In LatticeSC/M IPexpress GUI for System Bus, why is the usr_clk input absent when MPI is enabled?
When the MPI interface is enabled, by default the "mpi_clk" clock sources the System Bus clock (HCLK) ( "Systembus clock" option is set to MPI by default in the Global Tab of the System_Bus IPexpress GUI).
That is why no separate usr_clk input is initially created.
You can still choose to create a user clock port and make it the source of HCLK by changing the "Systembus Clock" option from MPI to USER in the Global tab of the System_Bus IPexpress GUI. The "usr_clk" port may not show up in the IPexpress UI, but it will be in the generated files.