ut71x: Fix incorrect resistance values on some DMMs.

The resistance values of some DMMs were incorrectly reported due to a
missing factor of 10 in the calculations.

Tested on Voltcraft VC-920/VC-940 and Tenma 72-9380A/72-7730/72-7732.

diff --git a/src/dmm/ut71x.c b/src/dmm/ut71x.c
index 0f0843e087ee2b7a577e738ac98124f37248bdc5..9a7d2eab93b0776bd56e1a75fcaf9fea86887cb1 100644 (file)
--- a/src/dmm/ut71x.c
+++ b/src/dmm/ut71x.c
@@ -44,7 +44,7 @@ static const int exponents[16][8] = {
        {  0,  -4,  -3,  -2, -1,  0,  0,  0 }, /* DC V */
        {  0,  -4,  -3,  -2, -1,  0,  0,  0 }, /* AC V */
        { -5,   0,   0,   0,  0,  0,  0,  0 }, /* DC mV */
-       {  0,  -1,   0,   1,  2,  3,  4,  0 }, /* Resistance */
+       {  0,  -2,  -1,   0,  1,  2,  3,  0 }, /* Resistance */
        {  0, -12, -11, -10, -9, -8, -7, -6 }, /* Capacitance */
        { -1,   0,   0,   0,  0,  0,  0,  0 }, /* Temp (C) */
        { -8,  -7,   0,   0,  0,  0,  0,  0 }, /* uA */
@@ -81,9 +81,21 @@ static int parse_value(const uint8_t *buf, struct ut71x_info *info, float *resul
                       buf[0], buf[1], buf[2], buf[3], buf[4]);
                return SR_ERR;
        }
+
        for (i = 0, intval = 0; i < num_digits; i++)
                intval = 10 * intval + (buf[i] - '0');
 
+       /*
+        * For measurements that only have 4000 instead of 40000 counts
+        * (resistance, continuity) we have to use an additional factor of 10.
+        *
+        * This seems to vary between DMMs. E.g. the Voltcraft VC920 and VC940
+        * have 4000 counts for resistance, whereas the Tenma 72-9380A,
+        * 72-7730 and 72-7732 have 40000 counts for resistance.
+        */
+       if (num_digits == 4)
+               intval *= 10;
+
        /* Apply sign. */
        intval *= info->is_sign ? -1 : 1;