tmp105: Correct handling of temperature limit checks

The TMP105 datasheet says that in Interrupt Mode (when TM==1) the device
signals an alert when the temperature equals or exceeds the T_high value and
then remains high until a device register is read or the device responds to
the SMBUS Alert Response address, or the device is put into Shutdown Mode.
Thereafter the Alert pin will only be re-signalled when temperature falls
below T_low; alert can then be cleared in the same set of ways, and the
device returns to its initial "alert when temperature goes above T_high"
mode. (If this textual description is confusing, see figure 3 in the
TI datasheet at https://www.ti.com/lit/gpn/tmp105 .)

We were misimplementing this as a simple "always alert if temperature is
above T_high or below T_low" condition, which gives a spurious alert on
startup if using the "T_high = 80 degrees C, T_low = 75 degrees C" reset
limit values.

Implement the correct (hysteresis) behaviour by tracking whether we
are currently looking for the temperature to rise over T_high or
for it to fall below T_low. Our implementation of the comparator
mode (TM==0) wasn't wrong, but rephrase it to match the way that
interrupt mode is now handled for clarity.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Message-id: 20201110150023.25533-3-peter.maydell@linaro.org

1 files changed, 7 insertions, 0 deletions

diff --git a/hw/misc/tmp105.h b/hw/misc/tmp105.h
index e5198fce80..7c97071ad7 100644
--- a/hw/misc/tmp105.h
+++ b/hw/misc/tmp105.h
@@ -43,6 +43,13 @@ struct TMP105State {
     int16_t limit[2];
     int faults;
     uint8_t alarm;
+    /*
+     * The TMP105 initially looks for a temperature rising above T_high;
+     * once this is detected, the condition it looks for next is the
+     * temperature falling below T_low. This flag is false when initially
+     * looking for T_high, true when looking for T_low.
+     */
+    bool detect_falling;
 };
 
 #endif