ADT7462ACPZ-R7 datasheet(26/81 Pages) ONSEMI

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ADT7462

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Status and Mask Registers and ALERT

Status Registers

Each measured temperature and voltage has an associated

high and low limit. The measured values are compared with

these programmable limits. The results of these

comparisons are stored in the status registers. A Logic 0 in

the status register represents an in−limit comparison, while

a Logic 1 represents an out−of−limit comparison.

Once a status bit is set, it remains set until the status

bit is cleared if the error condition has gone away. The status

registers are duplicated to accommodate situations where

there are two SMBus masters. If one master reads the host

status registers and consequently clears them, the second

master has no way of knowing what bits were set and what

bits were cleared. The second SMBus master can read from

the duplicate BMC status registers to determine which status

bits were set.

Table 19 is a list of the status registers and corresponding

addresses.

Table 19. Status Registers

Host

Address

BMC

Address

Thermal Status Register 1

0xB8

0xC0

Thermal Status Register 2

0xB9

0xC1

Thermal Status Register 3

0xBA

−

Voltage Status Register 1

0xBB

0xC3

Voltage Status Register 2

0xBC

0xC4

Fan Status Register 1

0xBD

0xC5

Digital Status Register 1

0xBE

0xC6

GPIO Status Register

0xBF

−

ALERT Output

The ADT7462 has an SMBus ALERT output that is

asserted when one of the status bits is set. This is to alert the

master that an out−of−limit measurement has taken place or

that there is a fault on one of the fan channels.

An ALERT is generated as a result of a status bit being set

in any of the registers.

Figure 38. ALERT and Status Bit Behavior

HIGH LIMIT

TEMPERATURE

STICKY STATUS BIT

TEMP BACK IN LIMIT

(STATUS BIT STAYS SET)

CLEARED ON READ

(TEMP BELOW LIMIT)

SMBALERT

Figure 38 shows how the ALERT output and “sticky”

status bits behave. When a limit is exceeded, the

corresponding status bit is set to 1. The status bit remains set

until the error condition goes away and the status register is

read. The status bits are referred to as sticky because they

remain set until read by software. This ensures that an

out−of−limit event cannot be missed, if software is polling

the device periodically. Note that the ALERT output remains

low for the entire duration that a reading is out of limit and

until the status register has been read.

Mask Registers

The user has the option of masking any of the individual

status bits that generate an ALERT. This is achieved by

setting the appropriate bit in the mask registers. The ALERT

output is not asserted on the setting of a status bit if it has

been masked. The status bit itself is not affected and

continues to be set when an out−of−limit condition exists.

Table 20 is a list of the mask registers and corresponding

addresses.

Table 20. Mask Registers

Thermal Mask Register 1

0x30

Thermal Mask Register 2

0x31

Voltage Mask Register 1

0x32

Voltage Mask Register 2

0x33

Fan Mask Register

0x34

Digital Mask Register

0x35

GPIO Mask Register

0x36

Fan Control

Fan Drive Using PWM Control

The ADT7462 uses pulse−width modulation (PWM) to

control fan speed. Control relies on varying the duty cycle

(or on/off ratio) of a square wave applied to the fan to vary

the fan speed. The advantage of using PWM control is that

it uses a very simple external circuit. The specific circuit

used depends upon the type of fan.

There are three main fan types in use: 2−wire fans, 3−wire

fans, and 4−wire fans. The 2−wire fan has only power and

ground connections. The 3−wire fan has power and ground

connections and a TACH output to indicate the speed of the

fan. The 4−wire fan has power and ground connections, a

TACH output, and a PWM input. The PWM input is

connected directly to the PWM drive of the ADT7462 and

is used to control the speed of the fans.

For 2−wire and 3−wire fans, the low frequency PWM

drive signal should be selected. For 4−wire fans, the high

frequency PWM drive signal should be selected.

Using the ADT7462 with 2−Wire Fans

Figure 39 shows the most typical circuit used with a

2−wire fan and illustrates how a 2−wire fan can be connected

to the ADT7462. The low frequency PWM mode must be

selected when using a 2−wire fan.

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