What are temperature-sensing diodes pins (DXP and DXN), and what considerations must be taken when using them?
In Virtex-4 FPGA, the temperature-sensing diodes pins are marked TDN and TDP.
You can access the Virtex device temperature-sensing diode through the DXP (Anode) and DXN (Cathode) pins. The DXP and DXN pins are connected to a diode-connected transistor, which creates a remote sensor (these additional pins are not available on all packages). In Virtex-4 FPGAs, you can access the temperature-sensing diode through TDP (Anode) and TDN (Cathode) pins. The DXP/DXN or TDP/TDN pins are dedicated pins and cannot be accessed through the software tools. These pins are attached to the substrate/die itself. They are always available, and you do not need to perform anything special in your design to use them. If the temperature-sensing feature is not used, the DXP and DXN pins can be left unconnected or grounded.
The temperature-sensing diode is one part of a two-part system that incorporates a Temperature Sensor Interface. To complete the system, you need a temperature sensor device, which connects to the Virtex device by means of the DXP and DXN pins. On most temperature sensing devices, corresponding pins connect to DXP and DXN. You can then set the upper and lower limits, and the temperature sensor creates an interrupt when these bounds are exceeded (the interrupt is an output pin on the sensor). You can then use this output to turn "Off" the clock or turn "On" a fan to reduce heat.
The accuracy of the temperature measurement does not depend on the diode (DXN/DXP pins), since the voltage vs. temperature curve is the physical nature of the diode. The accuracy relies upon the temperature sensor device, which translates the IV vs. temperature curves into a temperature reading. The data sheet for the temperature sensors listed below contains the accuracy specifications.
Examples of Temperature Sensors
Maxim Remote/Local Temperature Sensor
The following are links to product specifications for some of the Maxim temperature sensors: