TCA8418E_15 Datasheet, PDF (36/50 Pages) Texas Instruments – I2C Controlled Keypad Scan IC With Integrated ESD Protection

TCA8418E
SCPS222C – MAY 2010 – REVISED OCTOBER 2015
9 Application and Implementation
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NOTE
Information in the following applications sections is not part of the TI component
specification, and TI does not warrant its accuracy or completeness. TI’s customers are
responsible for determining suitability of components for their purposes. Customers should
validate and test their design implementation to confirm system functionality.
9.1 Application Information
9.1.1 Ghosting Considerations
The TCA8418E supports multiple key presses accurately. Applications requiring three-key combinations (such as
<Ctrl><Alt><Del>, or any other combinations) must ensure that the three keys are wired in appropriate key
positions to avoid ghosting (or appearing like a 4th key has been pressed).
To avoid ghosting, it is best to keep 3-button combinations that will be pressed on separate rows and columns.
Consider the situation with the keypad described in Figure 28
R0
1
2
3
R1
11 12 13
R2
21 22 23
R3
31 32 33
C0 C1 C2
Figure 28. Example Keypad
In the keypad setup in Figure 28, there is a 4x3 keypad matrix, connected to ROW0-ROW3, and COL0-COL2. All
of the ROWs are configured as inputs with pullup resistors. The COLs are configured as outputs, driving low.
When a key press is made, one of the ROW inputs will be pulled low, letting the TCA8418E know that a key has
been pressed, and the TCA8418E will then start the key scanning algorithm. During this algorithm, It will sweep
the output low across the columns, such that only 1 column is driven low at a time. While this is done to each
column, the TCA8418E will read the ROW inputs, to determine which keys on a column are being pressed.
Ghosting can occur when multiple keys are pressed that can make it appear that additional keys (which are not
being pressed) are being pressed.
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