Hello everyone,
I have succesfully used the SIMCOM 800L with Arduino UNO, but my project requirements state that I have to use as MCU the STM32F401RE programming it with ST library HAL. The problem that arises is that apparently I cannot receive any response from the GSM module to my AT commands. SIM800L is connected to the network, its LED blinks every 3 seconds, and is correctly suppllied with a 3.7V LiPo battery. I also made a voltage divider to reduce the input voltage to the RX pin of the GSM module as indicated in the product datasheet. Both polling and interrupt based receive function don't get any answer from the module. I also set the UART to send messages in 8bits with no parity and 1 stop bit, as I found checking what Arduino does by default with SoftSerial.
Has anyone met my same problems and found a solution?
Without seeing code, I doubt anybody will be able to help you.
Further, this is an Arduino forum 
The fact that you can program STMs using the Arduino IDE does not mean that people know something about them. I suggest that you try to find a dedicated forum if you don't get any replies.
The STM32 runs at 3.3V- you shouldn't need a voltage divider to interface it to the SIM800L.
Thanks for the replies,
I also have this doubt, but I was just following the GSM board datasheet that suggests to use the divider even with 3.3 USART interfaces because the module accepts as high input the voltage range 2.5-2.8.