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ADM1029ARQ

Dual PWM Fan Controller and Temperature Monitor for High Availability Systems

Analog Devices 

ADM1029

In the case of the ADM1029, write operations contain either

one or two bytes, and read operations contain one byte, and

perform the following functions:

To write data to one of the device data registers or read data

from it, the Address Pointer Register must be set so that the

correct data register is addressed, data can be written into that

register or read from it. The first byte of a write operation always

contains an address that is stored in the Address Pointer Regis-

ter. If data is to be written to the device, the write operation

contains a second data byte that is written to the register

selected by the address pointer register.

This is illustrated in Figure 4a. The device address is sent over

the bus followed by R/W set to 0. This is followed by two data

bytes. The first data byte is the address of the internal data

register to be written to, which is stored in the Address Pointer

Register. The second data byte is the data to be written to the

internal data register.

When reading data from a register there are two possibilities:

1. If the ADM1029’s Address Pointer Register value is unknown

or not the desired value, it is first necessary to set it to the

correct value before data can be read from the desired data

register. This is done by performing a write to the ADM1029

as before, but only the data byte containing the register address

is sent, as data is not to be written to the register. This is

shown in Figure 4b.

A read operation is then performed consisting of the serial

bus address, R/W bit set to 1, followed by the data byte read

from the data register. This is shown in Figure 4c.

2. If the Address Pointer Register is known to be already at the

desired address, data can be read from the corresponding

data register without first writing to the Address Pointer

Register, so Figure 4b can be omitted.

Note: although it is possible to read a data byte from a data

register without first writing to the Address Pointer Register,

if the Address Pointer Register is already at the correct value,

it is not possible to write data to a register without writing to

the Address Pointer Register, because the first data byte of a

write is always written to the Address Pointer Register.

ALERT RESPONSE ADDRESS

The ADM1029 has an interrupt (INT) output that is asserted

low when a fault condition occurs. Several INT outputs can be

wire OR’d to a common interrupt line. When the host processor

receives an interrupt request, it would normally need to read the

interrupt status register of each device to identify which device

had made the interrupt request. However, the ADM1029 sup-

ports the optional Alert Response Address function of the SMBus

protocol. When the host processor receives an interrupt request

it can send a general call address (0001100) over the bus. The

device asserting INT will then send its own slave address back

to the host processor, so the device asserting INT can be identi-

fied immediately.

If more than one device is asserting INT, all devices will try to

respond with their slave address, but an arbitration process

ensures that only the lowest address will be received by the host.

After sending its slave address, the first device will then clear its

INT output. The host can then check if the INT is still low and

send the general call again if necessary until all devices asserting

INT have responded.

The ARA function can be disabled by setting Bit 2 of the Con-

figuration Register (address 01h).

TEMPERATURE MEASUREMENT SYSTEM

LOCAL TEMPERATURE MEASUREMENT

The ADM1029 contains an on-chip bandgap temperature sensor,

whose output is digitized by the on-chip ADC. The temperature

data is stored in the Local Temp Value Register (address A0h).

As both positive and negative temperatures can be measured, the

temperature data is stored in two’s complement format, as shown

in Table II. Theoretically, the temperature sensor and ADC can

measure temperatures from –128°C to +127°C with a resolution

of 1°C, but temperatures outside the operating temperature

range of the device cannot be measured by the internal sensor.

REMOTE TEMPERATURE MEASUREMENT

The ADM1029 can measure the temperature of one or two

remote diode-connected transistors, connected to Pins 13 and

14 and/or 16 and 17. The data from the temperature measure-

ments is stored in the Remote 1 and Remote 2 Temp Value

Registers (addresses A1h and A2h).

If two remote temperature measurements are not required, Pins

16 and 17 can be reconfigured as general-purpose logic I/O

pins, as explained later.

The forward voltage of a diode or diode-connected transistor,

operated at a constant current, exhibits a negative temperature

coefficient of about –2 mV/°C. The absolute value of VBE varies

from device to device and individual calibration is required to

null this out so, unfortunately, the technique is unsuitable for

mass production.

The technique used in the ADM1029 is to measure the change

in VBE when the device is operated at two different currents.

This is given by:

where:

∆VBE = KT/q × ln(N)

K is Boltzmann’s constant

q is charge on the carrier

T is absolute temperature in Kelvins

N is ratio of the two currents

Figure 5 shows the input signal conditioning used to measure

the output of a remote temperature sensor. This figure shows

the external sensor as a substrate transistor, provided for tem-

perature monitoring on some microprocessors, but it could equally

well be a discrete transistor.

If a discrete transistor is used, the collector will not be grounded,

and should be linked to the base. If a PNP transistor is used, the

base is connected to the D– input and the emitter to the D+

input. If an NPN transistor is used, the emitter is connected to

the D– input and the base to the D+ input.

REV. 0

–9–

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