MC908QT4MPE datasheet(49/184 Pages) FREESCALE

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MC68HC908QY/QT Family Data Sheet, Rev. 5

Freescale Semiconductor

Chapter 4

Auto Wakeup Module (AWU)

4.1 Introduction

This section describes the auto wakeup module (AWU). The AWU generates a periodic interrupt during

stop mode to wake the part up without requiring an external signal. Figure 4-1 is a block diagram of the

AWU.

4.2 Features

Features of the auto wakeup module include:

•

One internal interrupt with separate interrupt enable bit, sharing the same keyboard interrupt vector

and keyboard interrupt mask bit

•

Exit from low-power stop mode without external signals

•

Selectable timeout periods

•

Dedicated low-power internal oscillator separate from the main system clock sources

4.3 Functional Description

The function of the auto wakeup logic is to generate periodic wakeup requests to bring the microcontroller

unit (MCU) out of stop mode. The wakeup requests are treated as regular keyboard interrupt requests,

with the difference that instead of a pin, the interrupt signal is generated by an internal logic.

Writing the AWUIE bit in the keyboard interrupt enable register enables or disables the auto wakeup

interrupt input (see Figure 4-1). A logic 1 applied to the AWUIREQ input with auto wakeup interrupt

request enabled, latches an auto wakeup interrupt request.

Auto wakeup latch, AWUL, can be read directly from the bit 6 position of port A data register (PTA). This

is a read-only bit which is occupying an empty bit position on PTA. No PTA associated registers, such as

PTA6 data direction or PTA6 pullup exist for this bit.

Entering stop mode will enable the auto wakeup generation logic. An internal RC oscillator (exclusive for

the auto wakeup feature) drives the wakeup request generator. Once the overflow count is reached in the

generator counter, a wakeup request, AWUIREQ, is latched and sent to the KBI logic. See Figure 4-1.

Wakeup interrupt requests will only be serviced if the associated interrupt enable bit, AWUIE, in KBIER

is set. The AWU shares the keyboard interrupt vector.

The overflow count can be selected from two options defined by the COPRS bit in CONFIG1. This bit was

“borrowed” from the computer operating properly (COP) using the fact that the COP feature is idle (no

MCU clock available) in stop mode. The typical values of the periodic wakeup request are (at room

temperature):

•

COPRS = 0: 650 ms @ 5 V, 875 ms @ 3 V

•

COPRS = 1: 16 ms @ 5 V, 22 ms @ 3 V

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