ADT7476AARQZ-R datasheet(12/69 Pages) ONSEMI

12 / 69 page

ADT7476A

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Alert Response Address

Alert response address (ARA) is a feature of SMBus

devices, allowing an interrupting device to identify itself to

the host when multiple devices exist on the same bus.

The SMBALERT output can be used as either an interrupt

output or an SMBALERT. One or more outputs can be

connected to a common SMBALERT line connected to the

master. If a device’s SMBALERT line goes low, the

following procedure occurs:

1. SMBALERT is pulled low.

2. The master initiates a read operation and sends the

alert response address (ARA = 0001 100). This is

a general call address that must not be used as a

specific device address.

3. The device whose SMBALERT output is low

responds to the alert response address, and the

master reads its device address. The address of this

device is now known and can be interrogated per

usual.

4. If more than one device’s SMBALERT output is

low, the one with the lowest device address has

priority in accordance with normal SMBus

arbitration.

5. Once the ADT7476A responds to the alert

response address, the master must read the status

registers, and SMBALERT is cleared only if the

error condition goes away.

SMBus Timeout

The ADT7476A includes an SMBus timeout feature. If

there is no SMBus activity for 35 ms, the ADT7476A

assumes the bus is locked and releases the bus. This prevents

the device from locking or holding the SMBus expecting

data. Some SMBus controllers cannot handle the SMBus

timeout feature, so if necessary, it can be disabled.

Table 6. CONFIGURATION REGISTER 1 (REG. 0x40)

Bit

Description

[6] TODIS

0: SMBus Timeout Enabled (Default)

1: SMBus Timeout Disabled

Virus Protection

To prevent rogue programs or viruses from accessing

critical ADT7476A register settings, the lock bit can be set.

Setting Bit 1 of Configuration Register 1 (0x40) sets the

lock bit and locks critical registers. In this mode, certain

registers can no longer be written to until the ADT7476A is

powered down and powered up again. For more information

on which registers are locked see Table 49.

Voltage Measurement Input

The ADT7476A has four external voltage measurement

channels. It can also measure its own supply voltage, VCC.

Pin 20 to Pin 23 can measure 5.0 V, 12 V, and 2.5 V

supplies, and the processor core voltage VCCP (0 V to 3 V

input). The VCC supply voltage measurement is carried out

through the VCC pin (Pin 4). The 2.5 V input can be used to

monitor a chipset supply voltage in computer systems.

Analog-to-Digital Converter

All analog inputs are multiplexed into the on-chip,

successive-approximation,

analog-to-digital

converter,

which has a resolution of 10 bits. The basic input range is 0 V

to 2.25 V, but the inputs have built-in attenuators to allow

measurement of 2.5 V, 3.3 V, 5.0 V, 12 V, and the processor

core voltage VCCP without any external components. To

allow the tolerance of these supply voltages, the ADC

produces an output of 3/4 full scale (768 dec or 300 hex) for

the nominal input voltage, giving it adequate headroom to

cope with overvoltages.

Input Circuitry

The internal structure for the analog inputs is shown in

Figure 24 The input circuit consists of an input protection

diode, an attenuator, plus a capacitor to form a first-order

low-pass filter that gives input immunity to high frequency

noise.

Figure 24. Structure of Analog Inputs

17.5 k

52.5 k

VCCP

35 pF

45 k

94 k

+2.5VIN

30 pF

68 k

71 k

VCC

30 pF

93 k

47 k

+5VIN

30 pF

183.6 k

30 k

+12VIN

30 pF

MUX

Table 7. VOLTAGE MEASUREMENT REGISTERS

Description

Default

0x20

2.5 V Reading

0x00

0x21

VCCP Reading

0x00

0x22

VCC Reading

0x00

0x23

5.0 V Reading

0x00

0x24

12 V Reading

0x00

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