STM32 Power Mode.

from : https://www.st.com/content/ccc/resource/technical/document/application_note/27/58/8e/81/79/fb/4f/ac/DM00108286.pdf/files/DM00108286.pdf/jcr:content/translations/en.DM00108286.pdf

standby mode 等於是 reboot，因為 RAM 也沒電了。