Add support for i2c slave address

Marlin/Configuration_adv.h

 // @section i2cbus
 
 //#define EXPERIMENTAL_I2CBUS
+#define I2C_SLAVE_ADDRESS  0 // Set non-zero to act as a slave
 
 #endif // CONFIGURATION_ADV_H

Marlin/Marlin_main.cpp

   void enableStepperDrivers() { pinMode(STEPPER_RESET_PIN, INPUT); }  // set to input, which allows it to be pulled high by pullups
 #endif
 
+#if ENABLED(EXPERIMENTAL_I2CBUS) && I2C_SLAVE_ADDRESS > 0
+
+  void i2c_listener(int bytes) {
+    i2c.receive(bytes);        // just echo all bytes received to serial
+  }
+
+#endif
+
 /**
  * Marlin entry-point: Set up before the program loop
  *  - Set up the kill pin, filament runout, power hold
       for (uint8_t i = 0; i < MIXING_STEPPERS; i++)
         mixing_virtual_tool_mix[t][i] = mixing_factor[i];
   #endif
+
+  #if ENABLED(EXPERIMENTAL_I2CBUS) && I2C_SLAVE_ADDRESS > 0
+    i2c.onReceive(i2c_listener);
+  #endif
 }
 
 /**

Marlin/example_configurations/Cartesio/Configuration_adv.h

 // @section i2cbus
 
 //#define EXPERIMENTAL_I2CBUS
+#define I2C_SLAVE_ADDRESS  0 // Set non-zero to act as a slave
 
 #endif // CONFIGURATION_ADV_H

Marlin/example_configurations/Felix/Configuration_adv.h

 // @section i2cbus
 
 //#define EXPERIMENTAL_I2CBUS
+#define I2C_SLAVE_ADDRESS  0 // Set non-zero to act as a slave
 
 #endif // CONFIGURATION_ADV_H

Marlin/example_configurations/Hephestos/Configuration_adv.h

 // @section i2cbus
 
 //#define EXPERIMENTAL_I2CBUS
+#define I2C_SLAVE_ADDRESS  0 // Set non-zero to act as a slave
 
 #endif // CONFIGURATION_ADV_H

Marlin/example_configurations/Hephestos_2/Configuration_adv.h

 // @section i2cbus
 
 //#define EXPERIMENTAL_I2CBUS
+#define I2C_SLAVE_ADDRESS  0 // Set non-zero to act as a slave
 
 #endif // CONFIGURATION_ADV_H

Marlin/example_configurations/K8200/Configuration_adv.h

 // @section i2cbus
 
 //#define EXPERIMENTAL_I2CBUS
+#define I2C_SLAVE_ADDRESS  0 // Set non-zero to act as a slave
 
 #endif // CONFIGURATION_ADV_H

Marlin/example_configurations/K8400/Configuration_adv.h

 // @section i2cbus
 
 //#define EXPERIMENTAL_I2CBUS
+#define I2C_SLAVE_ADDRESS  0 // Set non-zero to act as a slave
 
 #endif // CONFIGURATION_ADV_H

Marlin/example_configurations/RigidBot/Configuration_adv.h

 // @section i2cbus
 
 //#define EXPERIMENTAL_I2CBUS
+#define I2C_SLAVE_ADDRESS  0 // Set non-zero to act as a slave
 
 #endif // CONFIGURATION_ADV_H

Marlin/example_configurations/SCARA/Configuration_adv.h

 // @section i2cbus
 
 //#define EXPERIMENTAL_I2CBUS
+#define I2C_SLAVE_ADDRESS  0 // Set non-zero to act as a slave
 
 #endif // CONFIGURATION_ADV_H

Marlin/example_configurations/TAZ4/Configuration_adv.h

 // @section i2cbus
 
 //#define EXPERIMENTAL_I2CBUS
+#define I2C_SLAVE_ADDRESS  0 // Set non-zero to act as a slave
 
 #endif // CONFIGURATION_ADV_H

Marlin/example_configurations/WITBOX/Configuration_adv.h

 // @section i2cbus
 
 //#define EXPERIMENTAL_I2CBUS
+#define I2C_SLAVE_ADDRESS  0 // Set non-zero to act as a slave
 
 #endif // CONFIGURATION_ADV_H

Marlin/example_configurations/delta/biv2.5/Configuration_adv.h

 // @section i2cbus
 
 //#define EXPERIMENTAL_I2CBUS
+#define I2C_SLAVE_ADDRESS  0 // Set non-zero to act as a slave
 
 #endif // CONFIGURATION_ADV_H

Marlin/example_configurations/delta/generic/Configuration_adv.h

 // @section i2cbus
 
 //#define EXPERIMENTAL_I2CBUS
+#define I2C_SLAVE_ADDRESS  0 // Set non-zero to act as a slave
 
 #endif // CONFIGURATION_ADV_H

Marlin/example_configurations/delta/kossel_mini/Configuration_adv.h

 // @section i2cbus
 
 //#define EXPERIMENTAL_I2CBUS
+#define I2C_SLAVE_ADDRESS  0 // Set non-zero to act as a slave
 
 #endif // CONFIGURATION_ADV_H

Marlin/example_configurations/delta/kossel_pro/Configuration_adv.h

 // @section i2cbus
 
 //#define EXPERIMENTAL_I2CBUS
+#define I2C_SLAVE_ADDRESS  0 // Set non-zero to act as a slave
 
 #endif // CONFIGURATION_ADV_H

Marlin/example_configurations/delta/kossel_xl/Configuration_adv.h

 // @section i2cbus
 
 //#define EXPERIMENTAL_I2CBUS
+#define I2C_SLAVE_ADDRESS  0 // Set non-zero to act as a slave
 
 #endif // CONFIGURATION_ADV_H

Marlin/example_configurations/makibox/Configuration_adv.h

 // @section i2cbus
 
 //#define EXPERIMENTAL_I2CBUS
+#define I2C_SLAVE_ADDRESS  0 // Set non-zero to act as a slave
 
 #endif // CONFIGURATION_ADV_H

Marlin/example_configurations/tvrrug/Round2/Configuration_adv.h

 // @section i2cbus
 
 //#define EXPERIMENTAL_I2CBUS
+#define I2C_SLAVE_ADDRESS  0 // Set non-zero to act as a slave
 
 #endif // CONFIGURATION_ADV_H

Marlin/twibus.cpp

 #include <Wire.h>
 
 TWIBus::TWIBus() {
-  Wire.begin(); // We use no address so we will join the BUS as the master
+  #if I2C_SLAVE_ADDRESS == 0
+    Wire.begin();                  // No address joins the BUS as the master
+  #else
+    Wire.begin(I2C_SLAVE_ADDRESS); // Join the bus as a slave
+  #endif
   this->reset();
 }
 
 void TWIBus::reset() {
-  this->addr = 0;
   this->buffer_s = 0;
   this->buffer[0] = 0x00;
 }
 
-void TWIBus::address(uint8_t addr) {
+void TWIBus::address(const uint8_t addr) {
   this->addr = addr;
 
   #if ENABLED(DEBUG_TWIBUS)
-    debug(PSTR("sendto"), this->addr);
+    debug(PSTR("address"), this->addr);
   #endif
 }
 
-void TWIBus::addbyte(char c) {
+void TWIBus::addbyte(const char c) {
   if (buffer_s >= sizeof(this->buffer)) return;
   this->buffer[this->buffer_s++] = c;
 
   Wire.write(this->buffer, this->buffer_s);
   Wire.endTransmission();
 
-    // Reset the buffer after sending the data
+  // Reset the buffer after sending the data
   this->reset();
 }
 
-void TWIBus::reqbytes(uint8_t bytes) {
+void TWIBus::reqbytes(const uint8_t bytes) {
   if (!this->addr) return;
 
   #if ENABLED(DEBUG_TWIBUS)
     debug(PSTR("reqbytes"), bytes);
   #endif
 
+  // requestFrom() is a blocking function
   millis_t t = millis() + this->timeout;
   Wire.requestFrom(this->addr, bytes);
+  while (Wire.available() < bytes && PENDING(millis(), t)) { /*nada*/ }
 
-    // requestFrom() is a blocking function
-  while (Wire.available() < bytes) {
-    if (ELAPSED(millis(), t)) break;
-    else continue;
-  }
+  this->relaydata(bytes);
 
+  // Reset the buffer after sending the data
+  this->reset();
+}
+
+void TWIBus::relaydata(uint8_t bytes) {
   SERIAL_ECHO_START;
   SERIAL_ECHOPAIR("i2c-reply: from:", this->addr);
-  SERIAL_ECHOPAIR(" bytes:", Wire.available());
+  SERIAL_ECHOPAIR(" bytes:", bytes);
   SERIAL_ECHOPGM (" data:");
-
-    // Protect against buffer overflows if the number of received bytes
-    // is less than the number of requested bytes
-  uint8_t wba = Wire.available();
-  for (int i = 0; i < wba; i++) SERIAL_CHAR(Wire.read());
+  while (bytes-- && Wire.available()) SERIAL_CHAR(Wire.read());
   SERIAL_EOL;
-
-  // Reset the buffer after sending the data
-  this->reset();
 }
 
 #if ENABLED(DEBUG_TWIBUS)

Marlin/twibus.h

 
 #include "macros.h"
 
+#include <Wire.h>
+
 // Print debug messages with M111 S2 (Uses 236 bytes of PROGMEM)
 //#define DEBUG_TWIBUS
 
+typedef void (*twiSlaveFunc_t)(int bytes);
+
 /**
  * TWIBUS class
  *
   private:
     /**
      * @brief Timeout value in milliseconds
-     * @details For blocking operations this constant value will set the max
-     * amount of time Marlin will keep waiting for a reply. Useful is something
-     * goes wrong on the bus and the SDA/SCL lines are held up by another device.
+     * @details Maximum amount of time (ms) to wait for a reply.
+     *          Useful if something goes wrong on the bus and the
+     *          SDA/SCL lines are held up by another device.
      */
     const int timeout = 5;
 
     /**
      * @brief Target device address
-     * @description This stores, until the buffer is flushed, the target device
-     * address, take not we do follow Arduino 7bit addressing.
+     * @description The target device address. Persists until changed.
+     *              
      */
     uint8_t addr = 0;
 
     /**
      * @brief Number of bytes on buffer
-     * @description This var holds the total number of bytes on our buffer
-     * waiting to be flushed to the bus.
+     * @description Number of bytes in the buffer waiting to be flushed to the bus.
      */
     uint8_t buffer_s = 0;
 
     /**
      * @brief Internal buffer
-     * @details This is a fixed buffer, TWI command cannot be longer than this
+     * @details A fixed buffer. TWI commands can be no longer than this.
      */
     char buffer[30];
 
   public:
     /**
      * @brief Class constructor
-     * @details Initialized the TWI bus and clears the buffer
+     * @details Initialize the TWI bus and clear the buffer
      */
     TWIBus();
 
     /**
      * @brief Reset the buffer
-     * @details Brings the internal buffer to a known-empty state
+     * @details Set the buffer to a known-empty state
      */
     void reset();
 
     /**
      * @brief Send the buffer data to the bus
-     * @details Flushed the buffer into the bus targeting the cached slave device
-     * address.
+     * @details Flush the buffer to the bus at the target address.
      */
     void send();
 
     /**
      * @brief Add one byte to the buffer
-     * @details Adds the byte to the buffer in a sequential way, if buffer is full
-     * the request is silently ignored.
+     * @details Add a byte to the end of the buffer.
+     *          Silently fails if the buffer is full.
      *
      * @param c a data byte
      */
-    void addbyte(char c);
+    void addbyte(const char c);
 
     /**
-     * @brief Sets the target slave address
-     * @details The target slave address is stored so it can be later used when
-     * the complete packet needs to be sent over the bus.
+     * @brief Set the target slave address
+     * @details The target slave address for sending the full packet.
      *
      * @param addr 7-bit integer address
      */
-    void address(uint8_t addr);
+    void address(const uint8_t addr);
+
+    /**
+     * @brief Request data from the slave device
+     * @details Request a number of bytes from a slave device.
+     *          This implementation simply sends the data to serial
+     *          in a parser-friendly format.
+     *
+     * @param bytes the number of bytes to request
+     */
+    void reqbytes(const uint8_t bytes);
 
     /**
-     * @brief Request data from slave device
-     * @details Requests data from a slave device, when the data is received it will
-     * be relayed to the serial line using a parser-friendly formatting.
+     * @brief Relay data from the slave device to serial
+     * @details Relay a number of bytes from the bus to
+     *          serial in a parser-friendly format.
      *
      * @param bytes the number of bytes to request
      */
-    void reqbytes(uint8_t bytes);
+    void relaydata(uint8_t bytes);
+
+    #if I2C_SLAVE_ADDRESS > 0
+
+      /**
+       * @brief Receive bytes (passively)
+       * @details Receive bytes sent to our slave address.
+       *          and simply echo them to serial.
+       */
+      inline void receive(uint8_t bytes) { relaydata(bytes); }
+
+      /**
+       * @brief Register a slave handler
+       * @details Set a handler to receive data from the bus,
+       *          so we can act as a slave.
+       *
+       * @param handler A function to handle receiving bytes
+       */
+      inline void onReceive(const twiSlaveFunc_t handler) { Wire.onReceive(handler); }
+
+    #endif
 
     #if ENABLED(DEBUG_TWIBUS)