TCL (Tool Command Language) scripting plays a critical role in VLSI Physical Design (PD). Almost every major EDA tool—Cadence Innovus, Synopsys ICC2, PrimeTime, Design Compiler, and Tempus—relies heavily on TCL for automation, reporting, optimization, and design analysis.
This blog covers the Top 25 TCL Scripting Interview Questions and Answers for Physical Design, helping freshers and experienced engineers confidently face PD interviews.
Why TCL Is Important in Physical Design
- Automates repetitive PD tasks
- Controls EDA tools efficiently
- Extracts reports (timing, power, area, congestion)
- Enables custom design flows
- Improves productivity and accuracy
1. What is TCL scripting?
TCL (Tool Command Language) is a high-level, interpreted scripting language widely used in EDA tools. In Physical Design, TCL is used to automate tasks such as floorplanning, placement, routing, timing analysis, and report generation.
2. Why is TCL widely used in Physical Design tools?
TCL is lightweight, easy to learn, and tightly integrated with EDA tools. It allows engineers to control tool behavior, customize flows, and extract design data efficiently without manual intervention.
3. What are variables in TCL? How do you declare them?
Variables store values in TCL and are dynamically typed.
Syntax:
set var_name value
Example:
set clk_period 2.5
4. How do you print output in TCL?
The puts command is used to display output.
Example:
puts "Physical Design Flow Started"
5. What is a list in TCL?
A list is a collection of elements separated by spaces.
Example:
set cell_list {U1 U2 U3}
Lists are commonly used to store cells, nets, or pins in PD scripts.
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6. How do you access list elements in TCL?
Use the lindex command.
Example:
lindex $cell_list 0This returns the first element of the list.
7. What is the difference between set and expr?
| Command | Purpose |
|---|---|
| set | Assigns values |
| expr | Performs arithmetic operations |
Example:
set a 10
set b [expr $a + 5]
8. What are loops in TCL?
Loops execute commands repeatedly. Common loop types are for, foreach, and while.
Example (foreach):
foreach cell $cell_list {
puts $cell
}
9. How is TCL used in floorplanning?
TCL scripts define:
- Core area
- Aspect ratio
- IO pin placement
- Macro placement
Example:
create_floorplan -core_utilization 0.7
10. What is a procedure in TCL?
A procedure (proc) is a reusable block of code.
Syntax:
proc proc_name {args} {
commands
}
11. How do you write a procedure to print timing violations?
proc print_violations {} {
set vio [report_timing -max_paths 10]
puts $vio
}
12. What is foreach_in_collection?
It is used in Synopsys tools to iterate over design objects.
Example:
foreach_in_collection cell [get_cells] { puts [get_object_name $cell]
}
13. Difference between foreach and foreach_in_collection?
| Feature | foreach | foreach_in_collection |
|---|---|---|
| Data type | TCL lists | Tool collections |
| Speed | Slower | Faster |
| Usage | General TCL | Synopsys tools |
14. How do you capture tool command output in TCL?
Use square brackets [].
Example:
set area [report_area]
15. How do you write a TCL script to find high-fanout nets?
foreach net [get_nets] {
if {[get_attribute $net fanout] > 50} {
puts "High fanout net: [get_object_name $net]"
}
}
16. How is TCL used in placement optimization?
TCL scripts:
- Analyze congestion
- Move macros
- Fix overlaps
- Control placement strategies
Example:
place_opt
17. How do you read and write files in TCL?
Write to file:
set fp [open report.txt w]
puts $fp "Timing Report"
close $fp
18. What is the source command?
source executes another TCL script.
Example:
source setup.tcl
19. How do you pass arguments to a TCL script?
Using $argv and $argc.
Example:
set design_name [lindex $argv 0]
20. How do you check if a variable exists?
Answer:
if {[info exists var_name]} {
puts "Variable exists"
}
21. What is catch in TCL?
catch handles errors gracefully.
Example:
catch {read_verilog design.v} result
22. How is TCL used in CTS (Clock Tree Synthesis)?
TCL controls:
- Clock constraints
- Skew targets
- Buffer insertion
Example:
create_clock -period 2 -name CLK [get_ports clk]
23. How do you generate timing reports using TCL?
report_timing -delay_type max -max_paths 5
24. How do you optimize power using TCL scripts?
TCL automates:
- Power analysis
- Clock gating insertion
- Cell resizing
Example:
report_power
25. What are best practices for TCL scripting in Physical Design?
- Use modular scripts
- Add comments
- Handle errors using
catch - Avoid hardcoded values
- Use procedures for reusability
Conclusion
TCL scripting is an essential skill for Physical Design engineers, enabling automation, customization, and efficiency across the PD flow. Mastering TCL not only helps you crack interviews but also makes you highly productive in real projects.
If you are preparing for VLSI Physical Design roles, practicing these TCL questions and writing scripts regularly in Innovus or ICC2 will give you a strong edge.
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