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

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.

Figure 29. Incorrect 3 Button Combination

In Figure 29, keys 1, 2, and 11 are pressed, which causes a ghosting issue. Since R1 becomes pulled to ground through key 1 (which is pulled through key 2 when C1 is transmitting a low), when C1 is driving low, the TCA8418E will see a low signal at both R0 and R1. This will falsely trigger key 12 as being pressed (the key highlighted as yellow).

The reason for this is that keypad matrices will short the columns to the rows connected together. When C1 is driving low, the low gets transmitted onto R0 via key 2. Key 1 is being pressed, which also shorts C0 to ground. Key 11 is pressed, which then shorts R1 to C0. In this process, R1 is shorted to C1, which is the reason ghosting occurs.

Keypad matrices can support multiple key presses properly, if care is taken when choosing the layout. In Figure 30, we see a 3 button combination which will work as expected. Keys 1, 11, and 21 are pressed (this also is the combination that will set the <Ctrl><Alt><Del> interrupt, see Control-Alt-Delete Support for more information).

Figure 30. Correct 3 Button Combination

Figure 31 shows a typical application of the TCA8418E. In this specific example, a common 12 key number pad layout is used. This number pad has keys for numbers 0 to 9, *, and #.

9.2.1 Design Requirements

The system designer needs to know a few key pieces in order to design their system for the TCA8418E.

• The number of keys desired
• Whether the keys will be multiplexed or not
• The layout of the multiplexed keys
• Unused keys be tied to VCC through a pullup resistor (10 kΩ)

9.2.2 Detailed Design Procedure

9.2.2.1 Designing the Hardware Layout

The first steps towards designing a keypad array is to determine the desired layout, and to map each key to the appropriate value which will show up in the FIFO. For this example, the number pad below is the physical location of the keys that are desired. The layout is a 4 x 3 array, using rows 0-3 and columns 0-2. For this example, we will not assume any of the other pins will be used.

The following behavior is desired for this example design

• All keys in the keypad array to be added to the FIFO upon a key press
• Attempting to clear the interrupt before the proper registers have been cleared to de-assert the INT pin for 50 μs, then assert the INT pin.
• No additional pins are being used, other than the keypad array
• Keypad lock support, requiring that the unlock combination be ‘#, 1’ which must be pressed within 2 seconds of each other
• Keypad lock interrupt mask timer of 10 seconds to match the back light auto-turn off with 10 seconds of no interrupt
• Hardware debouncing to be enabled

Figure 32. Example Keypad

Since the TCA8418E will report keys pressed according to the values in the key value table, it will be important to know what the TCA8418E’s values for these key locations are.

According to the key event table, the key presses are assigned in the following way:

Table 26. Key Event Table

Keypad Button	1	2	3	4	5	6	7	8	9	*	0	#
Key Event Table Value (Decimal)	1	2	3	11	12	13	21	22	23	31	32	33

The schematic for this keypad layout is shown in Figure 33, with the key event table values. Note that no external pullup resistors are needed, because the TCA8418E has integrated pullup resistors.

Figure 33. Keypad Schematic

9.2.3 Application Curves

Figure 34. Initial Key Press to Interrupt Output

Figure 35. Zoom On Second Scan