MC68HC908QC16_07 Datasheet, PDF (117/274 Pages) Freescale Semiconductor, Inc – Microcontrollers

Chapter 12
Periodic Wakeup Module (PWU)
12.1 Introduction
This section describes the periodic wakeup (PWU) module.The PWU is available in all modes of operation
(run, wait, and stop) and performs two main functions:
• Generate periodic wakeup requests to bring the microcontroller unit (MCU) out of stop mode.
• Generate periodic interrupt requests during run and wait modes.
12.2 Features
Features of the periodic wakeup module include:
• Interrupt with separate interrupt enable bit, interrupt vector and interrupt mask bit
• Exit from low-power stop mode without external signals
• Programmable clock input
• Selectable timeout periods (40 μs to 3 minutes with an adjustment resolution of better than 1% for
periods over 4 ms)
• Dedicated low-power 32 kHz internal oscillator separate from the main system clock sources
• Option to allow bus clock source to run the PWU
• Accessible in all modes of operation (run, wait, and stop)
12.3 Functional Description
Figure 12-1 is a block diagram of the PWU.
The PWU module consists of a PWU counter whose count is reset once it equals the value stored in the
PWU modulo register (PWUMOD). The PWU counter clock, PWUCLOCK, frequency is selectable using
the PWU prescaler register (PWUP). The prescaler clock source can be selected using the PWUCLKSEL
bit in the PWU status and control register (PWUSC) and can either be the internal RC oscillator or the
BUSCLKX4 clock.The PWUCLKSEL bit, PWUMOD and PWUP registers can only be written to when the
PWUON bit is clear.
The PWUON bit in PWUSC is used to enable the PWU module. The SMODE bit in PWUSC is used to
allow an enabled PWU module to continue running in stop mode. BUSCLKX4 must be enabled to run in
stop mode if used as the clock source for the PWU when SMODE is set. See Chapter 4 Configuration
Registers (CONFIG1 and CONFIG2) on enabling BUSCLKX4 to run in stop mode.
The PWU counter when enabled will count until it reaches the value stored in the PWU modulo register
(PWUMOD), when they are equal the read-only PWU flag (PWUF) in PWUSC is latched. The PWU
interrupt enable bit, PWUIE, in PWUSC enables a PWU interrupt request. The PWUF can be cleared by
writing to the PWU acknowledge bit, PWUACK in PWUSC or by a PWU interrupt vector fetch.
MC68HC908QC16 • MC68HC908QC8 • MC68HC908QC4 Data Sheet, Rev. 3
Freescale Semiconductor
117