by Starting from this post, we will talk about commonly asked interview questions related to timing constraints or SDC. The most basic question is, what are design / library objects and how to manipulate these objects.
What Are Design / Library Objects?
These objects include:
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- Design: defined by top level module
- Port: input, output and inout ports in the top level design
- Clock
- Cell: submodule instances in the design
- Pin: submodule instance ports
- Net: wire or registers to connect pins
- Library name
- Library cell
We can use the following commands to retrieve these objects:
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# get design get_design # get ports get_ports all_inputs all_outputs # get clocks get_clocks all_clocks # get cell get_cells all_registers # get pins get_pins U1/A # get library name get_libs 16nm # get library cells get_lib_cells 16nm/AND* |
How To Manipulate These Objects?
The following example shows how to access collections of objects:
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# get a list of all inputs set input_list [all_inputs] # check number of inputs in the design sizeof_collection $input_list # print out all inputs query_objects $input_list |
The following example shows how to add to and remove from the collections:
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# add all outputs to $input_list set input_list [add_to_collection $input_list [all_outputs]] # remove all outputs from $input_list set input_list [remove_from_collection $input_list [all_outputs]] |
The following example shows how to filter collections:
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# get only AND* gates in the collections # “ref_name” is an attribute of the object, used as filtering criteria filter_collection [get_cells *] “ref_name =~ AND*” # another way to get filtered collections set and_gate_list [get_cells * -filter “ref_name=~ AND*”] |
Note, other than “=~”, there are other relational operators: “==”, “!=”, “>”, “<”, “>=”, “<=”, and “!~”.
The following example shows how to iterate through collections:
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# use “foreach_in_collection” to iterate over a collection foreach_in_collection and_gate [get_cells * -filter “ref_name=~ AND*”] { echo “Instance [get_object_name $and_gate] is an AND gate.” } |
Conclusions
In companies like Google and Facebook, interviewers prefer candidates with strong coding and scripting background. Getting familiar with these basic commands helps interviewees to succeed in coding questions.
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