Using SYSMON for Operational Junction Temperature Measurement

Designing Heatsinks and Thermal Solutions for Xilinx Devices (XAPP1377)

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Evaluating the in-system thermal performance should always be gauged by reading the system monitor circuitry (sometimes called SYSMON or XADC) in the device. This is an integrated ADC which can read an on-chip thermal diode as a means to get an accurate temperature reading. The diode is capable of providing the junction temperature of the device during operation. The temperature specifications for a device are always relative to the SYSMON readings and not to case or other measurement points. For the device to be considered to be operating within operating limits, the SYSMON temperature reading must take into account any uncertainties in the temperature measurement which can be obtained from the device datasheets. For example, an UltraScale+ device has a plus or minus accuracy of 3°C when using an external voltage reference. This means for the device to be considered to be operating in temperature specification, SYSMON should read at or below 97°C (100°C max operating temperature minus 3°C uncertainty). If the device supports temperature excursions, a SYSMON measurement reading is at or below 107° (110°C minus 3°C uncertainty). For Versal and later devices, the uncertainty into the device testing has been eliminated, thus simplifying the measurement to be interpreted without any uncertainty considerations. For Versal E and I-grade devices, a SYSMON reading of 100°C can be considered within specifications (110°C when considering excursion). SYSMON data can be accessed either within the design circuitry or using the JTAG debug interface and the Vivado software.