred memory. For example: reg [7:0] mem[0:255] /* synthesis ram_init_file = " my_init_file.mif" */;   ramstyle A Verilog HDL synthesis attribute that specifies the type of TriMatrix Memory block to use when implementing an inferred RAM. M512", "M4K", "M9K", "M144K", "MLAB", "M-RAM” For example: reg [0:7] my_ram[0:63] /* synthesis ramstyle = "M512" */;   translate_off or translate_on  Verilog HDL synthesis directives that direct Analysis & Synthesis to ignore portions of the design code that are specific to simulation and not relevant to logic synthesis. For example: parameter tpd = 2; // Generic delays // synthesis translate_off #tpd; // synthesis translate_on

感謝！
但是…有哪些是 Altera & Xilinx 共用的 語法？？

ISE中可以这样用 
/* synthesize syn_keep=1 */   is for wire 
/* synthesize syn_preserve=1 */   is for register

Xilinx的XST支持Synplicity的约束，所以更好的方法是如上的两种，前缀"syn_"应该表示是Synplicity的约束。

还有更多的综合约束，可以参考Xilinx XST User Guide中的“XST-Suppoerted” Third Party Constraints部分。
www.xilinx.com/support/documentation/sw_manuals/xilinx12_1/xst.pdf

对细节感兴趣的话，官方文档还是最好的资料。