MIC284 Datasheet, PDF (7/20 Pages) Micrel Semiconductor – Two-Zone Thermal Supervisor Advance Information

MIC284
set, it prevents the /INT output from sinking current. In I²C
and SMBus systems, the IM bit is therefore an interrupt mask
bit.
/CRIT: Over-temperature events are indicated to external
circuitry via this output. This output is open-drain and may be
wire-OR’ed with other open-drain signals. Most systems will
require a pull-up resistor or current source on this pin.
T1: This pin connects to an off-chip PN diode junction, for
monitoring the junction temperature at a remote location. The
remote diode may be an embedded thermal sensing junction
in an integrated circuit so equipped (such as Intel's Pentium
III), or a discrete 2N3906-type bipolar transistor with base and
collector tied together.
Temperature Measurement
The temperature-to-digital converter is built around a switched
current source and an eight-bit analog-to-digital converter.
Each diode's temperature is calculated by measuring its
forward voltage drop at two different current levels. An
internal multiplexer directs the MIC284's current source out-
put to either an internal or external diode junction. The
MIC284 uses two’s-complement data to represent tempera-
tures. If the MSB of a temperature value is zero, the
temperature is zero or positive. If the MSB is one, the
temperature is negative. More detail on this is given in the
"Temperature Data Format" section below. A “temperature
event” results if the value in either of the temperature result
registers (TEMPx) becomes greater than the value in the
corresponding temperature setpoint register (T_SETx). An-
other temperature event occurs if and when the measured
temperature subsequently falls below the temperature hys-
teresis setting in T_HYSTx.
During normal operation the MIC284 continuously performs
temperature-to-digital conversions, compares the results
against the setpoint registers, and updates the states of /INT,
/CRIT, and the status bits accordingly. The remote zone is
converted first, followed by the local zone. The states of /INT,
/CRIT, and the status bits are updated after each measure-
ment is taken. The remote diode junction connected to T1
may be embedded in an integrated circuit such as a CPU,
ASIC, or graphics processor, or it may be a diode-connected
discrete transistor.
Micrel
Diode Faults
The MIC284 is designed to respond in a failsafe manner to
hardware faults in the external sensing circuitry. If the
connection to the external diode is lost or the sense line (T1)
is shorted to VDD or ground, the temperature data reported
by the A/D converter will be forced to its full-scale value
(+127°C). This will cause a temperature event to occur if
T_SET1 or CRIT1 are set to any value less than 127°C (7F_h
= 0111 1111_b). An interrupt will be generated on /INT if so
enabled. The temperature reported for the external zone will
remain +127°C until the fault condition is cleared. This fault
detection mechanism requires that the MIC284 complete the
number of conversion cycles specified by Fault_Queue. The
part will therefore require one or more conversion cycles
following power-on or a transition from shutdown to normal
operation before reporting an external diode fault.
Serial Port Operation
The MIC284 uses standard SMBus Write_Byte and
Read_Byte operations for communication with its host. The
SMBus Write_Byte operation involves sending the device’s
slave address (with the R/W bit low to signal a write opera-
tion), followed by a command byte and a data byte. The
SMBus Read_Byte operation is similar, but is a composite
write and read operation: the host first sends the device’s
slave address followed by the command byte, as in a write
operation. A new start bit must then be sent to the MIC284,
followed by a repeat of the slave address with the R/W bit
(LSB) set to the high (read) state. The data to be read from
the part may then be clocked out.
The command byte is eight bits wide. This byte carries the
address of the MIC284 register to be operated upon, and is
stored in the part’s pointer register. The pointer register is an
internal write-only register. The command byte (pointer
register) values corresponding to the various MIC284 regis-
ter addresses are shown in Table 2. Command byte values
other than those explicitly shown are reserved, and should
not be used. Any command byte sent to the MIC284 will
persist in the pointer register indefinitely until it is overwritten
by another command byte. If the location latched in the
pointer register from the last operation is known to be correct
(i.e., points to the desired register), then the Receive_Byte
procedure may be used. To perform a Receive_Byte, the host
sends an address byte to select the MIC284, and then
retrieves the data byte. Figures 1 through 3 show the formats
for these procedures.
September 29, 2000
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MIC284