MAX7321
case of a bus lockup, terminating any serial transaction
to or from the MAX7321.
Any port can be configured as a logic input by setting
the port output logic-high (logic-high for an open-drain
output is high impedance). When the MAX7321 is read
through the serial interface, the actual logic levels at
the ports are read back.
The open-drain ports offer latching transition detection
when used as inputs. All input ports are continuously
monitored for changes. An input change sets 1 of 8 flag
bits that identify changed input(s). All flags are cleared
upon a subsequent read or write transaction to the
MAX7321.
A latching interrupt output, INT, is programmed to flag
logic changes on ports used as inputs. Data changes
on any input port forces INT to a logic-low. Changing
the I/O port level through the serial interface does not
cause an interrupt. The interrupt output INT is deassert-
ed when the MAX7321 is next accessed through the
serial interface.
Internal pullup resistors to V+ are selected by the
address select inputs, AD0 and AD2. Pullups are
enabled on the input ports in groups of four (see Table 3).
Use the slave address selection to ensure that I/O ports
used as inputs are logic-high on power-up. I/O ports with
internal pullups enabled default to a logic-high output
state. Ports with internal pullups disabled default to a
logic-low output state. Output port power-up logic
states are selected by the address select inputs AD0
and AD2. Ports default to logic-high or logic-low on
power-up in groups of four (see Table 3).
Initial Power-Up
On power-up, the transition detection logic is reset, and
INT is deasserted. The transition flags are cleared to
indicate no data changes. The power-up default states
of the eight I/O ports are set according to the I
2
C slave
address selection inputs, AD0 and AD2 (Table 3). For
I/O ports used as inputs, ensure that the default
states are logic-high so that the I/O ports power up
in the high-impedance state. All I/O ports configured
with pullups enabled also have a logic-high power-
up state.
Power-On Reset
The MAX7321 contains an integral power-on reset
(POR) circuit that ensures all registers are reset to a
known state on power-up. When V+ rises above V
POR
(1.6V max), the POR circuit releases the registers and
2-wire interface for normal operation. When V+ drops to
less than V
POR
, the MAX7321 resets all register con-
tents to the POR defaults (Table 3).
I
2
C Port Expander with 8 Open-Drain I/Os
6 _______________________________________________________________________________________
detection interrupt and selectable pullups.
levels.
detection interrupt and pullups on all ports.
All ports power up as inputs (or logic-high outputs).
open-drain output to logic-high.
