power management

Hi, I am working on BBB AM335x for power management stuff as i am very new to linux power management i need a bit help i am trying to achieve following using sysfs but i didnt got sucess i want to put my whole system into sleep state /idle state except WiFi and backlight, when i am writting* $echo mem > /sys/power/state* my whole system is going into sleep state similarly i have checked with */sys/class/backlight *but didnt worth. Can someone help me how i can do it ? How i can suspend and resume particular device only ?? Thanks