MIC284 datasheet(7/20 Pages) MICREL

7 / 20 page

September 29, 2000

MIC284

Micrel

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.

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.

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

= 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

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.

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