ut372: Implement initial protocol parser.

For now this only works correctly if the device is in the default
state showing current RPM. The flags are not checked.

src/dmm/ut372.c

@@ -22,18 +22,66 @@
  * UNI-T UT372 protocol parser.
  */
 
+#include <stdlib.h>
+#include <math.h>
 #include "libsigrok.h"
 #include "libsigrok-internal.h"
 
 #define LOG_PREFIX "ut372"
 
+char lookup[] = {
+	0x7B,
+	0x60,
+	0x5E,
+	0x7C,
+	0x65,
+	0x3D,
+	0x3F,
+	0x70,
+	0x7F,
+	0x7D
+};
+
+#define DECIMAL_POINT_MASK 0x80
+
 SR_PRIV gboolean sr_ut372_packet_valid(const uint8_t *buf)
 {
-	return FALSE;
+	return (buf[25] == '\r' && buf[26] == '\n');
 }
 
 SR_PRIV int sr_ut372_parse(const uint8_t *buf, float *floatval,
 		struct sr_datafeed_analog *analog, void *info)
 {
+	unsigned int i, j, segments, value, divisor;
+	char hex[3];
+
+	(void) info;
+
+	hex[2] = '\0';
+	value = 0;
+	divisor = 1;
+
+	for (i = 0; i < 5; i++) {
+		for (j = 0; j < 2; j++) {
+			hex[j] = buf[2*i + 1 + j];
+			if (hex[j] > 0x39)
+				hex[j] += 7;
+		}
+		segments = strtol(hex, NULL, 16);
+		for (j = 0; j < ARRAY_SIZE(lookup); j++) {
+			if (lookup[j] == (segments & ~DECIMAL_POINT_MASK)) {
+				value += j * pow(10, i);
+				break;
+			}
+		}
+		if (segments & DECIMAL_POINT_MASK)
+			divisor = pow(10, i);
+	}
+
+	*floatval = (float) value / divisor;
+
+	analog->mq = SR_MQ_FREQUENCY;
+	analog->unit = SR_UNIT_REVOLUTIONS_PER_MINUTE;
+
 	return SR_OK;
 }