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Tcl DISCUSSION
I am writing timing constraints and pin assignments for an FPGA design in Tcl. A line such as set_input_delay 2.0 -clock clk [get_ports data[3]] fails with invalid command name "3". Putting the name in double quotes gives the same error, and when I build names in a loop with "data[$i]" I get invalid command name "0".
How should bus bits be written in constraint files and scripts, and how do I loop over all bits of a bus?
The error comes from Tcl itself, before the tool sees anything. Square brackets mean command substitution, so in get_ports data[3] the interpreter tries to run a command called 3 and insert its result. Double quotes do not help, because command substitution is still active inside them.
Braces switch substitution off, so the standard spelling in constraint files is [get_ports {data[3]}]: the outer brackets call get_ports, the braces deliver the literal text data[3] to it. Escaping each bracket with a backslash, data\[3\], is equivalent.
For a computed index you need the variable substituted but the brackets left alone, which braces cannot do since they block $i as well. Two clean options: escape only the brackets inside quotes, get_ports "data\[$i\]", or build the name with format: get_ports [format {data[%d]} $i]. With i equal to 5 both produce data[5].
A loop over an 8-bit bus is then for {set i 0} {$i < 8} {incr i} { ... } with one of those forms in the body.
Often no loop is needed, because the object queries accept wildcards: [get_ports {data[*]}] selects every bit of the bus, and one constraint can be applied to the whole set. Keep the braces here too; without them Tcl would try to run a command named *.
There is a second layer to be aware of: the tool's own pattern matcher. In ordinary Tcl glob matching, brackets denote a character set, and tools differ in whether they treat brackets in object names literally or as pattern syntax. If a braced name still matches nothing, check the pattern-matching section of your tool's Tcl or SDC reference; some flows need the brackets escaped inside the braces as well.
Check what a query returned before relying on it. In Vivado the result can be used like a Tcl list, so llength [get_ports {data[*]}] should print 8 for an 8-bit bus, and foreach p [get_ports {data[*]}] { puts [get_property NAME $p] } lists the names. In Synopsys-style shells and in the Quartus timing analyzer the result is a collection handle, not a list: count it with sizeof_collection or get_collection_size respectively, and iterate with foreach_in_collection.
A query that silently matches nothing is the most common reason a constraint has no effect, so look for "no objects matched" warnings in the log as well.
Two habits prevent most of these problems. First, brace every object name and pattern in constraint files as a matter of routine, including ones without brackets, such as [get_ports {clk}]; it costs nothing and the file survives later renaming to a bus. Second, remember that hierarchical names of generated logic can contain brackets, dots and backslash-escaped identifiers, so copy names from the tool's own report or netlist view instead of guessing them, and test a new query on the tool's Tcl console before putting it in the constraint file.
