diff --git a/src/capi.cpp b/src/capi.cpp
index db2e027..3ae4c87 100644
--- a/src/capi.cpp
+++ b/src/capi.cpp
@@ -45,8 +45,8 @@ class StackmatStateReceiver : public Receiver<StackmatState> {
         }
 
         void receive(StackmatState state) {
-            bool validChecksum = state.checksum != state.computedChecksum();
-            LOG2("StackmatStateReceiver::receive() state.millis: %d validChecksum: %d", state.millis, validChecksum);
+            bool validChecksum = state.checksum == state.computedChecksum();
+            LOG2("StackmatStateReceiver::receive() state.millis: %d checksum: %d computedChecksum: %d", state.millis, state.checksum, state.computedChecksum());
             if(!validChecksum) {
                 return;
             }
diff --git a/src/stackmat.cpp b/src/stackmat.cpp
index 60d9507..a3c5617 100644
--- a/src/stackmat.cpp
+++ b/src/stackmat.cpp
@@ -8,54 +8,28 @@ LOG_HANDLE("stackmat")
 
 StackmatSynthesizer::StackmatSynthesizer() {}
 
-#define MILLIS_PER_MINUTE (60*1000)
-#define MILLIS_PER_SECOND (1000)
-#define MILLIS_PER_DECISECOND (100)
-#define MILLIS_PER_CENTISECOND (10)
-
 void StackmatSynthesizer::receive(StackmatState state) {
+    LOG2("state.millis: %d state.checksum: %d", state.millis, state.checksum);
     send(state.commandByte);
 
-    int checksum = 64;
-    int millis = state.millis;
-
     // minutes
-    int minutes_digit = millis / MILLIS_PER_MINUTE;
-    millis %= MILLIS_PER_MINUTE;
-    checksum += minutes_digit;
-    send('0' + minutes_digit);
+    send('0' + state.minutesDigit());
     
     // seconds
-    int seconds = millis / MILLIS_PER_SECOND;
-    millis %= MILLIS_PER_SECOND;
-
-    int dekaseconds_digit = seconds / 10;
-    checksum += dekaseconds_digit;
-    send('0' + dekaseconds_digit);
-
-    int seconds_digit = seconds % 10;
-    checksum += seconds_digit;
-    send('0' + seconds_digit);
+    send('0' + state.tensSecondsDigit());
+    send('0' + state.onesSecondsDigit());
 
     // decimal
-    int deciseconds_digit = millis / MILLIS_PER_DECISECOND;
-    millis = millis % MILLIS_PER_DECISECOND;
-    checksum += deciseconds_digit;
-    send('0' + deciseconds_digit);
+    send('0' + state.tenthsDigit());
 
-    int centiseconds_digit = millis / MILLIS_PER_CENTISECOND;
-    millis = millis % MILLIS_PER_CENTISECOND;
-    checksum += centiseconds_digit;
-    send('0' + centiseconds_digit);
+    send('0' + state.hundredthsDigit());
 
     if(state.generation == 3) {
-        int milliseconds_digit = millis;
-        checksum += milliseconds_digit;
-        send('0' + milliseconds_digit);
+        send('0' + state.thousandthsDigit());
     }
 
     // checksum
-    send(checksum);
+    send(state.checksum);
 
     // LF
     send('\n');
@@ -76,7 +50,7 @@ void StackmatInterpreter::reset() {
 }
 
 void StackmatInterpreter::receive(int byte) {
-    LOG2("StackmatInterpreter::receive(%d)", byte);
+    LOG2("byte: %d", byte);
     if(byte < 0) {
         // idle received, parse collected characters
         switch(receivedBytesLength) {
@@ -93,7 +67,7 @@ void StackmatInterpreter::receive(int byte) {
                 int i = 0;
                 state.commandByte = receivedBytes[i++];
 
-                int minuteDigit = (receivedBytes[i++] - '0');
+                int minutesDigit = (receivedBytes[i++] - '0');
                 int tensSecondsDigit = (receivedBytes[i++] - '0');
                 int onesSecondsDigit = (receivedBytes[i++] - '0');
                 int tenthsDigit = (receivedBytes[i++] - '0');
@@ -109,7 +83,7 @@ void StackmatInterpreter::receive(int byte) {
                 unsigned char cr = receivedBytes[i++];
 
                 state.millis = 0;
-                state.millis += minuteDigit * MILLIS_PER_MINUTE;
+                state.millis += minutesDigit * MILLIS_PER_MINUTE;
                 int seconds = 10 * tensSecondsDigit + onesSecondsDigit;
                 state.millis += seconds * MILLIS_PER_SECOND;
                 state.millis += tenthsDigit * MILLIS_PER_DECISECOND;
diff --git a/src/stackmat.h b/src/stackmat.h
index fc73bb8..18223ee 100644
--- a/src/stackmat.h
+++ b/src/stackmat.h
@@ -13,6 +13,7 @@ namespace fskube {
 #define LARGESTSIGNAL_BYTES GEN3SIGNAL_BYTES
 
 #define MILLIS_PER_MINUTE (60*1000)
+#define MILLIS_PER_TENSECONDS (10000)
 #define MILLIS_PER_SECOND (1000)
 #define MILLIS_PER_DECISECOND (100)
 #define MILLIS_PER_CENTISECOND (10)
@@ -26,29 +27,26 @@ struct StackmatState {
     unsigned int generation;
     unsigned char commandByte;
 
-    unsigned int minuteDigit() {
-        int minutes_digit = millis / MILLIS_PER_MINUTE;
-        return minutes_digit;
+    unsigned int minutesDigit() {
+        return ( millis / MILLIS_PER_MINUTE ) % 10;
     }
     unsigned int tensSecondsDigit() {
-        int seconds = millis / MILLIS_PER_SECOND;
-        return seconds / 10;
+        return ( ( millis % MILLIS_PER_MINUTE ) / MILLIS_PER_TENSECONDS ) % 10;
     }
     unsigned int onesSecondsDigit() {
-        int seconds = millis / MILLIS_PER_SECOND;
-        return seconds % 10;
+        return ( millis / MILLIS_PER_SECOND ) % 10;
     }
     unsigned int tenthsDigit() {
-        return ( millis % MILLIS_PER_SECOND ) / MILLIS_PER_DECISECOND;
+        return ( millis / MILLIS_PER_DECISECOND ) % 10;
     }
     unsigned int hundredthsDigit() {
-        return ( millis % MILLIS_PER_SECOND ) % MILLIS_PER_DECISECOND;
+        return ( millis / MILLIS_PER_CENTISECOND  ) % 10;
     }
     unsigned int thousandthsDigit() {
-        return millis % 1000;
+        return millis % 10;
     }
     unsigned char computedChecksum() {
-        return 64 + minuteDigit() + tensSecondsDigit() +
+        return 64 + minutesDigit() + tensSecondsDigit() +
             onesSecondsDigit() + tenthsDigit() + hundredthsDigit() +
             thousandthsDigit();
     }
diff --git a/test/ExtrapolationTest.py b/test/ExtrapolationTest.py
index 2cf841b..14eeed3 100755
--- a/test/ExtrapolationTest.py
+++ b/test/ExtrapolationTest.py
@@ -30,8 +30,10 @@ def test(self):
         # We only extrapolate the signal when we think that the timer is running.
         state = fskube.StackmatState()
         state.millis = 40
+        state.checksum = state.computedChecksum()
         stackmatSythesizer.receive(state)
         state.millis = 500
+        state.checksum = state.computedChecksum()
         stackmatSythesizer.receive(state)
         for sample in sampler.data:
             fskube.fskube_addSample(sample)
@@ -42,7 +44,6 @@ def test(self):
         sleepSecs = 1
         time.sleep(sleepSecs)
         currentSignalMillis = fskube.fskube_getState().millis
-        import pdb; pdb.set_trace()#<<<
         self.assertGreaterEqual(currentSignalMillis, lastSignalMillis + sleepSecs*1000)
 
 if __name__ == "__main__":