MAX7349AEG Maxim Integrated, MAX7349AEG Datasheet - Page 23

• Manufacturer Part Number: MAX7349AEG
• Description: Peripheral Drivers & Components - PCIs Low-EMI Key Switch & Sounder Controller
• Manufacturer: Maxim Integrated

When an alert sound or key sound is happening, the user
cannot write to the sounder. The MAX7347/MAX7348/
MAX7349 do not acknowledge a write to the sounder I
address. However, a read from the sounder will work cor-
rectly. An alert sound or key sound event terminates a
current user-programmed event and clears the queue. If
an alert sound or key sound event is currently being
processed, then a new alert sound or key sound event
will be put into the queue, replacing an existing queued
alert sound or key sound event, if one exists. User access
to the sounder is restored when the last alert sound or key
sound event is completed. Note this means that the buffer
bit (D0) for an alert sound or key sound command is
effectively ignored.
The MAX7347/MAX7348/MAX7349 are put into shut-
down mode by clearing bit D7 in the configuration reg-
ister (Table 12). In shutdown, the key-scan controller
and sounder controller are both disabled, and the
MAX7347/MAX7348/MAX7349 draw minimal current.
No additional supply current is drawn if any keys are
pressed. All switch matrix current sources are turned
off, and row outputs ROW0 to ROW7 and column out-
puts COL0 to COL7 become high impedance.
The alert input status may still be read in shutdown,
and an alert event can still cause an interrupt request if
this feature is enabled (Table 12). This means that alert
can be used for µC wakeup while the system sleeps
drawing minimum current.
Outputs configured as GPOs (COL2/PORT2 to
COL2/PORT7 and INT) may still be controlled in shut-
down and their output states can be changed under
software control at any time.
The sounder output may not be changed in shutdown,
even if it is effectively being used as a logic output. Writes
to the sounder during shutdown are ignored, and the
sounder FIFO is cleared on entering shutdown. However,
the sounder retains its output logic state for the duration
of shutdown, and so can be set low or high as desired by
writing 0x00 or 0x02, respectively, to the sounder register
(Table 12) before entering shutdown.
The MAX7347/MAX7348/MAX7349 may be taken out of
shutdown mode and put into operating mode by setting
bit D7 in the configuration register (Table 12). The key-
scan and sounder controller FIFOs are cleared, and key
monitoring starts. Note that rewriting the configuration
register with bit D7 high when bit D7 was already high
does not clear the FIFOs; the FIFOs are only cleared
when the MAX7347/MAX7348/MAX7349 are actually
coming out of shutdown.
______________________________________________________________________________________
2-Wire Interfaced Low-EMI Key Switch
Sounder Operation
Shutdown
2
C
and Sounder Controllers
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 for
software to detect which of the four keys is the ghost key.
The MAX7347/MAX7348/MAX7349 employ a proprietary
scheme that detects any three-key combination that
generates a fourth ghost key, and does not report any of
these four keys as being pressed. This means that
although ghost keys are never reported, many combina-
tions of three keys are effectively ignored when pressed
at the same time. Applications requiring three key com-
binations (such as <Ctrl><Alt><Del>) must ensure that
the 3 keys are not wired in positions that define the ver-
tices of a rectangle (Figure 11).
The MAX7347/MAX7348/MAX7349 use two techniques
to minimize EMI radiating from the key-switch wiring.
First, the voltage across the switch matrix never
exceeds 0.65V, irrespective of supply voltage V+. This
reduces the voltage swing at any node when a switch
is pressed to 0.65V 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.
The MAX7347/MAX7348/MAX7349 operate with a 2.4V
to 3.6V power-supply voltage. Bypass the power supply
to GND with a 0.047µF or higher ceramic capacitor as
close to the device as possible.
The MAX7347/MAX7348/MAX7349 are designed to be
insensitive to resistance either in the key switches or
the switch routing to and from the appropriate COLx
and ROWx up to 1kΩ. These controllers are therefore
compatible with low-cost membrane and conductive
carbon switches.
Applications Information
Power-Supply Considerations
Ghost-Key Elimination
Switch On-Resistance
Low-EMI Operation
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