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86 Datasheet
System Management Feature Specifications
controller continues to transmit data bytes until it terminates the sequence with a stop. All data
bytes will result in an acknowledge from the Scratch EEPROM. If more than eight bytes are
written, the internal address will “roll over” and the previous data will be overwritten.
In Table 6-5 through Table 6-7, ‘S’ represents the SMBus start bit, ‘P’ represents a stop bit, ‘R’
represents a read, ‘W’ represents a write bit, ‘A’ represents an acknowledge, and ‘///’ represents a
negative acknowledge. The shaded bits are transmitted by the processor information ROM or
Scratch EEPROM and the bits that are not shaded are transmitted by the SMBus host controller. In
the tables the data addresses indicate eight bits. The SMBus host controller should transmit eight
bits, but as there are only 128 addresses, the most significant bit is a don’t care.
6.5 Thermal Sensing Device
The Itanium 2 processor’s thermal sensing device provides a means of acquiring thermal data from
the processor. The accuracy of the thermal reading is expected to be better than ±3 °C. The thermal
sensing device is composed of control logic, SMBus interface logic, a precision analog to digital
converter, and a precision current source. The thermal sensing device drives a small current
through a thermal diode located on the processor core and measures the voltage generated across
the thermal diode by the current. With this information, the thermal sensing device computes a byte
of temperature data. Software running on the processor or on a micro-controller can use the
temperature data from the thermal sensing device to thermally manage the system.
The thermal sensing device provides a register with a data byte (seven bits plus sign) which
contains a value corresponding to the sampled output of the thermal diode in the Itanium 2
processor core. The value of the byte read from the thermal sensor gives an accurate reading of
processor core temperature at the time of the reading. This data can be used in conjunction with the
upper temperature reference byte (provided in the processor information ROM) for thermal
management purposes. The temperature data from the thermal sensor can be read out digitally
using an SMBus read command (see Section 6.6). The thermal sensor detects when SMBus power
is applied to the processor, and resets itself at power up.
The thermal sensing device also contains alarm registers to store thermal reference threshold data.
These values can be individually programmed on the thermal sensor. If the measured temperature
equals or exceeds the alarm threshold value, the appropriate bit is set in the thermal sensing device
status register, which is also brought out to the Itanium 2 processor system bus via the
Table 6-5. Current Address Read SMBus Packet
S
Device
Address
R A Data /// P
17 bits11 8 bits 1 1
Table 6-6. Random Address Read SMBus Packet
S
Device
Address
W A
Data
Address
AS
Device
Address
R A Data /// P
17 bits118 bits1 1 7 bits 1 1 8 bits 1 1
Table 6-7. Byte Write SMBus Packet
S
Device
Address
W A
Data
Address
ADataAP
17 bits018 bits18 bits11