Datasheet

AD7993/AD7994
Rev. 0 | Page 27 of 32
ALERT/BUSY PIN
The ALERT/BUSY pin may be configured as an alert output or
busy output, as shown in Table 12.
SMBus ALERT
The AD7993/AD7994 alert output is an SMBus interrupt line
for devices that want to trade their ability to master for an extra
pin. The AD7993/AD7994 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 AD7993/AD7994 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 AD7993/AD7994s 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
configuration register to 1 or 0, respectively.
The host device can process the ALERT interrupt and
simultaneously 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
I
2
C arbitration during the slave address transfer.
The ALERT output becomes active when the value in the
conversion result register exceeds the value in the DATA
HIGH
register or falls below the value in the DATA
LOW
register. It is
reset when a write operation to the configuration register sets
D1 and D0 to a 1, or when the conversion result returns N LSB
below or above the value stored in the DATA
HIGH
register or
DATA
LOW
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 V
DD
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.
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 AD7993-1/AD7994-1 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 31). 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 AD7993/AD7994 is used to set
the I
2
C address for the AD7993/AD7994 device. The AS pin
can be tied to V
DD
, 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 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
AD7993/AD7994 can work with a capacitive load up to 40 pF.
SDA
ACK. BY
AD7993/AD7994
START BY
MASTER
HS MODE MASTER CODE SERIAL BUS ADDRESS BYTE
NACK
191 9
0 1 A2 A1 A0XX1000
SCL
0
0
A3
X
Sr
FAST MODE
HIGH SPEED MODE
03472-0-031
Figure 31. Placing the Part into High Speed Mode