diff --git a/Makefile b/Makefile
index b7d4662..90513d9 100644
--- a/Makefile
+++ b/Makefile
@@ -1,6 +1,6 @@
 OBJS := aldl.o aldl_pi.o sdldash.o
-LDFLAGS :=  -lSDL2 -lSDL2_ttf -lwiringPi
-CFLAGS := -Wall
+LDFLAGS :=  -lSDL2 -lSDL2_ttf -lwiringPi -lpthread
+CFLAGS := -Wall -DPOLLING
 
 # The ALDL datastream we want
 OBJS += a179.o
diff --git a/aldl.c b/aldl.c
index e47d063..451033b 100644
--- a/aldl.c
+++ b/aldl.c
@@ -32,10 +32,14 @@
 #include <time.h>
 #include <unistd.h>
 
+#include <pthread.h>
+
 #include "aldl.h"
 
 /* shift register & sync word */
 #define SYNC_WORD 0x1ff
+#define DWELL_TIME 1000  /* microseconds after falling edge */
+#define DEBOUNCE_TIME 200 /* microseconds */
 #define MAX_STREAM_WORDS 255
 static uint16_t ALDLSR = 0;
 static uint8_t bits = 0;
@@ -79,24 +83,21 @@ static void aldl_datalog(uint8_t *data, int datalen)
     fprintf(datalog_handle, "\n");
 }
 
-/* ISR */
-void isrfn(void)
+static void process_bit(int val)
 {
-  int val;
-  delayfn(2000);  // To handle both variations, minimum of 0.5, max of 1.8ms
-  val = readbit();
   ALDLSR <<= 1;
   ALDLSR |= (val & 1);
 
   if ((ALDLSR & SYNC_WORD) == SYNC_WORD) {
+    /* New sync word, we've finished a sequence */
+    printf(" - %d\n", datalen);
+
     /* Received a sync word.. */
     ALDLSR = 0;
     if (!gotsync) {
         gotsync = 1;
 	return;  /* First one, we don't do anything */
     }
-    /* New sync word, we've finished a sequence */
-    printf(" - %d\n", datalen);
 
     /* Process ALDL payload */
     if (aldl_callback)
@@ -116,3 +117,83 @@ void isrfn(void)
     ALDLSR = 0;
   }
 }
+
+/* ISR */
+void isrfn(void)
+{
+  int val;
+  delayfn(DWELL_TIME);  // To handle both variations, minimum of 0.5, max of 1.8ms
+  val = !readbit();  // If hi, represents ALDL 0; if low, ALDL1.
+  process_bit(val);
+}
+
+#include <sys/time.h>
+
+int run_sampler = 1;
+
+long tv_diff(struct timeval *tv1, struct timeval *tv2)
+{
+  long usecs = (tv2->tv_sec - tv1->tv_sec) * 1000000;
+  usecs += (tv2->tv_usec - tv1->tv_usec);
+  return usecs;
+}
+
+/* Alternatively.. continually poll. */
+void poll_gpio(void)
+{
+  struct timeval tv, last_tv;
+  int pin, last_pin;
+  long total_us = 0;
+
+  gettimeofday(&last_tv, NULL);
+  last_pin = readbit();
+
+  while(run_sampler) {
+     /* Read */
+     gettimeofday(&tv, NULL);
+     pin = readbit();
+
+     /* Work out how long it's been since our last sample */
+     total_us += tv_diff(&last_tv, &tv);
+     last_tv = tv;
+
+     /* No edge.. or possibly false edge. */
+     if (pin == last_pin || total_us < DEBOUNCE_TIME)
+        continue;
+
+     if (pin == 1) {  /* ie Rising edge */
+//	printf ("%06d\n", total_us);
+	/* Rising edge.  tells us when the bit ends */
+	if (total_us > DWELL_TIME)
+	    process_bit(0);  /* long period high is logical 0 */
+        else
+	    process_bit(1);  /* short period high is logical 1 */
+     }
+     /* Edge transition, reset time and record new pin */
+     total_us = 0;
+     last_pin = pin;
+  }
+}
+
+static pthread_t poller;
+static int polling = 0;
+
+static void *thread_poller(void *arg)
+{
+   poll_gpio();
+   pthread_exit(NULL);
+}
+
+int thread_start(void)
+{
+  polling = 1;
+  return pthread_create(&poller, NULL, thread_poller, NULL);
+}
+void thread_shutdown(void)
+{
+  if (polling) {
+    run_sampler = 0;
+    pthread_join(poller, NULL);
+    polling = 0;
+  }
+}
diff --git a/aldl.h b/aldl.h
index 70a52ca..5fba9eb 100644
--- a/aldl.h
+++ b/aldl.h
@@ -28,9 +28,12 @@
  */
 
 /* API */
-void isrfn(void);
 int readbit(void);
 void delayfn(int ms);
+void isrfn(void);
+
+int thread_start(void);
+void thread_shutdown(void);
 
 extern int gotsync;
 extern void (*aldl_callback)(uint8_t *data, int datalen);
diff --git a/aldl_pi.c b/aldl_pi.c
index 9e71090..ecffa30 100644
--- a/aldl_pi.c
+++ b/aldl_pi.c
@@ -42,7 +42,7 @@ static char *datalog_fname = "DATALOG.csv";
 
 int readbit(void)
 {
-   return !digitalRead(ALDL_PIN_IN);  /* Logic low is ALDL '1' */
+   return digitalRead(ALDL_PIN_IN);  /* Logic low is ALDL '1' */
 }
 void delayfn(int us)
 {
@@ -65,7 +65,12 @@ int main (int argc, char **argv)
   /* Set up RPi GPIO */
   pinMode(ALDL_PIN_IN, INPUT);
   pullUpDnControl(ALDL_PIN_IN, PUD_DOWN);
+
+#if POLLING
+  thread_start();
+#else
   wiringPiISR (ALDL_PIN_IN, INT_EDGE_FALLING, &isrfn);
+#endif
 
   /* Initialize datalogging */
   datalog_init(datalog_fname);
@@ -82,5 +87,9 @@ int main (int argc, char **argv)
 
   datalog_close();
 
+#if POLLING
+  thread_start();
+#endif
+
   return 0;
 }