ADT7462ACPZ-R7 datasheet(23/81 Pages) ONSEMI

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ADT7462

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Voltage Measurement and Limit Registers

The corresponding register locations for voltage

measurements are listed in Table 13. Each voltage

measurement channel has a high and low voltage limit

associated with it. The voltage measurements are compared

with these limits. The results of these comparisons are stored

in status registers. A Logic 0 indicates an in−limit condition,

and a Logic 1 indicates an out−of−limit condition. The

ADT7462 can generate an ALERT, if configured to do so,

when a status bit is set. For more information on the status

registers and ALERT, see the Status and Mask Registers

ALERT section. A complete list of all the high and low

voltage limits in the ADT7462 and their default values is

contained in Table 13.

Table 13. Voltage Value and Limit Registers

Low Limit

High Limit

Voltage Value

Pin No.

Value Register Address

Default

+12V1

0xA3

0x6D

0x00

0x7C

0xFF

+12V2

0xA5

0x6E

0x00

0x7D

0xFF

+3.3V

0x96

0x70

0x00

0x68

0xFF

+1.8V or +2.5V

0x8B

0x45

0x40

0x49

0x95

+1.25V or +0.9V

0x8F

0x47

0x40

0x4B

0x95

+5V

0xA7

0x71

0x00

0x7E

0xFF

+12V3

0xA9

0x6F

0x00

0x7F

0xFF

VCCP1, +1.5V, +1.8V, +2.5V

0x90

0x72

0x20

0x69

0xFF

VCCP2, +1.5V, +1.8V, +2.5V

0x91

0x73

0x00

0x6A

0xFF

+1.2V1 (GBIT) or +3.3V

0x92

0x74

0x00

0x6B

0xFF

+1.2V2 (FSB_VTT) or VBATT

0x93

0x75

0x80

0x6C

0xFF

+1.5V1 (ICH)

0x94

0x76

0x00

0x50

0xA4

+1.5V2 (3GIO)

0x95

0x77

0x00

0x4C

0xA4

Battery Measurement Input (VBATT)

The VBATT input allows the condition of a CMOS backup

battery to be monitored. This is typically a lithium coin cell,

such as a CR2032. The VBATT input is accurate only for

voltages greater than 1.2 V. Note that when Pin 26 is

configured as a +1.2V input, voltages lower than 1.2 V are

not accurately measured. Input voltage and corresponding

voltage measured are shown in Figure 16.

Typically, the battery in a system is required to keep some

devices powered on when the system is in a powered−off

state. The VBATT measurement input is designed to minimize

battery drain. To reduce current drain from the battery, the

lower resistor of the VBATT attenuator is not connected,

except when a VBATT measurement is being made. The total

current drain on the VBATT pin is 80 nA typical (for a

maximum VBATT voltage = 4.0 V), so a CR2032 CMOS

battery functions in a system in excess of the expected 10

years. Note that when a VBATT measurement is not being

made, the current drain is reduced to 16 nA typical. Under

normal voltage measurement operating conditions, all

measurements are made in a round−robin format, and each

reading is actually the result of 16 digitally averaged

measurements. However, averaging is not carried out on the

VBATT measurement to reduce measurement time and,

therefore, reduce the current drain from the battery.

The VBATT current drain when a measurement is being

made is calculated by:

I +

VBATT

100 kW

tpulse

tperiod

where:

tPULSE is the VBATT measurement time (~711 ms typical).

tPERIOD is the time required to measure all analog inputs.

Monitoring cycle time depends on the ADT7462

configuration. Calculating the monitoring cycle time is

described in more detail in the ADC Information section.

VBATT Input Battery Protection

In addition to minimizing battery current drain, the VBATT

measurement circuitry is specifically designed with battery

protection in mind. Internal circuitry prevents the battery

from being back−biased by the ADT7462 supply or through

any other path under normal operating conditions. In the

unlikely event of a catastrophic ADT7462 failure, the

ADT7462 includes a second level of battery protection,

including a series 3 k

W resistor to limit current to the battery,

as recommended by UL (see Figure 37). Thus, it is not

necessary to add a series resistor between the battery and the

VBATT input; the battery can be connected directly to the

VBATT input to improve voltage measurement accuracy.

ADT7462ACPZ-R7 Datasheet(PDF) 23 Page - ON Semiconductor

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