I use SIM800c module and after sleep mode not worked to me I decided to use power on/off.
When module start work it make on and off by PWRKEY pin as described in manual.
But when I try to run it again without shutdown voltage it not work. No answer to AT commands at all !
PWRKEY is a toggle pin.
It turns the module ON if you pull it to GND if module is OFF.
It turns the module OFF if you pull it to GND if module is ON.
You should monitor status of GSM_STATUS pin to know if module is ON or OFF.
Sleep mode is very simple. wondering what problem you faced.
Problem was that modem going to POWER DOWN.
GSM_STATUS pin I can’t monitor because I use eval board that buy on Aliexpress…