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MIC184

Local/Remote Thermal Supervisor

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MIC184

the MIC184 may then be clocked out. There is one excep-

tion to this rule: If the location latched in the pointer register

from the last write operation is known to be correct (i.e.,

points to the desired register), then the “RECEIVE_DATA”

procedure may be used. To perform a RECEIVE_DATA, the

host sends an address byte to select the slave MIC184, and

then retrieves the appropriate number (one or two) of data

bytes. Figures 1 through 5 show the formats for these data

read and data write procedures.

The command byte is 8 bits (1 byte) wide. This byte carries

the address of the MIC184 register to be operated upon, and

is stored in the MIC184’s pointer register. The pointer register

is a write-only register, which is implemented for backward

compatibility to the National Semiconductor LM75 and similar

devices. The command byte (pointer register) values corre-

sponding to the various MIC184 register addresses are shown

in Table 2. Command byte values other than 0000 00XXb =

00h through 03h are reserved, and should not be used.

The CONFIG register is 8 bits (1 byte) wide. Therefore, com-

munications with the CONFIG register will at a minimum require

a READ_BYTE, WRITE_BYTE, or a RECEIVE_BYTE.

The TEMP, T_HYST, and T_SET registers are logically

nine bits wide. Note, though, that these registers are physi-

cally two bytes (one SMBus word) wide within the MIC184.

Properly communicating with the MIC184 involves a 16-bit

READ_WORD or RECEIVE_WORD from, or WRITE_WORD

to, these registers. This is a requirement of the I2C/SMBus

serial data protocols, which only allow data transfers to occur

in multiples of eight bits.

Temperature Data Format

The LSB of each 9-bit logical register represents 0.5°C. The

values are in a two’s complement format, wherein the most

significant bit (D8) represents the sign: “0” for positive tem-

peratures and “1” for negative temperatures. The seven least

significant bits of each 16-bit physical register are undefined.

Therefore, physical bits D6 through D0 of the data read

from these registers must be masked off, and the resulting

binary value right justified before using the data received.

It is also possible to read only the first byte of any of these

three registers, sacrificing 0.5°C of resolution in exchange

for somewhat simpler data handling. However, all writes

to the T_SET and T_HYST registers must be in the 16-bit

WRITE_WORD format. Table 3 shows examples of the data

format used by the MIC184 for temperatures.

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A/D Converter Timing

Whenever the MIC184 is not in its low power shutdown mode,

the internal A/D converter (ADC) attempts to make continuous

conversions unless interrupted by a bus transaction accessing

the MIC184. When the MIC184 is accessed, the conversion

in progress will be halted, and the partial result discarded.

When the access of the MIC184 is complete the ADC will

begin a new conversion cycle, with results valid tCONV after

that. Figure 6 shows this behavior. tCONV is twice as long for

external conversions as it is for internal conversions. This al-

lows the use of a filter capacitor on the A2/T1 input without a

loss of accuracy due to the resulting longer settling times.

Power-On

When power is initially applied, the MIC184’s internal registers

are set to default states which make the MIC184 completely

backward compatible with the LM75. Also at this time, the

levels on the address inputs A2, A1, and A0 are read to es-

tablish the device’s slave address. The MIC184’s power-up

default state can be summarized as follows:

• Normal-mode operation

(MIC184 not in shutdown)

• ZONE is set to internal

(on-chip temperature sensing)

• INT function is set to comparator mode

• INT output is set to active-low operation

• Fault_Queue depth = 1

• Interrupts are enabled (IM = 0)

• T_SET = +80°C; T_HYST = +75°C

In order to accommodate the use of A2/T1 as a dual-purpose

input, there is a weak pulldown on A2/T1 that will attempt to

sink ≈25µA from the pin to ground for tPOR following power-

up of the MIC184. This allows the MIC184 to pull A2/T1 to a

low state when a diode junction is connected from that pin

to ground, and latch a zero as the A2 address value. If A2 is

not to be used as a diode connection, it should be connected

to VDD or ground. Note that a fault in the external tempera-

ture sensor (if used) may not be reported until one or more

conversion cycles have been completed following power-on.

See DIODE FAULTS.

Shutdown Mode

Setting the SHDN bit in the configuration register halts the

otherwise continuous conversions by the A/D converter. The

MIC184

T emperature

+125° C

+25° C

+0.5° C

0° C

–0.5° C

–25° C

–40° C

–55° C

R aw B inary

Mas ked B inary

0111 1101 0XXX XXXX

0001 1001 0XXX XXXX

0000 0000 1XXX XXXX

0000 0000 0XXX XXXX

1111 1111 1XXX XXXX

1110 0111 0XXX XXXX

1101 1000 0XXX XXXX

1100 1001 0XXX XXXX

0 1111 1010b

0 0011 0010b

0 0000 0001b

0 0000 0000b

1 1111 1111b

1 1100 1110b

1 1011 0000b

1 1001 0010b

Table 3. Digital Temperature Format

10

Mas ked Hex

0F Ah

032h

001h

000h

1F F h

1C E h

1B 0h

192h

May 2006

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