TCA8418E_15 Datasheet, PDF (38/50 Pages) Texas Instruments – I2C Controlled Keypad Scan IC With Integrated ESD Protection

TCA8418E
SCPS222C – MAY 2010 – REVISED OCTOBER 2015
9.2 Typical Application
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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 #.
ROW0
1
2
3
ROW1
4
5
6
ROW2
7
8
9
ROW3
*
0
#
COL0 COL1 COL2
Figure 31. Typical Application Diagram
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
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