MAX7347/MAX7348/MAX7349
2-Wire Interfaced Low-EMI Key Switch
and Sounder Controllers
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too many interrupts. The key-switch status can be
checked at any time by reading the key-switch FIFO. A
1-byte read access returns both the first key-press
event in the FIFO (if there is one) and the FIFO status,
so it is easy to operate the MAX7347/MAX7348/
MAX7349 by polling. If the INT pin is not required, it
can be configured as an open-drain general-purpose
output (GPO) capable of driving an LED.
The MAX7349 monitors up to 64 keys. The MAX7348
monitors up to 40 keys. The MAX7347 monitors up to
24 keys (Table 1).
If the application requires fewer keys to be scanned, up
to six of the key-switch outputs can be configured as
open-drain GPOs capable of driving LEDs. For each
key-switch output used as a GPO, the number of key
switches that can be scanned is reduced by eight.
An alert logic input (MAX7349 only) can be configured
to deliver an automatic, customizable sound and/or an
interrupt on every falling edge of the logic input. The
logic state of the alert input can be read at any time.
Tone Generator
The piezo sounder controller generates a square wave
with the frequency of a musical tone under processor con-
trol. The selection of tones covers the 5th musical octave
(523.25Hz to 987.77Hz), plus seven other notes up to
2637Hz. The sounder output is also programmable to be
either high or low for the entire sound duration to operate
an electronic sounder, relay, or lamp instead of a piezo
transducer. The sound duration is programmable from
15.625ms in seven binary steps up to a maximum of 1s.
The piezo sounder controller interface uses a single 1-
byte access to its own separate slave address.
Commands are double-buffered to allow two commands
(2 bytes) to be stored and executed in succession. The
sounder controller performs the transition between
queued sound commands without click artifacts. The
controller can also autoloop between the two most
recent commands. Autolooping allows a wide range of
intermittent and two-tone sounds to be initiated, and
then run automatically without further intervention.
Key-Scan Controller
Key inputs are scanned statically, not dynamically, to
ensure low-EMI operation. As inputs only toggle in
response to switch changes, the key matrix can be
routed closer to sensitive circuit nodes.
The key controller debounces and maintains a FIFO of
key-press events (including autorepeated key presses,
if autorepeat is enabled). Figure 1 shows keys order.
Serial Interface
Figure 2 shows the 2-wire serial interface timing details.
Serial Addressing
The MAX7347/MAX7348/MAX7349 operate as slaves
that send and receive data through an I
2
C-compatible
2-wire interface. The interface uses a serial data line
(SDA) and a serial clock line (SCL) to achieve bidirec-
tional communication between master(s) and slave(s).
A master (typically a microcontroller) initiates all data
transfers to and from the MAX7347/MAX7348/MAX7349
and generates the SCL clock that synchronizes the
data transfer.
The MAX7347/MAX7348/MAX7349s’ SDA line operates
as both an input and an open-drain output. A pullup
resistor, typically 4.7kΩ, is required on SDA. The
MAX7347/MAX7348/MAX7349s’ SCL line operates only
as an input. A pullup resistor, typically 4.7kΩ, is required
on SCL if there are multiple masters on the 2-wire inter-
face, or if the master in a single-master system has an
open-drain SCL output.
Each transmission consists of a START condition
(Figure 3) sent by a master, followed by the MAX7347/
MAX7348/MAX7349 7-bit slave address plus R/W bit, a
register address byte, 1 or more data bytes, and finally
a STOP condition.
Start and Stop Conditions
Both SCL and SDA remain high when the interface is
not busy. A master signals the beginning of a transmis-
sion with a START (S) condition by transitioning SDA
from high to low while SCL is high. When the master
has finished communicating with the slave, it issues a
