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ST7 KEYPAD DECODING TECHNIQUES, IMPLEMENTING WAKE-UP ON KEYSTROKE

3 EXTERNAL INTERRUPTS

The MCU is in HALT mode. When a key is pressed, a low level is applied to the pin corre-

sponding to the row the key belongs (pins configured as inputs with pull-up). It’s a falling edge

applied to a pin of Port C which creates an external interrupt (EI1) and wakes up the MCU. The

MCU executes then the external interrupt subroutine (decoding the pressed key) and comes

back to its previous state (HALT state in the main program).

4 KEYPAD

The keypad used is a 4x4 matrixed keypad. Rows are connected to pins configured as inputs

with pull-up. So the initial state of these pins are a high level (1). When a key is pressed, a low

level is applied to the corresponding pin. For this reason, the keypad is coded as follows:

Table 1. Key values

KEY

row

value

column

value

KEY

row

value

column

value

1

0x0E

0x0E

7

0x0B

0x0E

2

0x0E

0x0D

8

0x0B

0x0D

3

0x0E

0x0B

9

0x0B

0x0B

F

0x0E

0x07

D

0x0B

0x07

4

0x0D

0x0E

A

0x07

0x0E

5

0x0D

0x0D

0

0x07

0x0D

6

0x0D

0x0B

B

0x07

0x0B

E

0x0D

0x07

C

0x07

0x07

12

3

F

45

6

E

7

8

9

D

A

0

B

C

PC0

PC1

PC2

PC3

PA0

PA1

PA2

PA3