STA Tool Command - report_timing -max_paths_count (OpenSTA -group_count) : Part 1

As, I have shared last time that in this series of articles, we are going to discuss about the different command of STA Tools along with the concepts. We have used OpenSTA which is an open source tool and can be downloaded from OpenSTA. It’s commands are similar to Primetime (Industry standard tool). By using OpenSTA you can understand the working of tool properly. Here, I have made some of the comparison between the commands of Primetime & OpenSTA (which I am going to use in another few articles). The command which I have selected is report_timing (in Primetime) & report_checks (in OpenSTA). The report_checks / report_timing command reports paths in the design

Primetime	OpenSTA
report_timing [-group group_name][-max_paths count][-nworst paths_per_endpoint]	report_checks [-path_group group_names][-group_count path_count][-endpoint_count endpoint_path_count]
group group_name:Specifies the path groups from which timing paths are selected for reporting based on other specified options for reports.	path_group: List of path group names to report. All path groups are reported if this option is not specified
max_paths count:Specifies the number of paths to be reported per path group	group_count: The number of paths to report in each path group
nworst paths_per_endpoint: Specifies the number of paths to be reported per endpoint per path group	endpoint_count: The number of paths to report for each endpoint

In the previous Article, we have discussed in great detail about the report_timing -group (report_checks -path_group). How to create clock group and how tool by default reports only 1 report per path group. But now you can ask a question – what if you need all the possible reports with respect to 1 path group. We are going to discuss the same in this Article.

Below is an example to understand the reporting of the command (Same example which we have used last time).

In this circuit, depending upon the way we create path_group, we can get timing reports. But in general, there are several Timing path in any circuit. As a STA engineer, we need to analyse all of them.
Below commands and reporting structure can remind you by default options.

Case(1):

create_clock -name clk -period 10 {clk1 clk2 clk3}
set_input_delay -clock clk 0 {in1 in2}

report_checks -format summary -path_group clk

Startpoint           Endpoint        Slack
--------------------------------------------------------------------------------
r2/Q (DFF_X1)        r3/D (DFF_X1)       9.43

report_checks -format short -path_group clk

Startpoint: r2 (rising edge-triggered flip-flop clocked by clk)
Endpoint: r3 (rising edge-triggered flip-flop clocked by clk)
Path Group: clk
Path Type: max

Same reporting, I can achieve by adding one more switch with value 1 (i.e. -group_count 1)

Case(1):

create_clock -name clk -period 10 {clk1 clk2 clk3}
set_input_delay -clock clk 0 {in1 in2}

report_checks -format summary -path_group clk -group_count 1

Startpoint           Endpoint        Slack
--------------------------------------------------------------------------------
r2/Q (DFF_X1)        r3/D (DFF_X1)       9.43

report_checks -format short -path_group clk -group_count 1

Startpoint: r2 (rising edge-triggered flip-flop clocked by clk)
Endpoint: r3 (rising edge-triggered flip-flop clocked by clk)
Path Group: clk
Path Type: max

If you have noticed the difference, we have used one more argument / switch group_count.

group_count The number of paths to report in each path group

But it’s not that simple. Let’s understand this command more in detail using different test cases and different scenerios.
In above example, only 1 path is reported with respect to clock group clk? Why ONE, because I have given group_count == 1. What if I give group_count = 2? let’s check

Case(1):

create_clock -name clk -period 10 {clk1 clk2 clk3}
set_input_delay -clock clk 0 {in1 in2}

report_checks -format summary -path_group clk -group_count 2

Startpoint              Endpoint           Slack
--------------------------------------------------------------------------------
r2/Q (DFF_X1)           r3/D (DFF_X1)           9.43
in1 (input)                r1/D (DFF_X1)           9.85

we obtain 2 worst reports from path group clk

Similarly, we can check for group_count = 3

report_checks -format summary -path_group clk -group_count 3

Startpoint              Endpoint           Slack
--------------------------------------------------------------------------------
r2/Q (DFF_X1)           r3/D (DFF_X1)           9.43
in1 (input)                r1/D (DFF_X1)           9.85
in2 (input)                r2/D (DFF_X1)           9.85

What if, I give group_count = 4? I am sure, you will say 4 paths should be reported. Let’s check

report_checks -format summary -path_group clk -group_count 4

Startpoint              Endpoint           Slack
--------------------------------------------------------------------------------
r2/Q (DFF_X1)           r3/D (DFF_X1)           9.43
in1 (input)                r1/D (DFF_X1)           9.85
in2 (input)                 r2/D (DFF_X1)           9.85

Here we see only 3 paths.... Why??

Let’s check for group_count = 5

report_checks -format summary -path_group clk -group_count 5

Startpoint              Endpoint           Slack
--------------------------------------------------------------------------------

r2/Q (DFF_X1)           r3/D (DFF_X1)           9.43
in1 (input)                 r1/D (DFF_X1)           9.85
in2 (input)                 r2/D (DFF_X1)          9.85

Here also only three paths. Why is this thing happening.

Let’s check for group_count = 6

report_checks -format summary -path_group clk -group_count 6

Startpoint              Endpoint           Slack
--------------------------------------------------------------------------------
r2/Q (DFF_X1)           r3/D (DFF_X1)           9.43
in1 (input)                 r1/D (DFF_X1)           9.85
in2 (input)                 r2/D (DFF_X1)           9.85

Here also the same,

I have summarized the result in the table

group_count	No. Of Reports
1	1
2	2
3	3
4	3
5	3
6	3
7	3
8	3
9	3
10	3

Now the question is although we have increased group_count but number of paths reported is three only? As per the definition, it should also increase. I am leaving his question to you right now and let’s see if you can think/guess something about this. Else we obviously discuss it later.

Let’s try to understand it by creating 2 different clock group and play with value of group_count.


Consider case(2) to understand group_count clearly

Command:
create_clock -name clk -period 10 {clk1 clk3}
create_clock -name clkc -period 10 {clk2}
set_input_delay -clock clk 0 {in1}
set_input_delay -clock clkc 0 {in2}

report_checks -format summary -path_group {clk clkc} -group_count 1

Startpoint              Endpoint           Slack
--------------------------------------------------------------------------------
r2/Q (DFF_X1)           r3/D (DFF_X1)           9.43
in2 (input)                r2/D (DFF_X1)           9.85

report_checks -format short -path_group {clk clkc} -group_count 1

Startpoint: r2 (rising edge-triggered flip-flop clocked by clkc)
Endpoint: r3 (rising edge-triggered flip-flop clocked by clk)
Path Group: clk
Path Type: max

Startpoint: in2 (input port clocked by clk)
Endpoint: r2 (rising edge-triggered flip-flop clocked by clkc)
Path Group: clkc
Path Type: max

Since group count is one therefore one path from each path group is reported i.e. one path from clock group clk and other from clock group clkc.

Let’s make group_count 2 and observe the changes in report

report_checks -format summary -path_group {clk clkc} -group_count 2

Startpoint              Endpoint           Slack
--------------------------------------------------------------------------------
r2/Q (DFF_X1)           r3/D (DFF_X1)           9.43
in1 (input)                 r1/D (DFF_X1)           9.85
in2 (input)                 r2/D (DFF_X1)           9.85

report_checks -format short -path_group {clk clkc} -group_count 2

Startpoint: r2 (rising edge-triggered flip-flop clocked by clkc)
Endpoint: r3 (rising edge-triggered flip-flop clocked by clk)
Path Group: clk
Path Type: max

Startpoint: in1 (input port clocked by clk)
Endpoint: r1 (rising edge-triggered flip-flop clocked by clk)
Path Group: clk
Path Type: max

Startpoint: in2 (input port clocked by clk)
Endpoint: r2 (rising edge-triggered flip-flop clocked by clkc)
Path Group: clkc
Path Type: max

In this case we observe thee paths, two from the clock group clk and one from the clock group clkc
You must be thinking why only one path is reported from clock group clkc. Just think over it

Now, I’m increasing the group_count

report_checks -format summary -path_group {clk clkc} -group_count 3

Startpoint              Endpoint           Slack
--------------------------------------------------------------------------------
r2/Q (DFF_X1)           r3/D (DFF_X1)           9.43
in1 (input)                 r1/D (DFF_X1)           9.85
in2 (input)                 r2/D (DFF_X1)           9.85

report_checks -format short -path_group {clk clkc} -group_count 3

Startpoint: r2 (rising edge-triggered flip-flop clocked by clkc)
Endpoint: r3 (rising edge-triggered flip-flop clocked by clk)
Path Group: clk
Path Type: max

Startpoint: in1 (input port clocked by clk)
Endpoint: r1 (rising edge-triggered flip-flop clocked by clk)
Path Group: clk
Path Type: max

Startpoint: in2 (input port clocked by clk)
Endpoint: r2 (rising edge-triggered flip-flop clocked by clkc)
Path Group: clkc
Path Type: max

We should get here 3 paths from clock group clk and three path from clock group clkc but overall we are getting only three paths
why?? Still not able to get / guess answer? 😊

Again, I am trying summarize my analysis

I have summarized the result in the table

Clock Group	group_count	Total No. Of Reports
2	1	2
2	2	3
2	3	3
2	4	3
2	5	3
2	...	3

Let’s discuss another circuit and see if you can figure out but that we will discuss in part 2 of this article. :)

-By Surabhi Singh
(Btech-EC:- 2019 Passout)
(ABES Engineering College)

-Supervised By Puneet Mittal
(Founder & Director)
(VLSI Expert Private Limited)