MCIMX27 Datasheet, PDF (814/1650 Pages) Freescale Semiconductor, Inc – Multimedia Applications Processor

Keypad Port (KPP)
Switches with
diode
Column pulled
down
Column not
pulled down
Three real
key presses
Pulled down
row
The path of zero pull down
gets stopped at this point
Pulled down
row
The path of the
zero pull down
cannot reach the
wrong row
Diode prevents ghost key
press
Figure 25-11. Matrix with “Ghost” Key Protections
25.4.7 3-Point Contact Keys Support
The KPP module supports interfacing to a matrix consisting of 3-point contact keys. As shown in
Figure 25-12, two points of such a key are connected to keypad lines, while a third point is connected to
ground (low logic). The keypad lines should be configured as input and a pull-up should to be present on
these lines. When such a key is pressed, corresponding keypad lines go low and an interrupt is generated.
There is no need to perform a scanning routine for identification of pressed key as it can be done by reading
the keypad data-register. A limitation with such a matrix is that for every key at least one keypad row line
should be used.
25-14
MCIMX27 Multimedia Applications Processor Reference Manual, Rev. 0.2
Freescale Semiconductor