
AD7992
–18–
REV. PrH
PRELIMINARY TECHNICAL DATA
Figure 14. Reading two bytes of data from the Conversion
Result Register
Figure 13. Reading a single byte of data from a selected
register
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.
The ALERT pin has an open-drain configuration which
allows the ALERT outputs of several AD7992 devices to
be wired-AND 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 de-
fault is active low. The ALERT function can be disabled
or enabled 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 which pulled
the ALERT low will acknowledge the ARA (Alert Re-
sponse Address). If more than one device pulls the
ALERT pin low, the highest priority (lowest address)
device will win 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 to a 1, or when the
conversion result returns N LSBs 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
Limit Registers section)
The ALERT output requires an external pull-up resistor.
This can be connected to a voltage different from V
DD
provided the maximum voltage rating of the ALERT out-
put pin is not exceeded. The value of the pull-up resistor
depends on the application, but should be as large as pos-
sible to avoid excessive sink currents at the ALERT out-
put.
SDA
NO ACK. BY
MASTER
START BY
MASTER
FRAME 1
SERIAL BUS ADDRESS BYTE
FRAME 2
SINGLE DATA BYTE FROM AD7992
ACK. BY
AD7992
1
9
1
9
D7
D6
D5
D2
D1
D0
R/
A0
A1
A2
A3
0
1
SCL
STOP BY
MASTER
D4
D3
0
SDA
ACK. BY
MASTER
START BY
MASTER
FRAME 1
SERIAL BUS ADDRESS BYTE
FRAME 2
MOST SIGNIFICANT DATA BYTE FROM AD7992
ACK. BY
AD7992
1
9
1
9
Alert_
Flag
D10
D9
D8
R/
A0
A1
A2
A3
0
0
SCL
1
D11
NO ACK. BY
MASTER
FRAME 3
LEAST SIGNIFICANT DATA BYTE FROM AD7992
1
9
D7
D6
D5
D2
D1
D0
STOP BY
MASTER
D4
D3
SCL (CONTINUED)
SDA (CONTINUED)
CH ID0
0
0