SIM800L GSM with Arduino Mega 2560 — Serial2, 4 V supply and AT tests

The SIM800L lets a project send SMS alerts and take calls — the classic “farm motor via missed call” or “tank full SMS” build. On the Mega it goes on its own hardware port, Serial2 (pins 17/16), keeping Serial1 free for a GPS.

The sketch checks the basics in order — module alive, SIM ready, signal, network registration — then turns the Serial Monitor into an AT terminal. Set the monitor to Both NL & CR and try AT+CMGF=1 and SMS commands from there.

Wiring

Also needed: LM2596 buck converter set to 4.0 V, fed from a 9–12 V 2 A adapter, 1000 µF / 10 V electrolytic capacitor, 10 kΩ + 20 kΩ resistors, Micro-SIM with 2G service.

Every connection
Arduino Mega 2560	SIM800L GSM/GPRS module (2G)	Wire	Notes
LM2596 buck set to 4.0 V (2 A) · +	VCC	red	1000 µF capacitor across VCC/GND at the module. Never from the Mega’s 5V or 3.3V pin.
LM2596 buck set to 4.0 V (2 A) · -	GND	black
GND	GND	black	Common ground: Mega GND, buck − and SIM800L GND.
D17	TXD	green	SIM800L TX → RX2.
D16	RXD	yellow	Via 10 kΩ / 20 kΩ divider — TX2 → SIM800L RX; the divider brings 5 V down to about 3.3 V.

3.4–4.4 V, peaks up to 2 A part, 5 V board. The SIM800L’s serial pins work at about 2.8 V. Its TXD is read fine by 3.3 V and 5 V boards; its RXD must be protected from a 5 V Arduino TX with a divider (10 kΩ / 20 kΩ). ESP32 and Pico can connect directly.

Board quirks

The SIM800L draws 2 A peaks when it registers or sends. Any weaker supply makes it reset in a loop (the LED blinks fast, then restarts). Set the buck to 4.0 V with a meter before connecting the module.
It is a 2G-only module. Use a SIM from an operator that still runs 2G in your area; Jio SIMs (4G-only) never register.
The Mega’s Serial2 (RX2 = 17, TX2 = 16) does the UART work in hardware, so AT replies and incoming SMS text arrive intact while the sketch is busy.
Network LED: blinking every 1 s = searching; every 3 s = registered; fast = GPRS data.
Fit the spring antenna (or the u.FL antenna) — without it signal (AT+CSQ) stays below 10.

Code

sim800l-mega.inoArduino C++Open in Playground

// SIM800L GSM module on an Arduino Mega 2560 using hardware Serial2.
// SIM800L TXD → D17 (RX2), RXD ← D16 (TX2) through a 10 kΩ / 20 kΩ divider, VCC from a 4.0 V
// 2 A buck converter (NOT the Mega), GND common. Checks the module, SIM and network, then
// becomes a bridge: type AT commands in the Serial Monitor (set to "Both NL & CR").
void sendAT(const __FlashStringHelper *cmd, unsigned long waitMs) {
  Serial.print(F(">> "));
  Serial.println(cmd);
  Serial2.println(cmd);
  unsigned long start = millis();
  while (millis() - start < waitMs) {
    while (Serial2.available()) Serial.write(Serial2.read());
  }
}

void setup() {
  Serial.begin(115200);
  Serial2.begin(9600);  // SIM800L auto-bauds; 9600 is the safe choice
  delay(3000);          // module boot time
  sendAT(F("AT"), 500);          // expect OK
  sendAT(F("ATE0"), 500);        // echo off
  sendAT(F("AT+CPIN?"), 500);    // READY = SIM found and unlocked
  sendAT(F("AT+CSQ"), 500);      // signal: 10-31 is usable, 99 = none
  sendAT(F("AT+CREG?"), 500);    // 0,1 = home network, 0,5 = roaming
  sendAT(F("AT+COPS?"), 1000);   // operator name
  Serial.println(F("Bridge mode - type AT commands"));
}

void loop() {
  while (Serial.available()) Serial2.write(Serial.read());
  while (Serial2.available()) Serial.write(Serial2.read());
}

// SIM800L GSM module on an Arduino Mega 2560 using hardware Serial2.
// SIM800L TXD → D17 (RX2), RXD ← D16 (TX2) through a 10 kΩ / 20 kΩ divider, VCC from a 4.0 V
// 2 A buck converter (NOT the Mega), GND common. Checks the module, SIM and network, then
// becomes a bridge: type AT commands in the Serial Monitor (set to "Both NL & CR").
void sendAT(const __FlashStringHelper *cmd, unsigned long waitMs) {
  Serial.print(F(">> "));
  Serial.println(cmd);
  Serial2.println(cmd);
  unsigned long start = millis();
  while (millis() - start < waitMs) {
    while (Serial2.available()) Serial.write(Serial2.read());
  }
}

void setup() {
  Serial.begin(115200);
  Serial2.begin(9600);  // SIM800L auto-bauds; 9600 is the safe choice
  delay(3000);          // module boot time
  sendAT(F("AT"), 500);          // expect OK
  sendAT(F("ATE0"), 500);        // echo off
  sendAT(F("AT+CPIN?"), 500);    // READY = SIM found and unlocked
  sendAT(F("AT+CSQ"), 500);      // signal: 10-31 is usable, 99 = none
  sendAT(F("AT+CREG?"), 500);    // 0,1 = home network, 0,5 = roaming
  sendAT(F("AT+COPS?"), 1000);   // operator name
  Serial.println(F("Bridge mode - type AT commands"));
}

void loop() {
  while (Serial.available()) Serial2.write(Serial.read());
  while (Serial2.available()) Serial.write(Serial2.read());
}

Library: TinyGSM (Volodymyr Shymanskyy).

Expected serial output

Serial Monitor · 115200Output

>> AT
OK
>> AT+CPIN?
+CPIN: READY
>> AT+CSQ
+CSQ: 18,0
>> AT+CREG?
+CREG: 0,1
Bridge mode - type AT commands

>> AT
OK
>> AT+CPIN?
+CPIN: READY
>> AT+CSQ
+CSQ: 18,0
>> AT+CREG?
+CREG: 0,1
Bridge mode - type AT commands

Common errors

Module keeps restarting

Supply too weak or the voltage drops during transmit bursts. Full fix.

+CREG: 0,2 forever

Searching but never registering: no 2G network for that SIM, a weak antenna, or the SIM is not activated.

Garbage or nothing in reply to AT

TXD/RXD swapped, or the divider is on the wrong wire. The divider belongs on the Mega TX → module RXD side.