AN2421 Datasheet, PDF (3/6 Pages) STMicroelectronics – Using the STMPE801 as a keypad controller

AN2421
1
Design objectives
Design objectives
The keyboard controller presented in this application note complies with the following design
objectives:
● Implementation of an 8*8 keypad controller using 8 GPIOs from the Baseband and 8
GPIOs from the STMPE801
● Requires no external interrupts
● Minimum traffic on the I²C bus
● Ability to support hot-key (wake-up from suspend) features
2
Keypad scanning operation
The rows (X-n) are used as inputs and are pulled up to V_CC using a weak pull-up resistor
(47kOhm). The columns (Y-n) are used as outputs and are normally driven to V_CC. An X-Y
matrix keypad works in the following sequence:
1. Y-0 is driven to GND.
2. KPC Software scans X-0 to X-n. A "LOW" indicates that key Y-0, X-n is pressed.
3. Y-0 is driven to V_CC. Scanning of column 1 completes.
4. Proceed to scanning of column Y+1.
To minimize I2C traffic between the baseband and the STMPE801, the GPIOs in baseband
(BB_GPIO) are used as inputs and the STMPE801 GPIOs (801_GPIO) are used as
outputs. This enables KPC Software to operate without causing the STMPE801 to return
interrupts. There is no reading operation from the STMPE801.
3/6