When the temperature diode is used with an external sensing component to monitor junction temperature, I/O switching might affect the accuracy of the measurement.
Cross-talk on the printed circuit board, ground bounce, and other types of noise coupling are responsible for this problem. The readings might be higher or lower than the actual temperature, or they might be stuck at any value in between, depending on the characteristics and coupling of the noise.
Xilinx recommends that your device design include a "quiet" mode. This will allow you to run the design, stop it, read the temperature, and then resume the operation.
You should confirm the readings from the temperature diode with a thermal analysis of the device. The power consumed, multiplied by the Tja (thermal resistance of the junction to ambient) must be calculated for the heatsink, case, and airflow in the system. You can compare these to the readings to confirm the accuracy of any temperature diode results.
For information on using a PCB layout to minimize noise, refer to the temperature sensor component's data sheet. Also, refer to (Xilinx Answer 5738) for more information on temperature sensor components.