timing path的概念

从register clock/input port开始，经过一些combinational logic，终止在register data/output port，组合一下共有四种类型的path：in2reg,in2out,reg2reg,reg2out

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timing analysis的相关概念

工具都是以path group为单位来分析和报告timing的。DC综合时可以对每个path group指定weight来做优化。

Timing Path Groups and Types
•    Timing paths are grouped into path groups according to the clock associated with the endpoint  of the path.
•    There is a default path group that includes all asynchronous paths.
•    There are two timing path types : max and min.
    •Path type: max        - reports timing paths that check setup violations.
    •Path type: min        - reports timing paths that check hold violations.
•    Design Compiler works primarily on the most critical path in each path group.

首先，每个create_clock会创建一个对应的path group；

另外，工具有一些default的path group，如PT中的clock_gating_default**:（path以combinational element为终点）、async_default*:（path以async的preset和clear为终点）、*default*:（剩下的所有clock，如以output为终点的clock）

最后，也可以在SDC中自己定义path group:  group_path -name out1bus -to [get_ports OUT_1*]，这样就可以报告该path group的timing。

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report_timing命令属性

Number of paths: -nworst显示每个end point最worst的N条path，-max_paths显示每个group的worst path的个数

Type of paths: -path_type full -delay_type max

Amout of detail...

Startpoints,endpoint

-path_type full_clock/full_clock expanded 显示完整的data path和clock path

                 full显示完整data path,

                 short 只显示一条data path的startpoint和endpoint

-transition_time 报告路径的transition time

-capacitance 报告路径的capacitance

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exception timing的rpt:

1)-exceptions dominant 报告路径中的exception constrained

2)-exceptions overridden 报告路径中被覆盖的exception constrained

3)-exceptions all 报告所有的exception path，包括unconstrained startpoint 和unconstrained endpoint

图1

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exceptions path的三种属性:方便进行filter

dominant_exception timing path的类型：false path, min_delay, max_delay, multicycle_path

endpoint_unconstrained_reason：no_capture_clock, dangling_end_point(floating point no timing constraints), danin_of_disable(disable timing arc)

startpoint_unconstrained_reason：no_launch_clock，dangling_start_point,fanout_of_disable

filter_collection $path {defined(dominant_exception)}

filter_collection $path {undefined(dominant_exception)}

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get_timing_path更好的删选timing path

set x [get_timing_paths -from A -through B -to C]

report_timing $x -exceptions all

Path Sepecification Methods

-from $startpoint

-to $endpoint

-through

report_timing -from [get_clocks ...] -though $startpoint

report_timing -through $endpoint  -to [get_clocks ...]

report_timing -through {B1 B2}

更加精细的方式：

-rise_from/-rise_to

-fall_from/-fall_through...

图2

set_false_path -to [get_clocks CLK]   ：取以CLK为终点的timing path，data端的FF的clock是CLK

                                                         path：a,b,c,d有效

set_false_path -rise_from [get_clocks CLK] ：以CLK为起点，且data端以clock的rising edge来触发

                                                         path：a,d有效

report_timing -rise_from [get_clock clk] -nworst 100

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对于delay的report：

report_delay_calculation -from BLK1/A -to BLK1/z