MAX7324
I
2
C Port Expander with Eight Push-Pull Outputs
and Eight Inputs
8 _______________________________________________________________________________________
address selection determines which inputs have
pullups applied. However, at power-up, the I
2
C SDA
and SCL bus interface lines are high impedance at the
inputs of every device (master or slave) connected to
the bus, including the MAX7324. This is guaranteed as
part of the I
2
C specification. Therefore, address inputs
AD0 and AD2 that are connected to SDA or SCL during
power-up appear to be connected to V+. The pullup
selection logic uses AD0 to select whether pullups are
enabled for ports I0–I3, and uses AD2 to select whether
pullups are enabled for ports I4–I7. The rule is that a
logic-high SDA, or SCL connection selects the pullups,
while a logic-low deselects the pullups (Table 2). The
pullup configuration is correct on power-up for a stan-
dard I
2
C configuration, where SDA and SCL are pulled
up to V+ by the external I
2
C pullups.
There are circumstances where the assumption that
SDA = SCL = V+ on power-up is not true—for example,
in applications in which there is legitimate bus activity
during power-up. Also, if SDA and SCL are terminated
with pullup resistors to a different supply voltage than
the MAX7324’s supply voltage, and if that pullup supply
rises later than the MAX7324’s supply, then SDA or
SCL may appear at power-up to be connected to GND.
In such applications, use the four address combina-
tions that are selected by connecting address inputs
AD0 and AD2 to V+ or GND (shown in bold in Tables 2
and 3). These selections are guaranteed to be correct
at power-up, independent of SDA and SCL behavior. If
one of the other 12 address combinations is used, an
unexpected combination of pullups might be asserted
until the first I
2
C transmission (to any device, not neces-
sarily the MAX7324) is put on the bus.
Port Inputs
Port inputs switch at CMOS logic levels as determined
by the expander’s supply voltage, and are overvoltage
tolerant to +6V, independent of the device’s supply
voltage.
Port-Input Transition Detection
All eight input ports are monitored for changes since
the expander was last accessed through the serial
interface. The state of the input ports is stored in an
internal “snapshot” register for transition monitoring.
The snapshot is continuously compared with the actual
input conditions, and if a change is detected for any
port input, then an internal transition flag is set for that
port. The eight port inputs are sampled (internally
latched into the snapshot register) and the old transi-
tion flags cleared during the I
2
C acknowledge of every
MAX7324 read and write access. The previous port
transition flags are read through the serial interface as
the second byte of a 2-byte read sequence.
