MC68HC908QY4_10 Datasheet, PDF (49/168 Pages) Freescale Semiconductor, Inc – M68HC08 Microcontrollers

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
MC68HC908QY/QT Family Data Sheet, Rev. 6
Freescale Semiconductor
49