TCA8418E_15 Datasheet, PDF (37/50 Pages) Texas Instruments – I2C Controlled Keypad Scan IC With Integrated ESD Protection

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Application Information (continued)
R0
R1
R2
R3
TCA8418E
SCPS222C – MAY 2010 – REVISED OCTOBER 2015
1
2
3
12 13
21 22 23
31 32 33
C0 C1 C2
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).
R0
2
3
R1
12 13
R2
22 23
R3
31 32 33
C0 C1 C2
Figure 30. Correct 3 Button Combination
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