PCF files contain physical constraints and are specified using the --pcf argument. Each (non blank) line contains a command; lines beginning with # are comments.
Two commands are supported: set_io and set_frequency.
set_io [-nowarn] [-pullup yes|no] [-pullup_resistor 3P3K|6P8K|10K|100K] port pin
Constrains named port port to package pin pin. -nowarn disables the warning if port does not exist. -pullup yes can be used to enable the built in pullup for all iCE40 devices. -pullup_resistor sets the pullup strength, and is available on iCE40 UltraPlus only.
set_frequency net frequency
Adds a clock constraint to a named net (any alias for the net can be used). frequency is in MHz.
Note that this is a non-standard extension, not supported by the vendor toolchain. It allows specifying clock constraints without needing the Python API.