Update MeterW1therm.cpp to catch erroneous temperature values

src/protocols/MeterW1therm.cpp

@@ -63,6 +63,55 @@ bool MeterW1therm::W1sysHWif::scanW1devices() {
 	return false;
 }
 
+// check for DS18B20-specific errors
+// For reference: https://github.com/cpetrich/counterfeit_DS18B20
+// Check byte 6 of scratchpad to recognize error readings.
+// Genuine DS18B20 compute byte 6 as the difference from
+// 00001111 & <byte 0> to 00010000; most clones always use 0x0c.
+// Genuine DS18B20 are still in the 00000xxxxxxx address range
+// as of 2023; clones are not.
+bool MeterW1therm::check_quirks(const std::string &device, char buffer[100]) {
+	// Check for sensor family DS18B20 via device == "28*":
+	if (device[0] != '2' || device[1] != '8')
+		return true;
+
+	if (!strncmp(buffer, "00 00 00 00 00 00 00 00 00", 26)) {
+		// According to DS18B20 datasheet, this scratchpad-reading MUST be wrong.
+		print(log_debug,
+			  "only zeroes from %s (%s), "
+			  "the 1wire-bus is probably unstable",
+			  "w1t", dev.c_str(), buffer);
+		return false;
+	}
+	// if (buffer == "?? ?? ?? ?? ?? ?? 0c*"), maybe an error has happened:
+	if (buffer[18] == '0' && buffer[19] == 'c') {
+		// only do this handling for genuine DS18B20, e.g. device="28-0000*":
+		if (device[3] == '0' && buffer[4] == '0' && buffer[5] == '0' && buffer[6] == '0') {
+			// if (buffer == "50 05*"), 0x0550/16 = 85 degrees Celsius
+			if (!strncmp(buffer, "50 05", 5)) {
+				// Sensor returned default boot-up value; did not complete temperature
+				// conversion.
+				print(log_debug,
+					  "85 degree error from %s (%s), "
+					  "the 1wire-bus is probably unstable",
+					  "w1t", dev.c_str(), buffer);
+				return false;
+			}
+			// (if buffer == "ff 07*"), 0x07ff/16 = 127.9375 degrees Celsius
+			if (!strncmp(buffer, "ff 07", 5)) {
+				// Sensor returned code for insufficient power during temperature conversion.
+				// This can happen in parasitic 2-wire circuits with floating Vcc pins.
+				print(log_debug,
+					  "insufficient power error from %s (%s), "
+					  "maybe Vcc was left floating",
+					  "w1t", dev.c_str(), buffer);
+				return false;
+			}
+		}
+	}
+	return true;
+}
+
 bool MeterW1therm::W1sysHWif::readTemp(const std::string &device, double &value) {
 	bool toret = false;
 	// read from /sys/bus/w1/devices/<device>/w1_slave
@@ -83,8 +132,11 @@ bool MeterW1therm::W1sysHWif::readTemp(const std::string &device, double &value)
 			print(log_debug, "CRC not ok from %s (%s)", "w1t", dev.c_str(), buffer);
 		} else {
 			// crc ok
-			// now parse t=<value>
 			if (fgets(buffer, 100, fp)) { // e.g. 07 01 55 00 7f ff 0c 10 18 t=16437
+				if (!check_quirks(device, buffer))
+					return false;
+
+				// now parse t=<value>
 				char *pos = strstr(buffer, "t=");
 				if (pos) {
 					pos += 2;