MIC384
set, it prevents the /INT output from sinking current. In I
2
C
and SMBus systems, the IM bit is therefore an interrupt mask
bit.
T1 and T2:
The T1 and T2 pins connect to off-chip PN diode
junctions, for monitoring the temperature at remote locations.
The remote diodes may be embedded thermal sensing
junctions in integrated circuits so equipped (such as Intel's
Pentium III), or discrete 2N3906-type bipolar transistors 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.
The temperature is calculated by measuring the forward
voltage of a diode junction at two different bias current levels.
An internal multiplexer directs the current source’s output to
either the internal or one of the external diode junctions. The
MIC384 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 any 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 MIC384 continuously performs
temperature-to-digital conversions, compares the results
against the setpoint and hysteresis registers, and updates
the state of /INT and the status bits accordingly. The remote
zones are converted first, followed by the local zone
(T1⇒T2⇒LOCAL). The states of /INT and the status bits are
updated after each measurement is taken.
Diode Faults
The MIC384 is designed to respond in a failsafe manner to
hardware faults in the external sensing circuitry. If the
connection to an external diode is lost or the sense line (T1
or T2) is shorted to V
DD
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 the setpoint register for the corresponding zone is set to any
Micrel
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 MIC384 complete the number of conversion
cycles specified by Fault_Queue (see below). 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 MIC384 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 MIC384,
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 MIC384 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 MIC384 regis-
ters 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 MIC384 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 MIC384, and then retrieves the
data byte. Figures 1 through 3 show the formats for these
procedures.
Command_Byte
Binary
0000 0000
b
0000 0001
b
0000 0010
b
0000 0011
b
0001 0000
b
0001 0010
b
0001 0011
b
0010 0000
b
0010 0010
b
0010 0011
b
Hex
00
h
01
h
02
h
03
h
10
h
12
h
13
h
20
h
22
h
23
h
Label
TEMP0
CONFIG
T_HYST0
T_SET0
TEMP1
T_SET1
TEMP2
T_HYST2
T_SET2
Target Register
Description
local temperature
configuration register
local temperature hysteresis
local temperature setpoint
remote zone 1 temperature
remote zone 1 temperature setpoint
remote zone 2 temperature
remote zone 2 temperature hysteresis
remote zone 2 temperature setpoint
T_HYST1 remote zone 1 temperature hysteresis
Table 2. MIC384 Register Addresses
September 2000
7
MIC384
MICREL [ MICREL SEMICONDUCTOR ]
