MAX7370 Maxim, MAX7370 Datasheet - Page 18

• Manufacturer Part Number: MAX7370
• Description: The MAX7370 I²C-interfaced peripheral provides microprocessors with management of up to 64 key switches, with optional GPIO and PWM-controlled LED drivers
• Manufacturer: Maxim

with I
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.
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
multiburst read or write.
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 keys are a phenomenon inherent with key-switch
matrices. When three switches located at the corners
of a matrix rectangle are pressed simultaneously, the
Figure 9. Ghost-Key Phenomenon
8 x 8 Key-Switch Controller and LED Driver/GPIOs
4). Autoincrement only functions when doing a
Command Address Autoincrementing
2
C Interface and High Level of ESD Protection
Operation with Multiple Masters
KEY-SWITCH MATRIX
Applications Information
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REGULAR KEYPRESS
EVENT
Ghost-Key Elimination
GHOST-KEY
EVENT
Reset from I
2
C
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
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
that can be pressed simultaneously as long as the keys
do not generate ghost-key events and the FIFO is not full.
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
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 10. Valid Three-Key Combinations
(Figure
KEY-SWITCH MATRIX
EXAMPLES OF VALID THREE-KEY COMBINATIONS
10). There is no limit on the number of keys
(Figure
KEY-SWITCH MATRIX
Low-EMI Operation
MAX7370
9). Because the key
CC
. This reduces