TCA8418E_15 Datasheet, PDF (19/50 Pages) Texas Instruments – I2C Controlled Keypad Scan IC With Integrated ESD Protection

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Key Press / Start
TCA8418E
SCPS222C – MAY 2010 – REVISED OCTOBER 2015
Generate KE_INT
Start Mask Timer
Countdown
YES
Mask Timer NO
Countdown
Expired
NO _{KP_LCK_TIMER} YES
[7:3] == 0
NO
First Unlock Key
Pressed
NO _{First Unlock Key}
Pressed
YES
Start Unlock1 to
Unlock2 Timer
YES
Start Unlock1 to
Unlock2 Timer
YES Unlock1 to
Unlock2 Timer
Expired
YES Mask Timer
Countdown
Expired
NO
NO Unlock1 to YES
Unlock2 Timer
Expired
Second Unlock ^NO
Key Pressed
Unlock Keypad
NO
Second Unlock ^NO
Key Pressed
YES
Unlock Keypad
YES
Generate
K_LCK_INT
Figure 21. Keypad Lock Flowchart
Generate
K_LCK_INT
8.3.3 Keypad Lock Interrupt Mask Timer
The TCA8418E features a Keypad Lock/Unlock feature which allows the user to stop the generation of key event
interrupts by locking the key pad. There is an interrupt mask timer feature with the keypad lock, which allows the
generation of a single interrupt when a key is pressed, primarily for the purpose of LCD backlighting. Note that
this interrupt mask timer can also be used to limit the number of interrupts generated for a given amount of time.
The interrupt mask timer is enabled by setting bits [7:3] of the KP_LCK_TIMER register. The value in this register
can be anywhere from 0 to 31 seconds (note that a value of 0 will disable this interrupt mask feature). When a
keypad is locked and the interrupt mask timer is set to a non-zero value, this will enable the interrupt mask timer.
This interrupt mask timer limits the amount of interrupts generated. Typically, this is used with the Keypad
Lock/Unlock feature for LCD back lights. It is easiest to explain this feature with the following example; A mobile
device has a LCD screen with a back light display which turns off after 10 seconds to save power. Normally, an
interrupt to the processor would tell this LCD back light to turn on. When the keypad is locked, no interrupts are
generated, so the back light will never turn on. This is where the interrupt mask feature is used. Please refer to
Figure 21. The procedure for an example is below.
1. Since the back light turns off after 10 seconds of no interrupts, the interrupt mask timer (
KP_LCK_TIMER[7:3] ) gets set to 10 seconds. Keypad is then locked.
2. When the first key press is detected, the TCA8418E sends an interrupt to the processor and starts a 10
second count down.
3. If the correct unlock sequence is not entered within the 10 seconds, no interrupts are sent and the back light
will turn off.
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