diff --git a/Makefile b/Makefile
index bd679c6..0e67b4a 100644
--- a/Makefile
+++ b/Makefile
@@ -1,6 +1,6 @@
 PRG            = rgbyteclock
-OBJ            = rgbyteclock.o timer.o lcd.o main.o rtc.o
-MCU_TARGET     = atmega16
+OBJ            = rgbyteclock.o timer.o lcd.o main.o rtc.o spi.o ringbuffer.o crc.o
+MCU_TARGET     = atmega164a
 OPTIMIZE       = -Os
 
 DEFS           =
@@ -30,6 +30,10 @@ main.o: rgbyteclock.o
 timer.o:timer.h
 lcd.o: lcd.h
 rtc.o: rtc.h
+spi.o: spi.h
+ringbuffer.o: ringbuffer.h
+crc.o: crc.h
+
 
 clean:
 	rm -rf *.o $(PRG).elf *.eps *.png *.pdf *.bak 
diff --git a/crc.c b/crc.c
new file mode 100644
index 0000000..b0833ef
--- /dev/null
+++ b/crc.c
@@ -0,0 +1,38 @@
+
+#include "crc.h"
+
+
+const uint16_t crc16_table[256] PROGMEM = {
+0x0000, 0x1189, 0x2312, 0x329b, 0x4624, 0x57ad, 0x6536, 0x74bf,
+0x8c48, 0x9dc1, 0xaf5a, 0xbed3, 0xca6c, 0xdbe5, 0xe97e, 0xf8f7,
+0x1081, 0x0108, 0x3393, 0x221a, 0x56a5, 0x472c, 0x75b7, 0x643e,
+0x9cc9, 0x8d40, 0xbfdb, 0xae52, 0xdaed, 0xcb64, 0xf9ff, 0xe876,
+0x2102, 0x308b, 0x0210, 0x1399, 0x6726, 0x76af, 0x4434, 0x55bd,
+0xad4a, 0xbcc3, 0x8e58, 0x9fd1, 0xeb6e, 0xfae7, 0xc87c, 0xd9f5,
+0x3183, 0x200a, 0x1291, 0x0318, 0x77a7, 0x662e, 0x54b5, 0x453c,
+0xbdcb, 0xac42, 0x9ed9, 0x8f50, 0xfbef, 0xea66, 0xd8fd, 0xc974,
+0x4204, 0x538d, 0x6116, 0x709f, 0x0420, 0x15a9, 0x2732, 0x36bb,
+0xce4c, 0xdfc5, 0xed5e, 0xfcd7, 0x8868, 0x99e1, 0xab7a, 0xbaf3,
+0x5285, 0x430c, 0x7197, 0x601e, 0x14a1, 0x0528, 0x37b3, 0x263a,
+0xdecd, 0xcf44, 0xfddf, 0xec56, 0x98e9, 0x8960, 0xbbfb, 0xaa72,
+0x6306, 0x728f, 0x4014, 0x519d, 0x2522, 0x34ab, 0x0630, 0x17b9,
+0xef4e, 0xfec7, 0xcc5c, 0xddd5, 0xa96a, 0xb8e3, 0x8a78, 0x9bf1,
+0x7387, 0x620e, 0x5095, 0x411c, 0x35a3, 0x242a, 0x16b1, 0x0738,
+0xffcf, 0xee46, 0xdcdd, 0xcd54, 0xb9eb, 0xa862, 0x9af9, 0x8b70,
+0x8408, 0x9581, 0xa71a, 0xb693, 0xc22c, 0xd3a5, 0xe13e, 0xf0b7,
+0x0840, 0x19c9, 0x2b52, 0x3adb, 0x4e64, 0x5fed, 0x6d76, 0x7cff,
+0x9489, 0x8500, 0xb79b, 0xa612, 0xd2ad, 0xc324, 0xf1bf, 0xe036,
+0x18c1, 0x0948, 0x3bd3, 0x2a5a, 0x5ee5, 0x4f6c, 0x7df7, 0x6c7e,
+0xa50a, 0xb483, 0x8618, 0x9791, 0xe32e, 0xf2a7, 0xc03c, 0xd1b5,
+0x2942, 0x38cb, 0x0a50, 0x1bd9, 0x6f66, 0x7eef, 0x4c74, 0x5dfd,
+0xb58b, 0xa402, 0x9699, 0x8710, 0xf3af, 0xe226, 0xd0bd, 0xc134,
+0x39c3, 0x284a, 0x1ad1, 0x0b58, 0x7fe7, 0x6e6e, 0x5cf5, 0x4d7c,
+0xc60c, 0xd785, 0xe51e, 0xf497, 0x8028, 0x91a1, 0xa33a, 0xb2b3,
+0x4a44, 0x5bcd, 0x6956, 0x78df, 0x0c60, 0x1de9, 0x2f72, 0x3efb,
+0xd68d, 0xc704, 0xf59f, 0xe416, 0x90a9, 0x8120, 0xb3bb, 0xa232,
+0x5ac5, 0x4b4c, 0x79d7, 0x685e, 0x1ce1, 0x0d68, 0x3ff3, 0x2e7a,
+0xe70e, 0xf687, 0xc41c, 0xd595, 0xa12a, 0xb0a3, 0x8238, 0x93b1,
+0x6b46, 0x7acf, 0x4854, 0x59dd, 0x2d62, 0x3ceb, 0x0e70, 0x1ff9,
+0xf78f, 0xe606, 0xd49d, 0xc514, 0xb1ab, 0xa022, 0x92b9, 0x8330,
+0x7bc7, 0x6a4e, 0x58d5, 0x495c, 0x3de3, 0x2c6a, 0x1ef1, 0x0f78};
+
diff --git a/crc.h b/crc.h
new file mode 100644
index 0000000..7e74288
--- /dev/null
+++ b/crc.h
@@ -0,0 +1,27 @@
+
+#ifndef CRC_H_
+#define CRC_H_
+
+#include <stdint.h>
+#include <avr/pgmspace.h>
+#include <avr/io.h>
+
+
+#define CRC_INIT_VALUE (0x0000)
+#define CRC_LEN (2)
+
+extern const uint16_t crc16_table[] PROGMEM;
+
+
+static inline uint16_t crc_step(uint16_t old_crc, uint8_t new_byte)
+{
+  uint16_t crc;
+  crc = (old_crc >> 8) ^ crc16_table[new_byte ^ (old_crc & 0xff)];
+  return crc;
+}
+
+
+
+
+
+#endif // CRC_H_
diff --git a/main.c b/main.c
index 4d06f49..226e855 100644
--- a/main.c
+++ b/main.c
@@ -13,12 +13,13 @@
 #include "timer.h"
 #include "lcd.h"
 #include "rgbyteclock.h"
-
+#include "spi.h"
 
 int main(void)
 {
   
   timer_init();
+  spi_slave_init();
   lcd_init();
 
   rgbyteclock();
diff --git a/ringbuffer.c b/ringbuffer.c
new file mode 100644
index 0000000..d84b07f
--- /dev/null
+++ b/ringbuffer.c
@@ -0,0 +1,98 @@
+/*
+ * ----------------------------------------------------------------------------
+ * "THE BEER-WARE LICENSE" (Revision 42):
+ * <struppi@struppi.name> wrote this file. As long as you retain this notice you
+ * can do whatever you want with this stuff. If we meet some day, and you think
+ * this stuff is worth it, you can buy me a beer in return.
+ * (c) 2008-2014 Stefan Rupp
+ * ----------------------------------------------------------------------------
+ */
+ 
+
+
+
+#include <stdlib.h>
+#include <inttypes.h>
+
+#include <util/atomic.h>
+
+#include "ringbuffer.h"
+
+
+void ringbuf_init(void *ringbuffer, uint8_t rb_size, void (*idle_func)(void))
+{
+  ringbuffer_t *rb = (ringbuffer_t *) ringbuffer;
+
+  rb->size = rb_size;
+  rb->free = rb_size;
+  rb->write_idx = 0;
+  rb->read_idx = 0;
+  rb->idle_func = idle_func;
+  return;
+}
+
+
+int8_t ringbuf_get_nb(void *ringbuffer, uint8_t *ch)
+{
+  ringbuffer_t *rb = (ringbuffer_t *) ringbuffer;
+  if ( rb->size - rb->free == 0) {
+    *ch = 0xff;
+    return 0;
+  }
+  ATOMIC_BLOCK(ATOMIC_RESTORESTATE) {
+    rb->free++;
+    *ch = rb->buffer[rb->read_idx++];
+    if ( rb->read_idx >= rb->size ) {
+      rb->read_idx = 0;
+    }
+  }
+  return 1;
+}
+
+
+int8_t ringbuf_get(void *ringbuffer, uint8_t *ch)
+{
+  ringbuffer_t *rb = (ringbuffer_t *) ringbuffer;
+  while ( rb->size - rb->free == 0) {
+    if (rb->idle_func != NULL) {
+      rb->idle_func();
+    }
+  }
+  rb->free++;
+  *ch = rb->buffer[rb->read_idx++];
+  if ( rb->read_idx >= rb->size ) {
+    rb->read_idx = 0;
+  }
+  return 1;
+}
+
+
+int8_t ringbuf_put_nb(void * ringbuffer, uint8_t ch)
+{
+  ringbuffer_t *rb = (ringbuffer_t *) ringbuffer;
+
+  if (rb->free > 0) {
+    ATOMIC_BLOCK(ATOMIC_RESTORESTATE) {
+      rb->free--;
+      rb->buffer[rb->write_idx++] = ch;
+      if (rb->write_idx >= rb->size) {
+        rb->write_idx = 0;
+      }
+    }
+    return 1;
+  }
+  return 0;
+}
+
+
+
+int8_t ringbuf_get_num_waiting( void * ringbuffer)
+{
+  ringbuffer_t *rb = (ringbuffer_t *) ringbuffer;
+  uint8_t waiting;
+  
+  waiting = rb->size - rb->free;
+  
+  return waiting;
+}
+
diff --git a/ringbuffer.h b/ringbuffer.h
new file mode 100644
index 0000000..3cfed13
--- /dev/null
+++ b/ringbuffer.h
@@ -0,0 +1,26 @@
+#if !defined __RINGBUFFER_H
+#define __RINGBUFFER_H
+
+#include <inttypes.h>
+
+typedef struct {
+  uint8_t size;
+  volatile uint8_t free;
+  volatile uint8_t write_idx;
+  volatile uint8_t read_idx;
+  
+  void (*idle_func)(void);
+  
+  uint8_t buffer[];
+} ringbuffer_t;
+
+
+#define RB_BUFSIZE(queue_len) (sizeof(ringbuffer_t)+(queue_len))
+
+void ringbuf_init(void *ringbuffer, uint8_t buf_size, void (*IdleFunc)(void));
+int8_t ringbuf_get(void *ringbuffer, uint8_t *ch);
+int8_t ringbuf_get_nb(void *ringbuffer, uint8_t *ch);
+int8_t ringbuf_put_nb(void *ringbuffer, uint8_t ch);
+int8_t ringbuf_get_num_waiting(void * ringbuffer);
+
+#endif
diff --git a/rtc.c b/rtc.c
index 835b3fd..1147508 100644
--- a/rtc.c
+++ b/rtc.c
@@ -7,6 +7,8 @@
  * (c) 2014 Stefan Rupp
  * ----------------------------------------------------------------------------
  */
+
+#if 0
  
 #include <avr/io.h>
 #include "rtc.h"
@@ -129,4 +131,5 @@ void timer_validate(uint8_t *hours, uint8_t *minutes, uint8_t *seconds, uint16_t
 */
 
 #endif
+#endif
 
diff --git a/spi.c b/spi.c
new file mode 100644
index 0000000..8ed979a
--- /dev/null
+++ b/spi.c
@@ -0,0 +1,215 @@
+/*
+ * ----------------------------------------------------------------------------
+ * "THE BEER-WARE LICENSE" (Revision 42):
+ * <struppi@struppi.name> wrote this file. As long as you retain this notice you
+ * can do whatever you want with this stuff. If we meet some day, and you think
+ * this stuff is worth it, you can buy me a beer in return.
+ * (c) 2013,2014 Stefan Rupp
+ * ----------------------------------------------------------------------------
+ */
+
+
+#include "spi.h"
+#include "ringbuffer.h"
+#include "crc.h"
+#include <stdlib.h>
+#include <string.h>
+#include <avr/interrupt.h>
+
+
+#define COMM_OBJECT_SIZE (16)
+#define NUM_COMM_OBJECTS (4)
+#define RB_SIZE (NUM_COMM_OBJECTS)
+#define SPI_COMM_PADDING (0x00)
+#define LEN_SIZE (1) // LEN is 1 byte long
+
+static uint8_t rbuff_used_rx[RB_BUFSIZE(RB_SIZE)];
+static uint8_t rbuff_used_tx[RB_BUFSIZE(RB_SIZE)];
+static uint8_t rbuff_unused[RB_BUFSIZE(RB_SIZE)];
+
+static uint8_t comm_objects[NUM_COMM_OBJECTS][COMM_OBJECT_SIZE];
+
+
+struct spi_comm_t {
+  uint8_t txout_buf[COMM_OBJECT_SIZE];
+  uint8_t rxin_buf[COMM_OBJECT_SIZE];
+  uint16_t crc;
+  uint8_t count;
+};
+static struct spi_comm_t spi_comm;
+
+
+static inline void set_ack_miso(uint8_t ack)
+{
+  if (ack) PORTC |= (1<<PC7);
+  else PORTC &= ~(1<<PC7);
+}
+
+
+
+static void set_crc(void* const buf)
+{
+  uint8_t* const data = buf;
+  uint8_t len = data[0];
+  uint16_t crc = CRC_INIT_VALUE;
+  uint16_t *crc_ptr;
+
+  for (int i=1; i<=len; ++i) {
+    crc = crc_step(crc, data[i]);
+  }
+
+  crc_ptr = buf + len + 1;
+  *crc_ptr = crc;
+  len += 2;
+  data[0] = len;
+}
+
+
+static inline uint8_t get_ack_mosi(void)
+
+{
+
+#if 0
+  return (PINB & (1<<PB5))?1:0;
+#else 
+  return 1;
+#endif
+}
+
+
+ISR(SPI_STC_vect)
+{
+  SPDR = spi_comm.txout_buf[spi_comm.count+1];
+  spi_comm.rxin_buf[spi_comm.count] = SPDR;
+  spi_comm.crc = crc_step(spi_comm.crc, spi_comm.rxin_buf[spi_comm.count]);
+  spi_comm.count++;
+
+
+}
+
+
+// This is the interrupt handler for the CS line.
+// in case of a falling edge, we prepare for the
+// SPI transfer that is about to come in in 100uS.
+// in case of an rising edge, we do some cleanup.
+ISR(PCINT1_vect)
+{
+  uint8_t tx_obj, rx_obj;
+
+  if (PINC & (1<<PC1)) {
+    // rising edge
+    if ( spi_comm.rxin_buf[0] != SPI_COMM_PADDING ) {
+      uint8_t len = spi_comm.rxin_buf[0];
+
+      uint8_t data_valid = (spi_comm.crc == 0x0000);
+      data_valid = 1; // FIXME: CRC not implemented on python side, so ignore for now
+      set_ack_miso(data_valid);
+
+      if (data_valid && ringbuf_get_nb(rbuff_unused, &rx_obj)) {
+	spi_comm.rxin_buf[0] = len-2; // strip CRC from data
+	if (len > COMM_OBJECT_SIZE) {len = COMM_OBJECT_SIZE;}
+        memcpy(comm_objects[rx_obj], spi_comm.rxin_buf, len);
+        ringbuf_put_nb(rbuff_used_rx, rx_obj);
+      }
+    }
+  }
+  else {
+    // falling edge
+    spi_comm.crc = CRC_INIT_VALUE;
+    spi_comm.count = 0;
+    uint8_t ack_mosi = get_ack_mosi();
+    if (ack_mosi) {
+      if ( ringbuf_get_nb(rbuff_used_tx, &tx_obj) ) {
+	uint8_t len = comm_objects[tx_obj][0];
+	if (len > COMM_OBJECT_SIZE) {len = COMM_OBJECT_SIZE;}
+	memcpy(spi_comm.txout_buf, comm_objects[tx_obj], len);
+	SPDR = comm_objects[tx_obj][0];
+	ringbuf_put_nb(rbuff_unused, tx_obj); // there ought always be enough space in this ringbuffer
+      }
+      else {
+	memset(spi_comm.txout_buf, SPI_COMM_PADDING, COMM_OBJECT_SIZE-1);
+	SPDR = SPI_COMM_PADDING;
+      }
+    }
+    else { // no ACK from master, so retransmit same frame once more. no need to copy anything, as txbuf wasn't touched.
+      SPDR = spi_comm.txout_buf[0];
+    }
+  }
+
+}
+
+
+int8_t spi_slave_init()
+{
+
+  ringbuf_init(rbuff_used_rx, RB_SIZE, NULL);
+  ringbuf_init(rbuff_used_tx, RB_SIZE, NULL);
+  ringbuf_init(rbuff_unused, RB_SIZE, NULL);
+
+  for (uint8_t i=0; i<NUM_COMM_OBJECTS; i++) {
+    ringbuf_put_nb(rbuff_unused, i);
+  }
+  
+  PCMSK1 |= (1<<PCINT9);
+  PCICR  |= (1<<PCIE1);
+  
+  /* Set MISO output, all others input */
+  DDRD |= (1<<PD2);
+  DDRD &= ~((1<<PD3)|(1<<PD4));
+  /* Enable SPI */
+  
+  SPCR = (1<<SPE) | (1<<SPIE);	// enable SPI
+  
+  DDRB &= ~(1<<PB5);  // ACK_MOSI
+  PORTB &= ~(1<<PB5);
+  
+  DDRC |= (1<<PC7); // ACK_MISO
+  set_ack_miso(0);
+  
+  return 1;
+}
+
+
+int8_t spi_prepare_to_send(void *data)
+{
+  uint8_t tx_object;
+  uint8_t len = ((uint8_t *)data)[0];
+
+  if ((len + LEN_SIZE + CRC_LEN) > COMM_OBJECT_SIZE) {
+    return -1;
+  }
+  if ( ringbuf_get_nb(rbuff_unused, &tx_object) ) {
+    set_crc(data);
+    memcpy(comm_objects[tx_object], data, len);
+    if ( ringbuf_put_nb(rbuff_used_tx, tx_object) ) {
+      return tx_object;
+    }
+    else {
+      return -tx_object;
+      //PORTB ^= (1<<PB3);
+    }
+  }
+  else {
+    return (-127);
+  }
+}
+
+
+int8_t spi_receive(void *data)
+{
+  uint8_t rx_object;
+  
+  if ( ringbuf_get_nb(rbuff_used_rx, &rx_object) ) {
+    memcpy(data, comm_objects[rx_object], COMM_OBJECT_SIZE);
+    if ( ringbuf_put_nb(rbuff_unused, rx_object) ) {
+      return rx_object;
+    }
+    else {
+      return (-rx_object);
+      //PORTB ^= (1<<PB3);
+    }
+  }
+  else {
+    return -127;
+  }
+}
diff --git a/spi.h b/spi.h
new file mode 100644
index 0000000..88d17fa
--- /dev/null
+++ b/spi.h
@@ -0,0 +1,15 @@
+
+#ifndef SPI_H_
+#define SPI_H_
+
+#include <stdint.h>
+#include <avr/io.h>
+
+
+int8_t spi_slave_init();
+int8_t spi_prepare_to_send(void *data);
+int8_t spi_receive(void *data);
+
+
+#endif // SPI_H_
+
diff --git a/timer.c b/timer.c
index ca9cec2..bda915c 100644
--- a/timer.c
+++ b/timer.c
@@ -19,7 +19,7 @@
  */
 static volatile uint32_t timer_time;
 
-ISR(TIMER0_COMP_vect) {
+ISR(TIMER0_COMPA_vect) {
   ++timer_time;
   return;
 }
@@ -46,11 +46,12 @@ void timer_init(void)
   // - Set Timer/Counter0 prescaler to 64 ==> (1<<CS01)|(1<<CS00)
   // - Set OCR2 to 249
   // - CTC ( i.e. clear counter, when COUNTER == OCR0A) ==> (1<<WGM01)
-  OCR0 = 249;
-  TCCR0 = (1<<CS01)|(1<<CS00)|(1<<WGM01);
-  
+  OCR0A = 249;
+  TCCR0B = (1<<WGM02);
+  TCCR0A = (1<<CS01)|(1<<CS00);  
+
   // Interrupts setzen
-  TIMSK |= (1<<OCIE0);
+  TIMSK0 |= (1<<OCIE0A);
 
   // Allow interrupts
   sei();
diff --git a/tos.h b/tos.h
new file mode 100644
index 0000000..c880bbb
--- /dev/null
+++ b/tos.h
@@ -0,0 +1,15 @@
+
+#ifndef TOS_H_
+#define TOS_H_
+
+#include <stdint.h>
+#include <avr/pgmspace.h>
+#include <avr/io.h>
+
+
+#define TOS_CONFIG     (0x11)
+#define TOS_CANFRAME   (0x21)
+#define TOS_PRINT      (0xef)
+
+
+#endif // TOS_H_