ST AN980 APPLICATION NOTE

AN980

APPLICATION NOTE

ST7 KEYPAD DECODING TECHNIQUES,

IMPLEMENTING WAKE-UP ON KEYSTROKE

by 8-Bit Micro Application Laboratory

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 ST72F264) is waked up by an external interrupt caused by someone pressed a key on the 4x4 matrixed keypad.

AN980/0802

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ST7 KEYPAD DECODING TECHNIQUES, IMPLEMENTING WAKE-UP ON KEYSTROKE

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 resistors 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

	4x100k
VDD	8x560
	PC0
	PC1
	PC2
	PC3
ST7	PA0
	PA1
	PA2
GND	PA3

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ST7 KEYPAD DECODING TECHNIQUES, IMPLEMENTING WAKE-UP ON KEYSTROKE

2 ST72F264 CONFIGURATION

The application has been validated with a ST72F264. Its configuration is described in this part. Refer to your datasheet for more details.

2.1 I/O CONTROL

Rows are connected to pins configured as inputs (Port C as input with pull up and interrupts). Columns are connected to pins configured as outputs (Port A).

External interrupts are caused by a low level applied to a pin of Port C (caused by a key pressed), they wake up the MCU which was in HALT mode.

Port B is configured as outputs to send the value of the pressed key on LEDS.

Please, refer to the Data Book to configure pins properly.

2.2 MISCELLANEOUS REGISTER

Bits 7 and 6 have to be set to configure events correctly: the external interrupt (EI1) has here to be caused by a falling edge only.

Please, refer to the datasheet for more details.

2.3 HALT MODE

The HALT instruction places the ST72F264 in its lowest power consumption mode. The core and all peripherals are frozen. In this mode, the internal oscillator is turned off, causing all internal processing to be halted. The data remain unchanged. During the HALT mode, external interrupts are still enabled. The MCU stays in this state until an external interrupt or a reset occurs. Then the internal oscillator is restarted and the core waits for 4096 CPU clock cycles (512 µs for a fCPU = 8MHz) before running the external interrupt subroutine. Then the MCU comes back to the main program (in our application to the HALT state).

Please, refer to the datasheet for more details.

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