vhdl-speedbar-display-mode

vhdl-speedbar-display-mode is a variable defined in `vhdl-mode.el'.
Its value is

files

Documentation:
Specifies the default displaying mode when opening speedbar.
Alternatively, the displaying mode can be selected from the speedbar menu or
by typing `f' (files), `h' (directory hierarchy) or `H' (project hierarchy).

You can customize this variable.