Datasheet
PowerPC 970FX RISC Microprocessor
5.8 Thermal Management Information
5.8.1 Thermal Management Pins
The PowerPC 970FX features an on-board temperature sensing diode, which connects to pins AA1
(DIODENEG) and Y1 (DIODEPOS).
This diode is calibrated at two points: a low temperature point and a high temperature point, both of which are
relative terms. For example, the low temperature point can be 30°C and the high temperature point can be
85°C or 105°C. The actual values used for this calibration are stored in the fuse data of each chip. These two
temperatures are applied externally with no self heating of the die, when the diode calibration is being done.
At these two temperatures, with no power applied (VDD = 0 V), a current of 100 μA is forced through the diode
and the voltage drop is logged. The voltages at these two temperatures are also stored in the fuse data for
each chip. Using these two temperature points and their corresponding voltage, a linear relationship is devel-
oped that correlates the voltage across the diode with a corresponding temperature. (See Figure 5-3
PowerPC 970FX Thermal Diode Implementation on page 76.) The chip temperature can be determined by
interpreting the voltage across the diode pins when a controlled 100 μA current is forced through the diode by
an external source. See the Thermal Diode Calibration Application Note for more details.
Other temperature sensors or monitoring hardware can also be implemented with the PowerPC 970FX and
mounted as close to the PowerPC 970FX as practical. If the external temperature-sensing hardware deter-
mines that an unsafe operating temperature has been reached, the THERM_INT input should be asserted to
initiate either power management or shutdown of the system.
Note: The unsafe operating temperature setting is application dependent.
5.8.2 Reading Thermal Diode Calibration Data through JTAG
To access the thermal diode calibration data stored in each processor, a sequence of JTAG commands must
be issued. By using JTAG commands, the data is accessed serially on the 970FX TDO pin and can also be
read using the I2C interface. Details for reading the thermal diode data are contained in the section about
accessing thermal diode calibration data in the PowerPC 970FX Power On Reset Application Note. The
temperature values and voltage values stored as fuse data must be interpreted as shown in Table 5-9
Thermal Diode Data Encoding on page 76.
This is a one-time only procedure. It is assumed the thermal diode calibration data stored in each processor is
captured and stored in system read-only memory for subsequent use. This procedure is not meant to be run
at every system startup because reading out this calibration data leaves the processor in an unusable state
until it is restarted.
During the POR sequence, the processor comes to a WAIT state to allow the service processor to scan the
MODE ring facility (address modifier x‘00C08000’, x‘00C04000’, or x‘00804001’). It is during this WAIT state
that the thermal diode data can be scanned out. Scanning the MODE ring is not necessary.
Note: For correct fuse operation, ensure that the SPARE2 and SPARE_GND pins are tied to GND.
Version 2.5
System Design Information
Page 75 of 78
March 26, 2007
IBM [ IBM ]
