关于时钟输入引脚为n时的调试

1、在xilinx fpga中，当输入时钟为单端时，手册上推荐时钟输入引脚为p，当输入时钟引脚为n时会对系统造成什么样的影响
2、新建工程
源码
module clk_test(
input wire clk_sys,
output wire clk_out1,
input wire clk_in1,
output wire clk_out2
);

wire clk_out1_bufg;

clk_wiz_0 clk_wiz_0_inst(
.clk_out1(clk_out1_bufg),
.clk_sys(clk_sys)
);

ODDR#(
.DDR_CLK_EDGE(“OPPOSITE_EDGE”), // “OPPOSITE_EDGE” or “SAME_EDGE”
.INIT(1’b0), // Initial value of Q: 1’b0 or 1’b1
.SRTYPE(“SYNC”) // Set/Reset type: “SYNC” or “ASYNC”
)
ODDR1_inst (
.Q(clk_out1), // 1-bit DDR output
.C(clk_out1_bufg), // 1-bit clock input
.CE(1’b1), // 1-bit clock enable input
.D1(1’b0), // 1-bit data input (positive edge)
.D2(1’b1), // 1-bit data input (negative edge)
.R(1’b0), // 1-bit reset
.S(1’b0) // 1-bit set
);

wire clk_in1_bufg;

BUFG BUFG_inst (
.O(clk_in1_bufg), // 1-bit output: Clock output
.I(clk_in1) // 1-bit input: Clock input
);

ODDR#(
.DDR_CLK_EDGE(“OPPOSITE_EDGE”), // “OPPOSITE_EDGE” or “SAME_EDGE”
.INIT(1’b0), // Initial value of Q: 1’b0 or 1’b1
.SRTYPE(“SYNC”) // Set/Reset type: “SYNC” or “ASYNC”
)
ODDR2_inst (
.Q(clk_out2), // 1-bit DDR output
.C(clk_in1_bufg), // 1-bit clock input
.CE(1’b1), // 1-bit clock enable input
.D1(1’b0), // 1-bit data input (positive edge)
.D2(1’b1), // 1-bit data input (negative edge)
.R(1’b0), // 1-bit reset
.S(1’b0) // 1-bit set
);
3、
4、管脚约束
set_property PACKAGE_PIN B17 [get_ports clk_out1]
set_property PACKAGE_PIN A17 [get_ports clk_in1]
set_property PACKAGE_PIN F22 [get_ports clk_sys]
set_property PACKAGE_PIN A18 [get_ports clk_out2]
set_property IOSTANDARD LVCMOS33 [get_ports clk_in1]
set_property IOSTANDARD LVCMOS33 [get_ports clk_out1]
set_property IOSTANDARD LVCMOS33 [get_ports clk_out2]
set_property IOSTANDARD LVCMOS33 [get_ports clk_sys]

set_property CLOCK_DEDICATED_ROUTE FALSE [get_nets clk_in1]
5、大致说明 全局时钟输入25Mhz，clk_out1输出50Mhz，板卡飞线clk_out1 ——>clk_in1 这样就完成了p输出n输入，完成后理论上clk_out2的输出也应该时50Mhz输出。
6、将程序下载到板卡，示波器测量clk_out2。

7、可知并未对功能造成实质的影响
8、目测问题就是时序不太好。
Resolution: Poor placement of an IO pin and a BUFG has resulted in the router using a non-dedicated path between the two. There are several things that could trigger this DRC, each of which can cause unpredictable clock insertion delays that result in poor timing. This DRC could be caused by any of the following: (a) a clock port was placed on a pin that is not a CCIO-pin (b)the BUFG has not been placed in the same half of the device or SLR as the CCIO-pin © a single ended clock has been placed on the N-Side of a differential pair CCIO-pin.
This is normally an ERROR but the CLOCK_DEDICATED_ROUTE constraint is set to FALSE allowing your design to continue. The use of this override is highly discouraged as it may lead to very poor timing results. It is recommended that this error condition be corrected in the design.
9、