I have a SIM900 shield that I want to control with a Raspberry Pi. I want to use GPIO pins on the pi and UART pins on the SIM900 shield. If I understand correctly the numerous tutorials available, people usually power the pi and the shield with separate power supply (USB for the pi, 2.5 jack for SIM900). Isn't it a way to power the shield from the pi with one of the power pins?
There is the documentation of the shield I use.