Datasheet

Accessing the MAX7321
The MAX7321 is accessed through an I
2
C interface. The
transition flags are cleared, and INT is deasserted each
time the device acknowledges the I
2
C slave address.
A single-byte read from the MAX7321 returns the status
of the eight I/O ports.
A 2-byte read returns first the status of the eight I/O ports
(as for a single-byte read), followed by the transition flags.
A multibyte read (more than 2 bytes before the I
2
C
STOP bit) repeatedly returns the port data, alternating
with the transition flags. As the port data is resampled
for each transmission, and the transition flags are reset
each time, a multibyte read continuously returns the
current data and identifies any changing ports.
If a port data change occurs during the read sequence,
INT is reasserted after the I
2
C STOP bit. The MAX7321
does not generate another interrupt during a single-byte
or multibyte read.
Port data is sampled during the preceding I
2
C acknowl-
edge bit (the acknowledge bit for the I
2
C slave address in
the case of a single-byte or 2-byte read).
A single-byte write to the MAX7321 sets the logic state
of all eight I/O ports.
A multibyte write to the MAX7321 repeatedly sets the
logic state of all eight I/O ports.
Reading from the MAX7321
A read from the MAX7321 starts with the master
transmitting the MAX7321’s slave address with the R/W
bit set high. The MAX7321 acknowledges the slave
address, and samples the ports during the acknowl-
edge bit. INT deasserts during the slave address
acknowledge.
Typically, the master reads 1 or 2 bytes from the
MAX7321, each byte being acknowledged by the master
upon reception with the exception of the last byte.
When the master reads 1 byte from the MAX7321 and
subsequently issues a STOP condition (Figure 6), the
MAX7321 transmits the current port data, clears the
change flags, and resets the transition detection. INT
deasserts during the slave acknowledge. The new
snapshot data is the current port data transmitted to the
master; therefore, port changes ocurring during the
transmission are detected. INT remains high until the
STOP condition.
The master can read 2 bytes from the MAX7321 and
then issue a STOP condition (Figure 7). In this case,
the MAX7321 transmits the current port data, followed
by the change flags. The change flags are then cleared,
and transition detection resets. INT goes high (high
impedance if an external pullup resistor is not fitted) during
the slave acknowledge. The new snapshot data is the
current port data transmitted to the master; therefore, port
changes occurring during the transmission are detected.
INT remains high until the STOP condition.
Figure 4. Acknowledge
Figure 3. Bit Transfer
SCL
SDA BY
TRANSMITTER
CLOCK PULSE
FOR ACKNOWLEDGMENT
START
CONDITION
SDA BY
RECEIVER
1 2 8 9
S
SDA
SCL
DATA LINE STABLE;
DATA VALID
CHANGE OF DATA
ALLOWED
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Maxim Integrated │ 10
MAX7321 I
2
C Port Expander with 8 Open-Drain I/Os