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AD7992

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ALERT/BUSY PIN

The ALERT/BUSY pin may be configured as an ALERT output

or BUSY output, as shown in Table 11.

SMBus ALERT

The AD7992 ALERT output is an SMBus interrupt line for

devices that want to trade their ability to master for an extra

pin. The AD7992 is a slave-only device and uses the SMBus

ALERT to signal the host device that it wants to talk. The

SMBus ALERT on the AD7992 is used as an out-of-conversion-

range indicator (a limit violation indicator).

The ALERT pin has an open-drain configuration that allows the

ALERT outputs of several AD7992s to be wire-AND’ed together

when the ALERT pin is active low. D0 of the configuration

The power-up default is active low. The ALERT function can be

disabled or enabled by setting D2 of the configuration register

to 0 or 1, respectively.

The host device can process the ALERT interrupt and simul-

taneously access all SMBus ALERT devices through the alert

response address. Only the device that pulled the ALERT low

acknowledges the ARA (alert response address). If more than

one device pulls the ALERT pin low, the highest priority (lowest

address) device wins communication rights via standard I2C

arbitration during the slave address transfer.

The ALERT output becomes active when the value in the

conversion result register exceeds the value in the DATAHIGH

selected channel. It is reset when a write operation to the

configuration register sets D1 to a 1, or when the conversion

result returns N LSBs below or above the value stored in the

DATAHIGH register or DATALOW register, respectively. N is the

value in the hysteresis register (see the Limit Registers section).

The ALERT output requires an external pull-up resistor that

can be connected to a voltage different from VDD provided the

maximum voltage rating of the ALERT output pin is not

exceeded. The value of the pull-up resistor depends on the

application, but should be as large as possible to avoid excessive

sink currents at the ALERT output.

PLACING THE AD7992-1 INTO HIGH SPEED MODE

High speed mode communication commences after the master

addresses all devices connected to the bus with the master code,

00001XXX, to indicate that a high speed mode transfer is to

begin. No device connected to the bus is allowed to acknowl-

edge the high speed master code; therefore, the code is followed

by a not acknowledge (see Figure 30). The master must then

issue a repeated start followed by the device address with a R/W

bit. The selected device then acknowledges its address.

All devices continue to operate in high speed mode until the

master issues a STOP condition. When the STOP condition is

issued, the devices all return to fast mode.

THE ADDRESS SELECT (AS) PIN

The address select pin on the AD7992 is used to set the I2C

address for the AD7992 device. The AS pin can be tied to VDD,

to AGND, or left floating. The selection should be made as close

as possible to the AS pin; avoid having long tracks introducing

extra capacitance onto the pin. This is important for the float

selection, because the AS pin has to charge to a midpoint after

the start bit during the first address byte. Extra capacitance on

the AS pin increases the time taken to charge to the midpoint

and may cause an incorrect decision on the device address.

When the AS pin is left floating, the AD7992 can work with a

capacitive load up to 40 pF.

SDA

ACK. BY

AD7992

START BY

MASTER

HS-MODE MASTER CODE

SERIAL BUS ADDRESS BYTE

NACK

SCL

FAST MODE

HIGH SPEED MODE

Figure 30. Placing the Part into High Speed Mode

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