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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.
This commit is contained in:
Uwe Hermann 2017-05-23 07:44:42 +02:00
parent 7db9027985
commit 45fcaf2cca
1 changed files with 13 additions and 1 deletions
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14
src/dmm/ut71x.c
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@ -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;