Rainbow Electronics MAX7360 - Manual (Page 17)

I 2 C-Interfaced Key-Switch Controller and LED
Driver/GPIOs with Integrated ESD Protection
MAX7360
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Figure 10. Ghost-Key Phenomenon Figure 11. Valid Three-Key Combinations
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 10). Because the key
appears to be pressed electrically, it is impossible to
detect which of the four keys is the ghost key.
The MAX7360 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
rectangle (Figure 11). 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 FIFO is not full.
Low-EMI Operation
The MAX7360 uses two techniques to minimize EMI
radiating from the key-switch wiring. First, the voltage
across the switch matrix never exceeds +0.55V 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 +0.55V maximum. 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.
Switch On-Resistance
The MAX7360 is designed to be insensitive to resistance,
either in the key switches, or the switch routing to and
from the appropriate COL_ and ROW_ up to 4kI (max).
These controllers are therefore compatible with low-cost
membrane and conductive carbon switches.
Hot Insertion
The INTI, INTK, SCL, and AD0 inputs and SDA remain
high impedance with up to +3.6V asserted on them when
the MAX7360 powers down (V
CC
= 0). I/O ports (PORT0–
PORT7) remain high impedance with up to +14V asserted
on them when not powered. Use the MAX7360 in hot-
swap applications.
Staggered PWM
The LED’s on-time in each PWM cycle are phase
delayed 45N into eight evenly spaced start positions.
Optimize phasing when using fewer than eight ports as
constant-current outputs by allocating the ports with the
most appropriate start positions. For example, if using
four constant-current outputs, choose PORT0, PORT2,
PORT4, and PORT6 because their PWM start positions
are evenly spaced. In general, choose the ports that
spread the PWM start positions as evenly as possible.
This optimally spreads out the current demand from the
ports’ load supply.
INTK/INTI
There are two interrupt outputs, INTK and INTI. Each
interrupt operates independently from the other. See
the Key-Switch Interrupt Register (0x03) and the GPIO
Port Interrupts (INTI) sections for additional information
regarding these two interrupts.
Power-Supply Considerations
The MAX7360 operates with a +1.62V to +3.6V power-
supply voltage. Bypass the power supply to GND with a
0.1FF or higher ceramic capacitor as close as possible
to the device.
REGULAR KEYPRESS
EVENT
GHOST-KEY
EVENT
KEY-SWITCH MATRIX
KEY-SWITCH MATRIX KEY-SWITCH MATRIX
EXAMPLES OF VALID THREE-KEY COMBINATIONS