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STR71x GPIO SCANNING A 4x4 MATRIX KEYPAD

3 SOFTWARE

3.1 KEYPAD

The keypad used is a 4x4 matrixed keypad. Rows are connected to the P2.8-11 pins config-

ured as Input Pull Up /Pull Down Weak Push-Pull mode and pulled low through pull-down re-

sistors. Columns are connected to the P1.8-11 pins configured as Output Push-Pull.

Biasing is achieved by setting high (3.3 V) each row for 5 ms duration every 20 ms. This gives

an update rate of 50 Hz. The 5 ms time-base can be generated using a timer overflow inter-

rupt.

When a key is pressed, a rising edge is applied to the row the key belong. The MCU executes

the XTI IRQ handler routine and decodes the pressed key (Table 1).

The keypad is coded as follows:

Table 1. Key codes

Key

Row value

Column value

Key Code

0

0x0800

0x0400

0x84

1

0x0100

0x0800

0x18

2

0x0100

0x0400

0x14

3

0x0100

0x0200

0x12

4

0x0200

0x0800

0x28

5

0x0200

0x0400

0x24

6

0x0200

0x0200

0x22

7

0x0400

0x0800

0x48

8

0x0400

0x0400

0x44

9

0x0400

0x0200

0x42

A

0x0100

0x0100

0x11

B

0x0200

0x0100

0x21

C

0x0400

0x0100

0x41

D

0x0800

0x0100

0x81

E

0x0800

0x0800

0x88

F

0x0800

0x0200

0x82