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-rw-r--r--Documentation/hwmon/lm7720
1 files changed, 18 insertions, 2 deletions
diff --git a/Documentation/hwmon/lm77 b/Documentation/hwmon/lm77
index 57c3a46d637..bfc915fe363 100644
--- a/Documentation/hwmon/lm77
+++ b/Documentation/hwmon/lm77
@@ -18,5 +18,21 @@ sensor incorporates a band-gap type temperature sensor,
10-bit ADC, and a digital comparator with user-programmable upper
and lower limit values.
-Limits can be set through the Overtemperature Shutdown register and
-Hysteresis register.
+The LM77 implements 3 limits: low (temp1_min), high (temp1_max) and
+critical (temp1_crit.) It also implements an hysteresis mechanism which
+applies to all 3 limits. The relative difference is stored in a single
+register on the chip, which means that the relative difference between
+the limit and its hysteresis is always the same for all 3 limits.
+
+This implementation detail implies the following:
+* When setting a limit, its hysteresis will automatically follow, the
+ difference staying unchanged. For example, if the old critical limit
+ was 80 degrees C, and the hysteresis was 75 degrees C, and you change
+ the critical limit to 90 degrees C, then the hysteresis will
+ automatically change to 85 degrees C.
+* All 3 hysteresis can't be set independently. We decided to make
+ temp1_crit_hyst writable, while temp1_min_hyst and temp1_max_hyst are
+ read-only. Setting temp1_crit_hyst writes the difference between
+ temp1_crit_hyst and temp1_crit into the chip, and the same relative
+ hysteresis applies automatically to the low and high limits.
+* The limits should be set before the hysteresis.