TCA8418 Datasheet, PDF(21/37 Page) Texas Instruments – I2C CONTROLLED KEYPAD SCAN IC WITH INTEGRATED ESD PROTECTION

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TCA8418

www.ti.com ......................................................................................................................................................................................... SCPS215 â SEPTEMBER 2009

Keypad Lock/Unlock

This user can lock the keypad through the lock/unlock feature in this device. Once the keypad is locked, it can

prevent the generation of key event interrupts and recorded key events. The unlock keys can be programmed

with any value of the keys in the keypad matrix or any GPI values that are part of the key event table. When the

keypad lock interrupt mask timer is enabled, the user will need to press two specific keys before an keylock

interrupt is generated or keypad events are recorded. After the keypad is locked, a key event interrupt is

generated any time a user presses a key. This first interrupt also triggers the processor to turn on the LCD and

display the unlock message. The processor will then read the lock status register to see if the keypad is

unlocked. The next interrupt (keylock interrupt) will not be generated unless both unlock keys sequences are

correct. If correct Unlock keys are not pressed before the mask timer expires, the state machine will start over

again.

Ghosting

Supports multiple key presses accurately. Applications requiring three-key combinations (such as

<Ctrl><Alt><Del>) must ensure that the three keys are wired in appropriate key positions to avoid ghosting (or

appearing like a 4th key has been pressed)

GPI Events

A column or row configured as GPI can be programmed to be part of the Key Event Table, hence becomes also

capable of generating Key Event Interrupt. A key Event Interrupt caused by a GPI follow the same process flow

as a Key Event Interrupt caused by a Key press.

GPIs configured as part of the Key Event Table allows for single key switches to be monitored as well as other

GPI interrupts. As part of the Event Table, GPIs are represented with decimal value of 97 (0x61 or 1100001) and

run through decimal value of 114 (0x72 or 1110010).

For a GPI that is set as active high, and is enabled in the Key Event Table, the state-machine will add an event

to the event count and event table whenever that GPI goes high. If the GPI is set to active low, a transition from

high to low will be considered a press and will also be added to the event count and event table. Once the

interrupt state has been met, the state machine will internally set an interrupt for the opposite state programmed

in the register to avoid polling for the released state, hence saving current. Once the released state is achieved,

it will add it to the event table. The press and release will still be indicated by bit 7 in the event register.

The GPI Events can also be used as unlocked sequences. When the GPI_EM bit is set, GPI events will not be

tracked when the keypad is locked. GPI_EM bit must be cleared for the GPI events to be tracked in the event

counter and table when the keypad is locked.

Bus Transactions

Data is exchanged between the master and TCA8418E through write and read commands.

Writes

Data is transmitted to the TCA8418E by sending the device address and setting the least significant bit (LSB) to

a logic 0. The command byte is sent after the address and determines which register receives the data that

follows the command byte. There is no limitation on the number of data bytes sent in one write transmission.

Product Folder Link(s): TCA8418

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