When the temperature point exceeds the OT upper threshold, the OT alarm is asserted. The OT alarm deasserts when the temperature falls below the OT lower threshold.
The default OT upper threshold value corresponds to a temperature of 122°C. The default value is used after reset and when the ALARM_UPPER_OT [3:0] bits = 0h. The default lower threshold value is 67°C.
The software can define its own upper and lower OT thresholds. Setting ALARM_UPPER_OT [3:0] to 3h overrides the default upper and lower thresholds with the temperature values defined by ALARM_UPPER_OT [15:4] and ALARM_LOWER_OT [15:4]. When overriding the default OT thresholds, ALARM_LOWER_OT  defines hysteresis mode when 0 or normal mode when 1.
For the PS SYSMON unit, the upper and lower OT threshold values are shared between the LPD and FPD over-temperature alarms. The register value for temperature translation algorithms is described in the UltraScale Architecture System Monitor User Guide (UG580) [Ref 6].
Writing 0000h to the ALARM_UPPER_OT register restores the default upper and lower OT temperatures and hysteresis mode. In the PS SYSMON unit, the LPD and FPD sensor channels both use the same threshold register, ALARM_UPPER_OT. The OT alarm signal causes a system error that is received by the PMU, reset units, and the CSU.