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14.4.4 Keypad Standby
There is no need for the CPU to scan the keypad continually. Between key presses,
the keypad can be left in a state that requires no software intervention until the next
key press is detected. To place the keypad in a standby state, software writes all col-
umn outputs low. Row inputs are left enabled. At this point the CPU can attend to
other tasks or revert to a low-power standby mode. The keypad port will interrupt the
CPU if any key is pressed.
Upon receiving a keypad interrupt, the CPU should set all the column strobes high
and begin a normal keypad scanning routine to determine which key was pressed. It
is important that open-drain drivers be used when scanning to prevent a possible DC
path between power and ground through two or more switches.
14.4.5 Glitch Suppression on Keypad Inputs
A glitch suppression circuit qualifies the keypad inputs to prevent noise from inadvert-
ently interrupting the CPU. The circuit is a four-state synchronizer clocked from a 256-
Hz clock source. This clock must continue to run in any low-power mode for which the
keypad is a wake-up source, as the CPU interrupt is generated from the synchronized
input. An interrupt is not generated until all four synchronizer stages have latched a
valid key assertion, effectively filtering out any noise less than 16 ms in duration. The
interrupt output is latched in an S-R latch and remains asserted until cleared by soft-
ware. The set input of the latch is clocked on the rising edge.
MMC2001
REFERENCE MANUAL
Freescale Semiconductor, Inc.
KEYPAD PORT
For More Information On This Product,
Go to: www.freescale.com
MOTOROLA
14-7
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