SEM_DRIVER

-- part of a SEM_DRIVER module of a DATA PATH module

-- priority routing
-- when in PASSIVE mode, turn on all YELLOW lights
if pi_active_mode = '0' then
  po_semaphore_ctrl_east  <= "010";
  po_semaphore_ctrl_west  <= "010";
  po_semaphore_ctrl_south <= "010";
  po_semaphore_ctrl_north <= "010";
else
  case sem_phase_sel is
    -- active direction (EW_GO or SN_GO state active)
    -- sees RED lights at this phase
    when "01" =>
      -- active_dir_s is an external signalization, an input of FSM
      -- this signal tells which crossroad direction executes its sequence next.

      -- '0' --> next_state <= SN_GO,
      -- south/north semaphores start their sequence: RED, YELLOW, GREEN, YELLOW

      -- '1' --> next_state <= EW_GO
      -- east/west semaphores start their sequence: RED, YELLOW, GREEN, YELLOW

      -- driver: cycle_end
      case active_dir_s is
        when '0' =>                     -- south/north direction sequence
          -- SN_GO.sem_phase: RED,
          -- south/north are RED
          po_semaphore_ctrl_south <= "100";
          po_semaphore_ctrl_north <= "100";
          -- east/west are GREEN
          po_semaphore_ctrl_east  <= "001";
          po_semaphore_ctrl_west  <= "001";
        when '1' =>                     -- east/west direction sequence
          -- south/north are GREEN
          po_semaphore_ctrl_south <= "001";
          po_semaphore_ctrl_north <= "001";
          -- east/west are RED
          po_semaphore_ctrl_east  <= "100";
          po_semaphore_ctrl_west  <= "100";
      end case;
    -- active direction sees GREEN light now
    when "11" =>
      case active_dir_s is
        when '0' =>                     -- south/north direction sequence
          -- SN_GO.sem_phase: GREEN,
          -- south/north are GREEN
          po_semaphore_ctrl_south <= "001";
          po_semaphore_ctrl_north <= "001";
          -- east/west are RED
          po_semaphore_ctrl_east  <= "100";
          po_semaphore_ctrl_west  <= "100";
        when '1' =>                     -- east/west direction sequence
          -- south/north are RED
          po_semaphore_ctrl_south <= "100";
          po_semaphore_ctrl_north <= "100";
          -- east/west are GREEN
          po_semaphore_ctrl_east  <= "001";
          po_semaphore_ctrl_west  <= "001";
      end case;
      -- NOTE: its not possible to turn on RED and YELLOW at the same time
      -- because colors are one-hot coded.

    -- "00" and "10" will result in all ligths yellow!
    -- "10" --> prepare state of semaphore, this is YELLOW by function
    -- "00" --> unexpected value, we need to cover all cases. Never gonna happen!
    when others =>
      po_semaphore_ctrl_east  <= "010";
      po_semaphore_ctrl_west  <= "010";
      po_semaphore_ctrl_south <= "010";
      po_semaphore_ctrl_north <= "010";
  end case;
end if;