From: Uwe Hermann <redacted>
Date: Tue, 23 May 2017 05:44:42 +0000 (+0200)
Subject: ut71x: Fix incorrect resistance values on some DMMs.
X-Git-Tag: libsigrok-0.5.0~55
X-Git-Url: https://sigrok.org/gitweb/?p=libsigrok.git;a=commitdiff_plain;h=45fcaf2ccad796b4bc4434d81b1d0e23f11a8eae

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 0f0843e0..9a7d2eab 100644
--- 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;