I am building a battery-powered datalogger that will communicate using cellular signal, based on the ATSAMD21 microcontroller and the SIM5320 cellular modem. To save power, an outside timer periodically switches the microcontroller on/off (not shown) and the microcontroller should be able to switch power to the modem. When the microcontroller is off, the modem should also be off.
I implemented this as shown in the schematic below, using a microcontroller output to switch power to the modem through a P-MOSFET. This part works perfectly.
I also added a 100k pullup resistor to the gate, which I hoped would keep the P_MOSFET off when the microcontroller is off. This part does NOT work--when the microcontroller is off, the gate voltage drops to ~0.8V and power runs to the modem. I tried using different pullup resistors, but even at 100 ohm the gate voltage increases to ~3V and the P-MOSFET is still on.
It looks to me like 'something' is driving the gate voltage toward GND, but what is it? Where did I go wrong?
If the gate voltage is above ~2V, the microcontroller actually powers on. Is it actually drawing power through an analog input/output pin?
It may be simpler to use an N-MOSFET instead, but I am unsure as to how it would work with the SIM5320 IC being connected to a 'GND' that is a few mV above real ground due to the mosfet resistance. Could an N-MOSFET work here? What are better ways to design this circuit?