AD7997 datasheet(27/32 Pages) AD

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AD7997/AD7998

Rev. 0 | Page 27 of 32

ALERT/BUSY PIN

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

as a busy output, as shown in Table 12.

SMBus ALERT

The AD7997/AD7998 ALERT output is an SMBus interrupt line

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

pin. The AD7997/AD7998 is a slave-only device that uses the

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

The SMBus ALERT on the AD7997/AD7998 is used as an out-

of-range indicator (a limit violation indicator).

The ALERT pin has an open-drain configuration that allows

the ALERT outputs of several AD7997/AD7998s to be wired-

AND’ed together when the ALERT pin is active low. D0 of the

configuration register is used to set the active polarity of the

ALERT output. The power-up default is active low. The ALERT

function can be enabled or disabled by setting D2 of the con-

figuration register to 1 or 0, respectively.

The host device can process the alert interrupt and simultane-

ously access all SMBus alert devices through the alert response

address. Only the device that pulled the alert low acknowledges

the alert response address (ARA). 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 LSB below or above the value stored in the

DATAHIGH register or the 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 maxi-

mum 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.

BUSY

When the ALERT/BUSY pin is configured as a BUSY output the

pin is used to indicate when a conversion is taking place. The

polarity of the BUSY pin is programmed through bit D0 in the

Configuration register.

PLACING THE AD7997-1/AD7998-1 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 acknowledge

the high speed master code; therefore, the code is followed by a

not-acknowledge (see Figure 31). The master must then issue a

repeated start followed by the device address with an R/W bit.

The selected device then acknowledges its address.

All devices continue to operate in high speed mode until such a

time as the master issues a stop condition. When the stop condi-

tion is issued, the devices all return to fast mode.

THE ADDRESS SELECT (AS) PIN

The address select pin on the AD7997/AD7998 is used to set

the I2C address for the AD7997/AD7998 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 on to the pin. This is

important for the float selection, as 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 AD7997/AD7998

can work with a capacitive load up to 40 pF.

SDA

ACK. BY

AD7997/AD7998

START BY

MASTER

HS MODE MASTER CODE

SERIAL BUS ADDRESS BYTE

NACK

SCL

FAST MODE

HIGH SPEED MODE

Figure 31. Placing the Part into High Speed Mode

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