diff --git a/include/avarix.h b/include/avarix.h
index 2b45d3b..b1fc199 100644
--- a/include/avarix.h
+++ b/include/avarix.h
@@ -12,7 +12,8 @@
 #define MAX(x,y) ((x) > (y) ? (x) : (y))
 /// Clamp a value to range [x;y]
 #define CLAMP(v,x,y) ((v) < (x) ? (x) : (v) > (y) ? (y) : (v))
-
+/// Get number of elements in an array
+#define LENGTHOF(a) ((sizeof(a)/sizeof((a)[0]))/(sizeof(a)%sizeof((a)[0]) == 0))
 
 #endif
 //@}
diff --git a/modules/i2c/i2c.c b/modules/i2c/i2c.c
index 1c47559..0c031ea 100644
--- a/modules/i2c/i2c.c
+++ b/modules/i2c/i2c.c
@@ -151,5 +151,4 @@ int8_t i2cm_recv(i2cm_t *m, uint8_t addr, uint8_t *data, uint8_t n)
   return i;
 }
 
-
 ///@endcond
diff --git a/modules/i2c/i2c.h b/modules/i2c/i2c.h
index 5b7ac68..b27c458 100644
--- a/modules/i2c/i2c.h
+++ b/modules/i2c/i2c.h
@@ -22,13 +22,69 @@ typedef struct i2cm_struct i2cm_t;
 /// I2C slave state
 typedef struct i2cs_struct i2cs_t;
 
-/// State of i2cx
-extern i2cT_t *const i2cx;
-
 #else
 
 typedef struct TWI_MASTER_struct i2cm_t;
-typedef struct i2cs_struct i2cs_t;
+
+typedef enum {
+  I2CS_STATE_NONE,
+  I2CS_STATE_READ,
+  I2CS_STATE_WRITE,
+
+} i2cs_state_t;
+
+#ifndef I2CS_RECV_BUFFER_SIZE
+# define I2CS_RECV_BUFFER_SIZE  32
+#endif
+
+#ifndef I2CS_SEND_BUFFER_SIZE
+# define I2CS_SEND_BUFFER_SIZE  32
+#endif
+
+/** @brief I2C slave master-write frame received
+ *
+ * @param buffer buffer containing the received bytes
+ * @param n number of bytes received from master
+ *
+ * This function is called when a master-write operation has completed
+ */
+typedef void (*i2cs_recv_callback_t)(uint8_t *buffer, uint8_t n);
+
+/** @brief I2C slave master-read operation was requested
+ *
+ * @param buffer buffer to provision
+ * @param maxsz maximum number of bytes which can be written to buffer
+ * @return number of bytes to send, returning 0 will result in a NACK from slave.
+ *
+ * This function is called when a master-read operation was requested by master
+ * and ask user to provision the buffer which will be sent.
+ */
+typedef uint8_t (*i2cs_prepare_send_callback_t)(uint8_t *buffer, uint8_t maxsz);
+
+/** @brief I2C slave transaction finished successfully or not
+ *
+ * This function is called when a STOP condition or any bus error has ended current transaction
+ */
+typedef void (*i2cs_reset_callback_t)(void);
+
+typedef struct {
+
+  i2cs_state_t state;
+
+  uint8_t recvd_bytes;
+  uint8_t recv_buffer[I2CS_RECV_BUFFER_SIZE];
+
+  uint8_t sent_bytes;
+  uint8_t bytes_to_send;
+  uint8_t send_buffer[I2CS_SEND_BUFFER_SIZE];
+
+  i2cs_recv_callback_t recv_callback;
+
+  i2cs_prepare_send_callback_t prepare_send_callback;
+
+  i2cs_reset_callback_t reset_callback;
+
+} i2cs_t;
 
 // Check for I2C enabled as both master and slave
 // Define pointers to internal structures
@@ -42,7 +98,8 @@ typedef struct i2cs_struct i2cs_t;
 #elif (defined I2CC_MASTER)
 # define i2cC  (&TWIC.MASTER)
 #elif (defined I2CC_SLAVE)
-extern i2cs_t *const i2cC;
+# define X_(p,s)  p ## C ## s
+# include "slavex.inc.h"
 #endif
 
 #if (defined I2CD_MASTER) && (defined I2CD_SLAVE)
@@ -50,7 +107,8 @@ extern i2cs_t *const i2cC;
 #elif (defined I2CD_MASTER)
 # define i2cD  (&TWID.MASTER)
 #elif (defined I2CD_SLAVE)
-extern i2cs_t *const i2cD;
+# define X_(p,s)  p ## D ## s
+# include "slavex.inc.h"
 #endif
 
 #if (defined I2CE_MASTER) && (defined I2CE_SLAVE)
@@ -58,7 +116,8 @@ extern i2cs_t *const i2cD;
 #elif (defined I2CE_MASTER)
 # define i2cE  (&TWIE.MASTER)
 #elif (defined I2CE_SLAVE)
-extern i2cs_t *const i2cE;
+# define X_(p,s)  p ## E ## s
+# include "slavex.inc.h"
 #endif
 
 #if (defined I2CF_MASTER) && (defined I2CF_SLAVE)
@@ -66,7 +125,8 @@ extern i2cs_t *const i2cE;
 #elif (defined I2CF_MASTER)
 # define i2cF  (&TWIF.MASTER)
 #elif (defined I2CF_SLAVE)
-extern i2cs_t *const i2cF;
+# define X_(p,s)  p ## F ## s
+# include "slavex.inc.h"
 #endif
 
 #endif
diff --git a/modules/i2c/slavex.inc.c b/modules/i2c/slavex.inc.c
index bcdd946..d0b457b 100644
--- a/modules/i2c/slavex.inc.c
+++ b/modules/i2c/slavex.inc.c
@@ -6,7 +6,193 @@
  * The X_(p,s) macro must be defined before including.
  * It is automatically undefined at the end of this file.
  */
+#include <avarix.h>
+#include <avr/interrupt.h>
+#include <stddef.h>
 
-#error Slave mode not supported yet
+#define I2CX(s) X_(I2C,s)
+#define i2cX(s) X_(i2c,s)
+#define TWIX X_(TWI,)
+#define twiX(s) X_(TWI,s)
 
+// declare i2cX singleton
+i2cs_t i2cX();
+
+void i2cX(_init)(void)
+{
+
+  i2cX().state = I2CS_STATE_NONE;
+
+  i2cX().reset_callback = NULL;
+  i2cX().prepare_send_callback = NULL;
+  i2cX().recv_callback = NULL;
+
+  // initialize hardware
+  TWIX.SLAVE.CTRLA = TWI_SLAVE_ENABLE_bm
+    | TWI_SLAVE_APIEN_bm
+    | TWI_SLAVE_DIEN_bm
+    | TWI_SLAVE_PIEN_bm
+    | ((I2CX(_INTLVL) << TWI_SLAVE_INTLVL_gp) & TWI_SLAVE_INTLVL_gm);
+  TWIX.SLAVE.ADDR = I2CX(_ADDRESS) << 1;
+}
+
+void i2cX(s_register_reset_callback)(i2cs_reset_callback_t f)
+{
+  INTLVL_DISABLE_BLOCK(I2CX(_INTLVL)) {
+    i2cX().reset_callback = f;
+  }
+}
+
+void i2cX(s_register_recv_callback)(i2cs_recv_callback_t f)
+{
+  INTLVL_DISABLE_BLOCK(I2CX(_INTLVL)) {
+    i2cX().recv_callback = f;
+  }
+}
+
+void i2cX(s_register_prepare_send_callback)(i2cs_prepare_send_callback_t f)
+{
+  INTLVL_DISABLE_BLOCK(I2CX(_INTLVL)) {
+    i2cX().prepare_send_callback = f;
+  }
+}
+
+// Interrupt handler
+ISR(twiX(_TWIS_vect))
+{
+
+  i2cs_t *i2cs = &i2cX();
+
+  uint8_t status = TWIX.SLAVE.STATUS;
+
+  if(status & TWI_SLAVE_BUSERR_bm || status & TWI_SLAVE_COLL_bm) {
+    // bus error happened
+    i2cs->state = I2CS_STATE_NONE;
+    if(i2cs->reset_callback) {
+      i2cs->reset_callback();
+    }
+    return;
+  }
+
+  else if(status & TWI_SLAVE_APIF_bm) {
+    // address / stop interrupt
+
+    // check previous state, call recvd callback
+    // if previous frame was a master-write
+    if(i2cs->state == I2CS_STATE_WRITE) {
+      if(i2cs->recv_callback) {
+        i2cs->recv_callback(i2cs->recv_buffer, i2cs->recvd_bytes);
+      }
+    }
+
+    if(status & TWI_SLAVE_AP_bm) {
+      // valid address interrupt
+      if(status & TWI_SLAVE_DIR_bm) {
+        // master read operation
+
+        i2cs->state = I2CS_STATE_READ;
+        if(i2cs->prepare_send_callback) {
+
+          i2cs->sent_bytes = 0;
+
+          // ask user to provision send buffer
+          uint8_t rsz = i2cs->prepare_send_callback(i2cs->send_buffer,
+                                          sizeof(i2cs->send_buffer));
+          if(rsz > 0) {
+            // user got some data to send
+            i2cs->bytes_to_send = MIN(rsz, sizeof(i2cs->send_buffer));
+            // ACK
+            TWIX.SLAVE.CTRLB = TWI_SLAVE_CMD_RESPONSE_gc;
+            return;
+          }
+          i2cs->bytes_to_send = 0;
+        }
+
+        // NACK, refuse read
+        TWIX.SLAVE.CTRLB = TWI_SLAVE_ACKACT_bm | TWI_SLAVE_CMD_RESPONSE_gc;
+        return;
+      }
+      else {
+        // master write operation
+        i2cs->state = I2CS_STATE_WRITE;
+
+        // clear recv buffer
+        i2cs->recvd_bytes = 0;
+        // confirm operation
+        TWIX.SLAVE.CTRLB = TWI_SLAVE_CMD_RESPONSE_gc;
+      }
+      return;
+    }
+    else {
+      // STOP condition interrupt
+      TWIX.SLAVE.CTRLB = TWI_SLAVE_CMD_COMPTRANS_gc;
+
+      i2cs->state = I2CS_STATE_NONE;
+      if(i2cs->reset_callback) {
+        i2cs->reset_callback();
+      }
+      return;
+    }
+  }
+
+  else if(status & TWI_SLAVE_DIF_bm) {
+    // data interruption
+    if(status & TWI_SLAVE_DIR_bm) {
+      // master read operation
+
+      if(i2cs->sent_bytes > 0 && (status & TWI_SLAVE_RXACK_bm)) {
+        // previous byte was NACKed by master
+        TWIX.SLAVE.CTRLB = TWI_SLAVE_CMD_COMPTRANS_gc;
+        i2cs->state = I2CS_STATE_NONE;
+        return;
+      }
+
+      if(i2cs->sent_bytes < i2cs->bytes_to_send)  {
+        // push byte
+        uint8_t byte = i2cs->send_buffer[i2cs->sent_bytes++];
+        TWIX.SLAVE.DATA = byte;
+
+        // ACK read, continue transmission
+        TWIX.SLAVE.CTRLB = TWI_SLAVE_CMD_RESPONSE_gc;
+      }
+      else {
+        // ACK read, end transmission
+        TWIX.SLAVE.CTRLB = TWI_SLAVE_CMD_COMPTRANS_gc;
+      }
+
+      return;
+    }
+    else {
+      // master write operation
+      uint8_t byte = TWIX.SLAVE.DATA;
+
+      if(i2cs->recvd_bytes < I2CS_RECV_BUFFER_SIZE) {
+        i2cs->recv_buffer[i2cs->recvd_bytes++] = byte;
+
+        // ACK write
+        TWIX.SLAVE.CTRLB = TWI_SLAVE_CMD_RESPONSE_gc;
+      }
+      else {
+        // NACK write, buffer is full
+        TWIX.SLAVE.CTRLB = TWI_SLAVE_ACKACT_bm | TWI_SLAVE_CMD_COMPTRANS_gc;
+      }
+
+      return;
+    }
+  }
+
+  // we encounter an unmanaged case
+  i2cs->state = I2CS_STATE_NONE;
+  i2cs->recvd_bytes = 0;
+  i2cs->sent_bytes = 0;
+  // reset client-side state
+  if(i2cs->reset_callback) {
+    i2cs->reset_callback();
+  }
+}
+
+#undef I2CX
+#undef i2cX
+#undef TWIX
+#undef twiX
 #undef X_
diff --git a/modules/i2c/slavex.inc.h b/modules/i2c/slavex.inc.h
new file mode 100644
index 0000000..9189b4a
--- /dev/null
+++ b/modules/i2c/slavex.inc.h
@@ -0,0 +1,25 @@
+/** @defgroup i2c I2C
+ * @brief I2C module
+ */
+//@{
+/**
+ * @file
+ * @brief I2C slave definitions
+ */
+#define i2cX(s) X_(i2c,s)
+
+// I2cX singleton
+extern i2cs_t i2cX();
+
+/** @brief Register f to be called whenever a master-read operation was requested and user need
+ * to provision the send buffer */
+void i2cX(s_register_prepare_send_callback)(i2cs_prepare_send_callback_t f);
+
+/** @brief Register f to be called whenever a master-write operation as finished */
+void i2cX(s_register_recv_callback)(i2cs_recv_callback_t f);
+
+/** @brief Register f to be called whenever the i2c transaction was terminated (on STOP or ERROR) */
+void i2cX(s_register_reset_callback)(i2cs_reset_callback_t f);
+
+#undef i2cX
+#undef X_