Convert oil_watchman to bitbuffer_search() + bitbuffer_manchester_decode()

It turns out the framing in pulse_demod_manchester_framed() is not done
by the RFM01/Si4320 modules themselves; it's device-specific. See discussion
in issue #224.

So convert to use FSK_PULSE_PCM and then look for the preamble/postamble,
and do the Manchester decoding, in the bitbuffer domain.

src/devices/oil_watchman.c

@@ -14,9 +14,15 @@
 #include "pulse_demod.h"
 #include "util.h"
 
+
+// Start of frame preamble is 111000xx
+static const unsigned char preamble_pattern = 0xe0;
+
+// End of frame is 00xxxxxx or 11xxxxxx depending on final data bit
+static const unsigned char postamble_pattern[2] = { 0x00, 0xc0 };
+
 static int oil_watchman_callback(bitbuffer_t *bitbuffer) {
-	bitrow_t *bb = bitbuffer->bb;
-	uint8_t *b = bb[0];
+	uint8_t *b;
 	uint32_t unit_id;
 	uint16_t depth = 0;
 	uint16_t binding_countdown = 0;
@@ -25,48 +31,67 @@ static int oil_watchman_callback(bitbuffer_t *bitbuffer) {
 	time_t time_now;
 	char time_str[LOCAL_TIME_BUFLEN];
 	data_t *data;
-
-	if (bitbuffer->bits_per_row[0] != 64 ||
-		b[0] != 0x28 || b[7] != crc8le(b, 7, 0x31, 0))
-			return 0;
+	unsigned bitpos = 0;
+	bitbuffer_t databits = {0};
 
 	time(&time_now);
 	local_time_str(time_now, time_str);
 
-	// The unit ID changes when you rebind by holding a magnet to the
-	// sensor for long enough; it seems to be time-based.
-	unit_id = (b[1] << 16) | (b[2] << 8) | b[3];
+	// Find a preamble with enough bits after it that it could be a complete packet
+	while ((bitpos = bitbuffer_search(bitbuffer, 0, bitpos, &preamble_pattern, 6)) + 136 <=
+	       bitbuffer->bits_per_row[0]) {
 
-	// 0x01: Rebinding (magnet held to sensor)
-	// 0x08: Leak/theft alarm
-	flags = b[4];
+		// Skip the matched preamble bits to point to the data
+		bitpos += 6;
 
-	// Not entirely sure what this is but it might be inversely
-	// proportional to temperature.
-	maybetemp = b[5] >> 2;
+		bitpos = bitbuffer_manchester_decode(bitbuffer, 0, bitpos, &databits, 64);
+		if (databits.bits_per_row[0] != 64)
+			continue;
 
-	if (flags & 1)
+		b = databits.bb[0];
+
+		// Check for postamble, depending on last data bit
+		if (bitbuffer_search(bitbuffer, 0, bitpos, &postamble_pattern[b[7] & 1], 2) != bitpos)
+			continue;
+
+		if (b[0] != 0x28 || b[7] != crc8le(b, 7, 0x31, 0))
+			continue;
+
+		// The unit ID changes when you rebind by holding a magnet to the
+		// sensor for long enough; it seems to be time-based.
+		unit_id = (b[1] << 16) | (b[2] << 8) | b[3];
+
+		// 0x01: Rebinding (magnet held to sensor)
+		// 0x08: Leak/theft alarm
+		// top three bits seem also to vary with temperature (independently of maybetemp)
+		flags = b[4];
+
+		// Not entirely sure what this is but it might be inversely
+		// proportional to temperature.
+		maybetemp = b[5] >> 2;
+
+		if (flags & 1)
 			// When binding, the countdown counts up from 0x51 to 0x5a
 			// (as long as you hold the magnet to it for long enough)
 			// before the device ID changes. The receiver unit needs
 			// to receive this *strongly* in order to change its
 			// allegiance.
 			binding_countdown = b[6];
-	else
+		else
 			// A depth reading of zero indicates no reading. Even with
 			// the sensor flat down on a table, it still reads about 13.
 			depth = b[6] | ((((uint16_t)b[5]) & 3) << 8);
 
-	data = data_make("time", "", DATA_STRING, time_str,
-			 "model", "", DATA_STRING, "Oil Watchman",
-			 "id", "", DATA_FORMAT, "%06x", DATA_INT, unit_id,
-			 "flags", "", DATA_FORMAT, "%02x", DATA_INT, flags,
-			 "maybetemp", "", DATA_INT, maybetemp,
-		     "binding_countdown", "", DATA_INT, binding_countdown,
-		     "depth", "", DATA_INT, depth,
-		     NULL);
-    data_acquired_handler(data);
-
+		data = data_make("time", "", DATA_STRING, time_str,
+				 "model", "", DATA_STRING, "Oil Watchman",
+				 "id", "", DATA_FORMAT, "%06x", DATA_INT, unit_id,
+				 "flags", "", DATA_FORMAT, "%02x", DATA_INT, flags,
+				 "maybetemp", "", DATA_INT, maybetemp,
+				 "binding_countdown", "", DATA_INT, binding_countdown,
+				 "depth", "", DATA_INT, depth,
+				 NULL);
+		data_acquired_handler(data);
+	}
 	return 0;
 };
 
@@ -83,8 +108,10 @@ static char *output_fields[] = {
 
 r_device oil_watchman = {
 	.name			= "Ultrasonic oil monitor",
-	.modulation		= FSK_PULSE_MANCHESTER_FRAMED,
+	.modulation		= FSK_PULSE_PCM,
 	.short_limit	= 250,
+	.long_limit     = 250, // NRZ
+	.reset_limit    = 1000,
 	.json_callback	= &oil_watchman_callback,
 	.disabled		= 0,
 	.fields			= output_fields,