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APPLICATION NOTE

ST7 KEYPAD DECODING TECHNIQUES, IMPLEMENTING

WAKE-UP ON KEYSTROKE

by Microcontroller Division Application Team

INTRODUCTION

The goal of this application note is to present an example of the use of the HALT mode.

In this application, the MCU (here a ST72251) is waked up by an external interrupt caused by

someone pressed a key on the 4x4 matrixed keypad.

1 ST7 / KEYBOARD INTERFACE

Rows are connected to inputs with pull-up and interrupts (Port C). Columns are connected to

Port A configured as output. The result of the interrupt (the value of the pressed key) is sent on

LEDS (Port B) and stored into the X register. In our configuration, we have to add 4 pull-up re-

sistors on Port A (from PA0 to PA3) to be able to apply a high level on the corresponding pads.

Figure 1. ST7 / keypad interface set-up

ST7

8x560

PC0

PC1

PC2

PC3

PA0

PA1

PA2

PA3

GND

4x100k