gsm-lte-modules

Goal: Interface cellular modems (SIM800L, SIM7000G, A7670) with microcontrollers to send text messages, perform HTTP(s) requests, or establish PPP TCP/IP bridges.

1. Power Requirements (Crucial)

Cellular modems emit bursts of RF transmission that pull up to 2A peaks.

• Connecting the VCC of a SIM800L directly to a 3.3V or 5V pin of an Arduino/Raspberry Pi will cause a brownout and infinite reboot loops.
• Rule: Use an external 3.7V - 4.2V LiPo battery or a beefy Buck Converter capable of 3A sustained, wired in parallel to the MCU.

2. UART and Logic Levels

• Modems usually operate at 2.8V logic.
• If using a 5V Arduino, you must use a logic level shifter or a resistor voltage divider on the RX pin of the modem string.

3. The AT Command Sequence

Modems are driven via serial AT commands. Terminate all commands with \r\n.