MAX7365 Maxim, MAX7365 Datasheet - Page 15

• Manufacturer Part Number: MAX7365
• Description: The MAX7365 is an I²C-interfaced peripheral that provides microprocessors with management of up to 56 key switches
• Manufacturer: Maxim

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 unex-
pected 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 generally
increments after each data byte is written or read
5). Autoincrement only functions when doing a 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
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—
Table 5. Autoincrement Rules
Keys FIFO
Autosleep
All other key
All other GPIOs
REGISTER
FUNCTION
Command Address Autoincrementing
Operation with Multiple Masters
Applications Information
���������������������������������������������������������������� Maxim Integrated Products 15
0x01 to 0x05
0x30 to 0x48
CODE (hex)
ADDRESS
0x00
0x06
Ghost-Key Elimination
Key-Switch Controller with GPIO Ports
AUTOINCREMENT
ADDRESS (hex)
Reset from I
Addr + 0x01
Addr + 0x01
0x00
0x00
(Table
2
C
1MHz I
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.
Figure 8. Ghost-Key Phenomenon
Figure 9. Valid Three-Key Combinations
(Figure
KEY-SWITCH MATRIX
2
EXAMPLES OF VALID THREE-KEY COMBINATIONS
9). There is no limit on the number of keys
C-Interfaced 8 x 7
KEY-SWITCH MATRIX
REGULAR KEYPRESS
EVENT
(Figure
KEY-SWITCH MATRIX
MAX7365
8). Because the key
GHOST-KEY
EVENT