ut71x: Fix incorrect resistance values on some DMMs.

[libsigrok.git] / src / dmm / ut71x.c

diff --git a/src/dmm/ut71x.c b/src/dmm/ut71x.c
index 442b6e12915f917f62f394e8f919be41ac2270c3..9a7d2eab93b0776bd56e1a75fcaf9fea86887cb1 100644 (file)
--- a/src/dmm/ut71x.c
+++ b/src/dmm/ut71x.c
@@ -14,8 +14,7 @@
  * GNU General Public License for more details.
  *
  * You should have received a copy of the GNU General Public License
- * along with this program; if not, write to the Free Software
- * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301 USA
+ * along with this program; if not, see <http://www.gnu.org/licenses/>.
  */
 
 /*
@@ -45,7 +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 */
@@ -82,9 +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;
 
@@ -117,7 +128,7 @@ static int parse_range(const uint8_t *buf, float *floatval, int *exponent)
 
        /* Apply respective exponent (mode-dependent) on the value. */
        *floatval *= powf(10, *exponent);
-       sr_dbg("Applying exponent %d, new value is %f.", *exponent, *floatval);
+       sr_dbg("Applying exponent %d, new value is %g.", *exponent, *floatval);
 
        return SR_OK;
 }
@@ -343,7 +354,7 @@ SR_PRIV int sr_ut71x_parse(const uint8_t *buf, float *floatval,
 
        handle_flags(analog, floatval, info);
 
-       analog->encoding->digits  = -exponent;
+       analog->encoding->digits = -exponent;
        analog->spec->spec_digits = -exponent;
 
        return SR_OK;