diff --git a/main.S b/main.S
index 2e951b9..2a9196a 100644
--- a/main.S
+++ b/main.S
@@ -9,12 +9,12 @@
 ;; ----------------------------------------------------
 ;; --- r4, r5 and r6 must not be used for any other ---
 ;; --- purpose                                      ---
-;; required for communication between drawscreen and isr
+;; required for communication between drawscreen and shifter isr
 #define SIGNAL_REGISTER r4
+#define SIGNAL_INIT_VALUE SIGNAL_OCTET_DONE
 #define SIGNAL_OCTET_DONE 0x01
 #define SIGNAL_ISR_ENABLE 0x02
 #define SIGNAL_ALL_DATA_DONE 0x04
-#define SIGNAL_INIT_VALUE SIGNAL_OCTET_DONE
 
 ;; required for handover of data between drawscreen and isr
 #define DATA_REGISTER r5
@@ -50,6 +50,12 @@
   .macro clear_debug
   bic   #BIT2, &P1OUT
   .endm
+  .macro set_signal_waiting_for_data
+  bis   #BIT3, &P1OUT
+  .endm
+  .macro clear_signal_waiting_for_data
+  bic   #BIT3, &P1OUT
+  .endm
 
   .section ".data"
 screendata:
@@ -87,12 +93,17 @@ init:
   ;; BIT0: data bit
   ;; BIT1: output enable
   ;; BIT2: debug
-  mov.b   #BIT0|BIT1|BIT2,&P1DIR
+  ;; BIT3: Signal waiting for data
+  ;; BIT6: UCB0SOMI - not used
+  mov.b   #BIT0|BIT1|BIT2|BIT3|BIT6,&P1DIR
   mov.b   #0,&P1OUT
-  ;; BIT4: long pulse
-  ;; BIT1: short pulse
-  mov.b   #BIT1|BIT4,&P2DIR
-  mov.b   #BIT1|BIT4,&P2SEL
+  ;; BIT1: timer, short pulse
+  ;; BIT4: timer, long pulse
+  ;; BIT5: spi, UCB0CLK
+  ;; BIT6: spi, UCB0SOMI
+  ;; BIT7: spi, UCB0SIMO
+  mov.b   #BIT1|BIT4|BIT5|BIT6|BIT7,&P2DIR
+  mov.b   #BIT1|BIT4|BIT5|BIT6|BIT7,&P2SEL
 
   ;; timer configuration
   ;;  configure and stop timer
@@ -108,6 +119,11 @@ init:
   mov.w   #OUTMOD_7,&TA1CCTL1
   mov.w   #OUTMOD_7,&TA1CCTL2
 
+  ;; spi configuration
+  ;;  USCI B to slave mode
+  mov.b   #0x00, &UCB0CTL0
+  mov.b   #0x00, &UCB0CTL1
+
   ;; make sure the isr will not immediately start
   mov.b   #SIGNAL_INIT_VALUE, SIGNAL_REGISTER
 
@@ -119,14 +135,24 @@ init:
 
 ;; ----------------------------------------------------
 mainloop:
-  call    #forwardscreen_init
+  ;call    #forwardscreen_init
   call    #resetscreen
 
 mainloop_draw:
   call    #drawscreen
-  call    #forwardscreen
+  ;call    #forwardscreen
+
+  ;call    #wait
+
+  ;; signal waiting for data
+  set_signal_waiting_for_data
+
+  ;; receive data via spi
+  call    #receivedata
+
+  ;; data has been received, clear signal
+  clear_signal_waiting_for_data
 
-  call    #wait
 
   jmp     mainloop_draw
 
@@ -136,7 +162,7 @@ wait:
   push    r11
   push    r12
 
-  mov.w   #0x0040, r11
+  mov.w   #0x0010, r11
 wait_continue_1:
   mov.w   #0xffff, r12
 wait_continue_2:
@@ -164,7 +190,7 @@ forwardscreen:
   mov.w   data_forward_pointer, r10
   mov.b   #_off, @r10
   inc.w   r10
-  mov.b   #_violet, @r10
+  mov.b   #_white, @r10
   cmp.w   r10, r8
   jnz     forwardscreen_done
   mov.w   #screendata, r10
@@ -195,6 +221,67 @@ resetscreen_continue:
   ret
 
 
+;; ----------------------------------------------------
+receivedata:
+  push    r7
+  push    r8
+  push    r9
+  push    r10
+
+  ;; screendata addresses
+  mov.w   #screendata, r7
+  mov.w   #screendataend, r8
+ 
+  ;; wait for first octet
+receivedata_wait_for_first_octet:
+  bit     #UCB0RXIFG, &UC0IFG
+  jz      receivedata_wait_for_first_octet
+
+  ;; get data from buffer register
+  mov.b   UCB0RXBUF, r9
+
+  ;; check whether value == 0xff (wait for the whole 
+  ;; set of data to fill the screendata)
+  cmp.b   #0xff, r9
+
+  ;; receive all data 
+  jz      receivedata_wait_for_all_data
+
+  ;; if it is not 0xff, it is the octet number which is the
+  ;; relative address within screendata
+receivedata_wait_for_octet:
+  bit     #UCB0RXIFG, &UC0IFG
+  jz      receivedata_wait_for_octet
+
+  ;; get octet from buffer register
+  mov.b   UCB0RXBUF, r10
+
+  ;; move it to the destination
+  mov.b   r10, screendata(r9) 
+  
+  ;; done
+  jmp     receivedata_end
+
+receivedata_wait_for_all_data:
+  ;; this is a bit dangerous, if the application controller
+  ;; sends too few data, we are in a dead lock
+
+
+
+
+
+receivedata_end:
+  pop     r10
+  pop     r9
+  pop     r8
+  pop     r7
+  ret
+
+
+
+
+
+
 ;; ----------------------------------------------------
 drawscreen:
   push    r7
@@ -272,26 +359,26 @@ drawscreen_data_done:
 ;; ----------------------------------------------------
 ; --- timer isr ---
   ;; r6: exclusively used by isr as bit-counter
-timer1_a0_isr:
+shifter_isr:
   ;; check isr enable bit
   bit     #SIGNAL_ISR_ENABLE, SIGNAL_REGISTER
-  jz      timer1_a0_isr_exit
+  jz      shifter_isr_exit
 
   ;; shift msb of data register r5 into carry flag and set or reset P1.0 accordingly
   rla.b   DATA_REGISTER
-  jnc     timer1_a0_isr_false_bit
+  jnc     shifter_isr_false_bit
   set_data_bit
-  jmp     timer1_a0_isr_end
-timer1_a0_isr_false_bit:
+  jmp     shifter_isr_end
+shifter_isr_false_bit:
   clear_data_bit
    
-timer1_a0_isr_end:
+shifter_isr_end:
   ;; enable output
   set_output_enable
 
   ;; roll bit-counter
   rla.b   BIT_COUNTER_REGISTER
-  jnc     timer1_a0_isr_exit
+  jnc     shifter_isr_exit
 
   ;; reset bit-counter
   mov.b   #BIT_COUNTER_INIT_VALUE, BIT_COUNTER_REGISTER
@@ -300,19 +387,19 @@ timer1_a0_isr_end:
 
   ;; check whether all data are processed
   bit     #SIGNAL_ALL_DATA_DONE,  SIGNAL_REGISTER
-  jz      timer1_a0_isr_exit
+  jz      shifter_isr_exit
   ;; disable isr
   bic     #SIGNAL_ISR_ENABLE,  SIGNAL_REGISTER
   ;; disable output
   clear_output_enable
 
-timer1_a0_isr_exit:
+shifter_isr_exit:
   reti
 
 
 ;; ----------------------------------------------------
   .section "__interrupt_vector_14","ax",@progbits
-  .word   timer1_a0_isr
+  .word   shifter_isr
 
   ;; .resetvec comes from linker
   .section ".resetvec","ax",@progbits