drivers: sensor: max31855: fixed sign bit positions

araneusdiadema · kartben · commit a7bddddb1e13 · 2025-04-25T14:05:18.000+02:00
MAX31855 driver had a wrong position of sign bits causing
invalid reading of negative temperature values. Fixed by
shifting position of sign bit by one bit.

Signed-off-by: Petr Vilím &lt;petr.vilim@proton.me&gt;
diff --git a/drivers/sensor/maxim/max31855/max31855.c b/drivers/sensor/maxim/max31855/max31855.c
@@ -1,5 +1,6 @@
 /*
  * Copyright (c) 2020 Christian Hirsch
+ * Copyright (c) 2025 Petr Vilím
  *
  * SPDX-License-Identifier: Apache-2.0
  */
@@ -72,7 +73,7 @@ static int max31855_channel_get(const struct device *dev, enum sensor_channel ch
 		temp = (temp >> THERMOCOUPLE_TEMPERATURE_POS) & 0x3fff;
 
 		/* if sign bit is set, make value negative */
-		if (temp & BIT(14)) {
+		if (temp & BIT(13)) {
 			temp |= THERMOCOUPLE_SIGN_BITS;
 		}
 
@@ -86,7 +87,7 @@ static int max31855_channel_get(const struct device *dev, enum sensor_channel ch
 		temp = (temp >> INTERNAL_TEMPERATURE_POS) & 0xfff;
 
 		/* if sign bit is set, make value negative */
-		if (temp & BIT(12)) {
+		if (temp & BIT(11)) {
 			temp |= INTERNAL_SIGN_BITS;
 		}
 

Original file line number	Diff line number	Diff line change
`@@ -1,5 +1,6 @@`
`1`	`1`	`/*`
`2`	`2`	`* Copyright (c) 2020 Christian Hirsch`
	`3`	`+ * Copyright (c) 2025 Petr Vilím`
`3`	`4`	`*`
`4`	`5`	`* SPDX-License-Identifier: Apache-2.0`
`5`	`6`	`*/`
`@@ -72,7 +73,7 @@ static int max31855_channel_get(const struct device *dev, enum sensor_channel ch`
`72`	`73`	`temp = (temp >> THERMOCOUPLE_TEMPERATURE_POS) & 0x3fff;`
`73`	`74`
`74`	`75`	`/* if sign bit is set, make value negative */`
`75`		`- if (temp & BIT(14)) {`
	`76`	`+ if (temp & BIT(13)) {`
`76`	`77`	`temp \|= THERMOCOUPLE_SIGN_BITS;`
`77`	`78`	`}`
`78`	`79`
`@@ -86,7 +87,7 @@ static int max31855_channel_get(const struct device *dev, enum sensor_channel ch`
`86`	`87`	`temp = (temp >> INTERNAL_TEMPERATURE_POS) & 0xfff;`
`87`	`88`
`88`	`89`	`/* if sign bit is set, make value negative */`
`89`		`- if (temp & BIT(12)) {`
	`90`	`+ if (temp & BIT(11)) {`
`90`	`91`	`temp \|= INTERNAL_SIGN_BITS;`
`91`	`92`	`}`
`92`	`93`