Datasheet

MAX7370
8 x 8 Key-Switch Controller and LED Driver/GPIOs
with I2C Interface and High Level of ESD Protection
18Maxim Integrated
Operation with Multiple Masters
When the device is operated on a two-wire interface with
multiple masters, a master reading the device uses a
repeated start between the write that sets the device’s
address pointer, and the read(s) that takes the data
from the location(s). This is because it is possible for
master 2 to take over the bus after master 1 has set
up the device’s address pointer but before master 1
has read the data. If master 2 subsequently resets the
device’s address pointer, master 1’s read can be from an
unexpected location.
Command Address Autoincrementing
Address autoincrementing allows the device to be
configured with fewer transmissions by minimizing the
number of times the command address needs to be
sent. The command address stored in the device gener-
ally increments after each data byte is written or read
(Table 4). Autoincrement only functions when doing a
multiburst read or write.
Applications Information
Reset from I
2
C
After a catastrophic event such as ESD discharge or
microcontroller reset, use bit D7 of the configuration reg-
ister (0x01) as a software reset for the key switches. Use
bit D4 of the GPIO global configuration register (0x40) as
a software reset for the GPIOs.
Ghost-Key Elimination
Ghost keys are a phenomenon inherent with key-switch
matrices. When three switches located at the corners
of a matrix rectangle are pressed simultaneously, the
switch that is located at the last corner of the rectangle
(the ghost key) also appears to be pressed. This occurs
because the potentials at the two sides of the ghost-key
switch are identical due to the other three connections—
the switch is electrically shorted by the combination of
the other three switches (Figure 9). Because the key
appears to be pressed electrically, it is impossible to
detect which of the four keys is the ghost key.
The device employs a proprietary scheme that detects
any three-key combination that generates a fourth ghost
key, and does not report the third key that causes a
ghost-key event. This means that although ghost keys
are never reported, many combinations of three keys
are effectively ignored when pressed at the same time.
Applications requiring three-key combinations (such as
<Ctrl><Alt><Del>) must ensure that the three keys are
not wired in positions that define the vertices of a rect-
angle (Figure 10). There is no limit on the number of keys
that can be pressed simultaneously as long as the keys
do not generate ghost-key events and the FIFO is not full.
Low-EMI Operation
The device uses two techniques to minimize EMI radiat-
ing from the key-switch wiring. First, the voltage across
the switch matrix never exceeds 0.5V if not in sleep
mode, independent of supply voltage V
CC
. This reduces
the voltage swing at any node when a switch is pressed
to 0.5V (max). Second, the keys are not dynamically
scanned, which would cause the key-switch wiring to
continuously radiate interference. Instead, the keys are
monitored for current draw (only occurs when pressed),
and debounce circuitry only operates when one or more
keys are actually pressed.
Figure 9. Ghost-Key Phenomenon Figure 10. Valid Three-Key Combinations
REGULAR KEYPRESS
EVENT
GHOST-KEY
EVENT
KEY-SWITCH MATRIX
KEY-SWITCH MATRIX KEY-SWITCH MATRIX
EXAMPLES OF VALID THREE-KEY COMBINATIONS