MAX7370 datasheet(18/37 Pages) MAXIM

18 / 37 page

�� Maxim Integrated Products 18

MAX7370

8 x 8 Key-Switch Controller and LED Driver/GPIOs

with I2C Interface and High Level of ESD Protection

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 I2C

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 VCC. 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

EXAMPLES OF VALID THREE-KEY COMBINATIONS

MAX7370 датащи(PDF) 18 Page - Maxim Integrated Products

MAX7370 датащи(HTML) 18 Page - Maxim Integrated Products

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37

датащи скачать

ссылки URL