MIC384
is 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 temperatures. 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). Another temperature event occurs if and
when the measured temperature subsequently falls below
the temperature hysteresis 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 value less than 127°C (7F
h
= 0111 1111
b
). An interrupt
will be generated on /INT if so enabled. The temperature
C o m m a n d _ B y te
B ina r y
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
L a be l
T E MP 0
C O N F IG
T _HY S T 0
T _S E T 0
T E MP 1
T _S E T 1
T E MP 2
T _HY S T 2
T _S E T 2
Micrel
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 operation),
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 registers
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.
T a r g e t R e g i s te r
D e s c r i p ti o n
loc a l te mpe ra ture
c onfigura tion re gis te r
loc a l te mpe ra ture hys te re s is
loc a l te mpe ra ture s e tpoint
re mote z one 1 te mpe ra ture
re mote z one 1 te mpe ra ture s e tpoint
re mote z one 2 te mpe ra ture
re mote z one 2 te mpe ra ture hys te re s is
re mote z one 2 te mpe ra ture s e tpoint
T _ H Y S T 1 re mote z one 1 te mpe ra ture hys te re s is
Table 2. MIC384 Register Addresses
September 2005
7
MIC384
MICREL [ MICREL SEMICONDUCTOR ]
