Ameba-D User Manual
User Manual All information provided in this document is subject to legal disclaimers. © REALTEK 2019. All rights reserved.
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Fig 16-8 4*3 keypad example
Shadow key can occur when multiple keys are pressed that can make it appear that additional keys (which are not being pressed) are being
pressed. In Fig 16-9, keys 11, 12, and 21 are pressed, which causes a shadow key issue. Since Row1 becomes pulled to ground through key 11
(which is pulled through key 12 when Column1 is transmitting a low), when Column1 is driving low, the RTL8721D sees a low signal at both
Row0 and Row1. This falsely triggers key 22 as being pressed (the key highlighted as yellow).
The reason for this is that keypad matrices short the columns to the rows connected together. When Column1 is driving low, the low gets
transmitted onto Row0 via key 12. Key 11 is being pressed, which also shorts Column0 to ground. Key 11 is pressed, which then shorts Row1 to
Column0. In this process, Row1 is shorted to Column1, which is the reason shadow Key occurs.
Keypad matrices can support multiple key presses properly, if care is taken when choosing the layout. In Fig 16-10, you see a 3-button
combination which works as expected.
Fig 16-9 Shadow key condition
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