RC-S2LP Wireless M-Bus transceiver overview, hardware connection, and test procedure

RC-S2LP Wireless M-Bus Transceiver

Introduction

The RC-S2LP is a sub-GHz RF transceiver module based on the STMicroelectronics S2-LP radio IC. It is commonly used in Wireless M-Bus (wM-Bus), smart metering, and low-power IoT gateway applications operating in the 868 MHz ISM band.

In the S0 board, the RC-S2LP is responsible for receiving and transmitting Wireless M-Bus frames from devices such as heat meters, water meters, and gas meters.

What the RC-S2LP Module Is Used For

The RC-S2LP enables:

Wireless M-Bus communication (T1, T2, C1, C2 modes)
Sub-GHz long-range communication
Low-power RF reception from battery-powered meters
Gateway-to-meter RF communication
Custom proprietary RF protocols

Typical applications include:

Smart utility meters
IoT gateways
Industrial telemetry
Building automation systems

Key Specifications

Feature	Description
RF IC	STMicroelectronics S2-LP
Frequency Band	868 MHz ISM band
Modulation	2-FSK, GFSK, OOK, MSK
Data Rate	Up to 500 kbps
Output Power	Up to +14 dBm
Sensitivity	Down to −130 dBm (mode-dependent)
Interface	SPI
Operating Voltage	1.8 V – 3.6 V
Low-Power Modes	Sleep, Standby

Hardware Connection

The RC-S2LP communicates with the host MCU using SPI and several control signals.

Required Signals

Signal	Description
MOSI	SPI Master Out
MISO	SPI Master In
SCK	SPI Clock
CS	SPI Chip Select
VCC	3.3 V supply
GND	Ground
RF	868 MHz antenna

Example Connection (ESP32)

RC-S2LP Pin	MCU Pin
MOSI	SPI MOSI
MISO	SPI MISO
SCK	SPI SCK
CS	GPIO
VCC	3.3V
GND	GND

Ensure a proper 868 MHz antenna is connected before transmitting.

Software Test Procedure

Test Objective

The test verifies that:

The RC-S2LP is powered correctly
SPI communication is functional
Registers can be read and written
RF transmission and reception work

SPI Communication Test

#include <SPI.h>

// SPI Pins for ESP32C6
#define PIN_SPI_MOSI 23
#define PIN_SPI_MISO 19
#define PIN_SPI_SCK  18
#define PIN_SPI_CS   5

// Optional Interrupt Pin
#define PIN_GDO0     4

// S2LP Command Strobes / Registers
#define CMD_SRES     0xFE // RESET
#define CMD_SNOP     0x00 // NO OP

// Define buffer size for TX / RX
#define BUFFER_SIZE 64

SPIClass *vspi = NULL;

void rcS2lpWriteReg(uint8_t reg, uint8_t value) {
  digitalWrite(PIN_SPI_CS, LOW);
  vspi->transfer(reg | 0x00);  // WRITE
  vspi->transfer(value);
  digitalWrite(PIN_SPI_CS, HIGH);
}

uint8_t rcS2lpReadReg(uint8_t reg) {
  digitalWrite(PIN_SPI_CS, LOW);
  vspi->transfer(reg | 0x80);  // READ
  uint8_t value = vspi->transfer(0);
  digitalWrite(PIN_SPI_CS, HIGH);
  return value;
}

void rcS2lpStrobe(uint8_t cmd) {
  digitalWrite(PIN_SPI_CS, LOW);
  vspi->transfer(cmd);
  digitalWrite(PIN_SPI_CS, HIGH);
}

void setup() {
  Serial.begin(115200);
  delay(1000);

  Serial.println("RC-S2LP Wireless M-Bus SPI Test");

  pinMode(PIN_SPI_CS, OUTPUT);
  digitalWrite(PIN_SPI_CS, HIGH);

  pinMode(PIN_GDO0, INPUT);

  // Initialize VSPI
  vspi = new SPIClass(VSPI);
  vspi->begin(PIN_SPI_SCK, PIN_SPI_MISO, PIN_SPI_MOSI);

  // Hardware reset RC-S2LP if needed (wire RESET pin)
  // pinMode(PIN_RESET, OUTPUT);
  // digitalWrite(PIN_RESET, LOW);
  // delay(100);
  // digitalWrite(PIN_RESET, HIGH);
  // delay(100);

  // Reset chip
  Serial.println("Sending chip reset...");
  rcS2lpStrobe(CMD_SRES);
  delay(100);

  // Read version / part number (example register)
  uint8_t partnum = rcS2lpReadReg(0x01); // PARTNUM register addr
  Serial.print("Part Number: 0x"); Serial.println(partnum, HEX);

  // Set some basic registers (Example — see datasheet)
  Serial.println("Configuring basic registers...");
  rcS2lpWriteReg(0x02, 0x01); // Example: Write some register

  Serial.println("Ready!");
}

void loop() {
  static uint32_t lastSend = 0;

  // Example: Transmit a test packet every 5 seconds
  if (millis() - lastSend > 5000) {
    lastSend = millis();

    Serial.println("Sending test packet...");

    // Build test packet
    uint8_t packet[] = "HELLO FROM ESP32-C6";
    uint8_t packetSize = sizeof(packet);

    // Send strobe for TX FIFO loading (depends on S2LP FIFO interface)
    digitalWrite(PIN_SPI_CS, LOW);
    vspi->transfer(0x66); // WRITE FIFO command (example)
    for (uint8_t i = 0; i < packetSize; i++) {
      vspi->transfer(packet[i]);
    }
    digitalWrite(PIN_SPI_CS, HIGH);

    // Strobe TX (example)
    rcS2lpStrobe(0x35); // STX

    Serial.println("Packet sent!");
  }

  // Example: Basic receive check
  if (digitalRead(PIN_GDO0) == HIGH) {
    Serial.println("Packet received!");
    uint8_t rxBuf[BUFFER_SIZE];

    // Read RX FIFO (example)
    digitalWrite(PIN_SPI_CS, LOW);
    vspi->transfer(0x7F); // READ FIFO command
    for (uint8_t i = 0; i < BUFFER_SIZE; i++) {
      rxBuf[i] = vspi->transfer(0);
    }
    digitalWrite(PIN_SPI_CS, HIGH);

    Serial.print("Data: ");
    for (uint8_t i = 0; i < BUFFER_SIZE; i++) {
      Serial.write(rxBuf[i]);
    }
    Serial.println();
  }
}

Wireless M-Bus Testing