FPGA设计时序约束十四、Set_External

一、序言

在时序约束中对clock的约束还存在一种特殊的延时约束set external delay。set external delay如字面含义，设置外部的时延值，但这个外部时延主要是指反馈时延，即信号从FPGA的output端口输出后经过外部电路回到输入端口的时延值。

二、Set External Delay

2.1 设置界面

进入Timing Constraints界面，在左侧Clock栏中选中Set External Delay，右侧即显示Set External Delay

设置界面中需设置反馈路径中的输出端口output port和输入端口input port，delay value为设置的时延值。勾选“Specify min/max values”后可设置max和min，即指进行setup或hold分析，“Add to existing external delay”勾选后对于约束的端口如果存在其他的外部时延约束，则会将其覆盖。

2.2 命令语法

命令格式

set_external_delay ‑from <args> ‑to <args> [‑min] [‑max] [‑add] [‑quiet] [‑verbose] <delay_value>

参数含义

三、工程示例

3.1 工程代码

设计中以PLL的反馈时钟输入输出为例设置set_external_delay

module set_external_delay(d1,rst,clk,clkfb,clkfb_out,out);
output out;
input d1,rst;
input clk,clkfb;
output clkfb_out;
reg ff1,ff2;
wire feedback,clk1;PLLE2_BASE #(.BANDWIDTH("OPTIMIZED"),  // OPTIMIZED, HIGH, LOW.CLKFBOUT_MULT(5),        // Multiply value for all CLKOUT, (2-64).CLKFBOUT_PHASE(0.0),     // Phase offset in degrees of CLKFB, (-360.000-360.000)..CLKIN1_PERIOD(0.0),      // Input clock period in ns to ps resolution (i.e. 33.333 is 30 MHz).// CLKOUT0_DIVIDE - CLKOUT5_DIVIDE: Divide amount for each CLKOUT (1-128).CLKOUT0_DIVIDE(1),.CLKOUT1_DIVIDE(1),.CLKOUT2_DIVIDE(1),.CLKOUT3_DIVIDE(1),.CLKOUT4_DIVIDE(1),.CLKOUT5_DIVIDE(1),// CLKOUT0_DUTY_CYCLE - CLKOUT5_DUTY_CYCLE: Duty cycle for each CLKOUT (0.001-0.999)..CLKOUT0_DUTY_CYCLE(0.5),.CLKOUT1_DUTY_CYCLE(0.5),.CLKOUT2_DUTY_CYCLE(0.5),.CLKOUT3_DUTY_CYCLE(0.5),.CLKOUT4_DUTY_CYCLE(0.5),.CLKOUT5_DUTY_CYCLE(0.5),// CLKOUT0_PHASE - CLKOUT5_PHASE: Phase offset for each CLKOUT (-360.000-360.000)..CLKOUT0_PHASE(0.0),.CLKOUT1_PHASE(0.0),.CLKOUT2_PHASE(0.0),.CLKOUT3_PHASE(0.0),.CLKOUT4_PHASE(0.0),.CLKOUT5_PHASE(0.0),.DIVCLK_DIVIDE(1),        // Master division value, (1-56).REF_JITTER1(0.0),        // Reference input jitter in UI, (0.000-0.999)..STARTUP_WAIT("FALSE")    // Delay DONE until PLL Locks, ("TRUE"/"FALSE"))PLLE2_BASE_inst (// Clock Outputs: 1-bit (each) output: User configurable clock outputs.CLKOUT0(),   // 1-bit output: CLKOUT0.CLKOUT1(clk1),   // 1-bit output: CLKOUT1.CLKOUT2(),   // 1-bit output: CLKOUT2.CLKOUT3(),   // 1-bit output: CLKOUT3.CLKOUT4(),   // 1-bit output: CLKOUT4.CLKOUT5(),   // 1-bit output: CLKOUT5// Feedback Clocks: 1-bit (each) output: Clock feedback ports.CLKFBOUT(clkfb_out), // 1-bit output: Feedback clock.LOCKED(LOCKED),     // 1-bit output: LOCK.CLKIN1(clk),     // 1-bit input: Input clock// Control Ports: 1-bit (each) input: PLL control ports.PWRDWN(PWRDWN),     // 1-bit input: Power-down.RST(rst),           // 1-bit input: Reset// Feedback Clocks: 1-bit (each) input: Clock feedback ports.CLKFBIN(clkfb)    // 1-bit input: Feedback clock);always@(posedge clk1)
begin
if(!rst)ff1<=1'b0;
else ff1<=d1;
endalways@(posedge clk1)
begin
if(!rst)ff2<=1'b0;
else ff2<=ff1; 
end 
assign out=ff2;endmodule

综合网表连接图如下图所示

输入输出引脚设置set_input_delay/set_output_delay,再是对PLL的反馈时钟输入端口和输出端口设置set_external_delay

create_clock -period 10.000 -name clk -waveform {0.000 5.000} [get_ports clk]
create_generated_clock -name clk1_gen -source [get_pins PLLE2_BASE_inst/CLKIN1] -divide_by 2 -add -master_clock clk [get_pins PLLE2_BASE_inst/CLKOUT1]
create_clock -period 6.000 -name clkfb -waveform {1.000 3.000} [get_ports clkfb]
set_input_delay -clock [get_clocks clkfb] 2.220 [get_ports d1]
set_output_delay -clock [get_clocks -of_objects [get_pins PLLE2_BASE_inst/CLKFBOUT]] 1.110 [get_ports clkfb_out]
set_external_delay -from [get_ports clkfb_out] -to [get_ports clkfb] -min 1.330