diff --git a/main.c b/main.c
index 4751808..52718f2 100644
--- a/main.c
+++ b/main.c
@@ -12,14 +12,13 @@
  * conversion result from the ADC is saved and held until the 
  * COMM_SAMPLE_HOLD_BIT goes to low again.
  * 
- * When the COMM_ENABLE_BIT is pulled to high and an earlier ADC value 
- * is currently held, it is checked whether the current ADC value is
- * by SPACE_MARK_THRESHOLD greater then the held value.
- * If it is greater, the COMM_RESULT_BIT is set to high, otherwise
- * it is set to low.
+ * While the COMM_SAMPLE_HOLD_BIT is pulled to high  it is checked 
+ * whether the current ADC value is by SPACE_MARK_THRESHOLD greater 
+ * then the held value. If it is greater, the COMM_RESULT_BIT is set 
+ * to low, otherwise it is set to high.
  * 
- * If either the COMM_ENABLE_BIT or the COMM_SAMPLE_HOLD_BIT goes to 
- * low the COMM_RESULT_BIT is set to low.
+ * If COMM_SAMPLE_HOLD_BIT goes to low the COMM_RESULT_BIT is set to
+ * high.
  */
 
 
@@ -36,15 +35,45 @@
 
 #define COMM_PORT_OUT P2OUT
 #define COMM_PORT_IN P2IN
+#define COMM_PORT_REN P2REN
 #define COMM_PORT_DIR P2DIR
 #define COMM_RESULT_BIT BIT2         // P2.2
 #define COMM_SAMPLE_HOLD_BIT BIT3    // P2.3
-#define COMM_ENABLE_BIT BIT4         // P2.4
 
 // derived from the calculation in mbus-converter.ods
 // assumes a 25Ohm shunt resistor and a 10mA swing
 #define SPACE_MARK_THRESHOLD 100
 
+// SPI debug codes
+#define DBG_START      0xffffffff
+#define DBG_HOLD       0x01000000
+#define DBG_SAMPLE     0x02000000
+#define DBG_RX_SLOPE   0x04000000
+
+
+// 32bit-value to transmit via SPI
+volatile uint32_t spiTxBuf = 0;
+
+// ISR for SPI transmitter
+__attribute__((interrupt(USCIAB0TX_VECTOR)))
+void spiTxIsr(void) {
+    static uint8_t oc = 0;
+    oc++;
+    if (oc < 4) {
+        UCA0TXBUF = (uint8_t)((spiTxBuf >> (8 * oc)) & 0x00ff);
+    } else {
+        IE2 &= ~UCA0TXIE;
+        oc = 0;
+    }
+}
+
+void writeSpi(uint32_t m) {
+    spiTxBuf = m;
+    UCA0TXBUF = (uint8_t)(spiTxBuf & 0x00ff);
+    IE2 |= UCA0TXIE;
+}
+
+
 
 // ISR to read and process result from adc
 __attribute__((interrupt(ADC10_VECTOR)))
@@ -59,19 +88,21 @@ void adcIsr(void)
         if (! holdFlag) {
             holdValue = currentValue;
             holdFlag = true;
-        }
-    } else {
-        holdFlag = false;
-    }
-
-    if ((COMM_PORT_IN & COMM_ENABLE_BIT) && holdFlag) {
-        if (currentValue > (holdValue + SPACE_MARK_THRESHOLD)) {
-            COMM_PORT_OUT &= ~COMM_RESULT_BIT;
+            writeSpi(DBG_HOLD | holdValue);
         } else {
-            COMM_PORT_OUT |= COMM_RESULT_BIT;
+            if (currentValue > (holdValue + SPACE_MARK_THRESHOLD)) {
+                COMM_PORT_OUT &= ~COMM_RESULT_BIT;
+                // writeSpi(DBG_RX_SLOPE | currentValue);
+            } else {
+                COMM_PORT_OUT |= COMM_RESULT_BIT;
+            }
         }
     } else {
-        COMM_PORT_OUT |= COMM_RESULT_BIT;
+        if (holdFlag) {
+            holdFlag = false;
+            COMM_PORT_OUT |= COMM_RESULT_BIT;
+            writeSpi(DBG_SAMPLE);
+        }
     }
 
 #ifdef DEBUG
@@ -99,14 +130,26 @@ void setup() {
 
     // communication
     COMM_PORT_DIR |= COMM_RESULT_BIT;
-    COMM_PORT_DIR &= ~(COMM_ENABLE_BIT | COMM_SAMPLE_HOLD_BIT);
+    COMM_PORT_DIR &= ~COMM_SAMPLE_HOLD_BIT;
+    COMM_PORT_REN |= COMM_SAMPLE_HOLD_BIT;
+    COMM_PORT_OUT &= ~COMM_SAMPLE_HOLD_BIT;
     COMM_PORT_OUT |= COMM_RESULT_BIT;
 
     // adc
-    ADC10CTL0 = SREF1 | REFON | ADC10ON | ADC10IE | MSC;
+    ADC10CTL0 = SREF_1 | REFON | ADC10ON | ADC10IE | MSC;
     ADC10CTL1 = INCH_3 | CONSEQ_2;
     ADC10AE0 = BIT3;
 
+    // spi
+    UCA0CTL1 = UCSWRST | UCSSEL_2;
+    UCA0CTL0 = UCMST | UCSYNC;
+    UCA0BR0 = 8;
+    UCA0BR1 = 0;
+    P1SEL |= BIT1 | BIT2 | BIT4;
+    P1SEL2 |= BIT1 | BIT2 | BIT4;
+
+    UCA0CTL1 &= ~UCSWRST;
+
     __enable_interrupt();
 
     // start the adc
@@ -124,6 +167,8 @@ void loop() {
 int main() {
     setup();
 
+    writeSpi(DBG_START);
+
     while (1) {
         loop();
     }