MC68HC908QC16_07 Datasheet, PDF (118/274 Pages) Freescale Semiconductor, Inc – Microcontrollers

Periodic Wakeup Module (PWU)
PWUCLKSEL
BUSCLKX4
EN
PWU CLOCK SOURCE
32 kHz
PWUP
1M
U
0X
PRESCALER
V_DD
PWUMOD
=?
COUNTER = PWUMOD
PWU COUNTER
CLK
EN RST
D
Q
E
R
INTERNAL RC
OSCILLATOR
PWU CLOCK
PWUCLKSEL
RESET
PWUACK
PWUIE
PWUF
PWUIREQ
STOP
SMODE
PWUON
RESET
VECTOR FETCH
DECODER
Note:
PWUCLKSEL bit, PWUMOD and PWUP registers can only be written to with PWUON=0
STOP is an internal MCU signal and when STOP = 0, indicates the MCU is in stop mode.
RESET is an internal MCU signal, indicates the MCU has taken a reset.
BUSCLKX4 is an internal MCU clock source, used to create the bus frequency for the MCU.
PWUIREQ is an internal MCU signal, used to request an interrupt to the MCU.
Figure 12-1. Periodic Wakeup Interrupt Request Generation Logic
Once the modulo value has been reached, the counter will be reset to $00. The PWU counter is also reset
when the module is disabled, or when the MCU enters stop mode with SMODE clear.
The periodic wakeup RC oscillator is highly dependent on operating voltage and temperature and
consequently would provide limited accuracy if used as a time-keeping function.
12.4 Interrupts
The following PWU source can generate interrupt requests:
• PWU flag (PWUF) — The PWUF bit is set when the counter reaches the modulo value
programmed in the PWUMOD register. The PWU interrupt enable bit, PWUIE, enables PWU
interrupt requests. PWUF and PWUIE are in the PWUSC register.
12.5 Low-Power Modes
The WAIT and STOP instructions put the MCU in low power-consumption standby modes.
12.5.1 Wait Mode
The PWU module remains active in wait mode while the PWUON bit in PWUSC is set. Setting PWUIE in
PWUSC enables PWU interrupts to bring the MCU out of wait mode. If the PWU is not required during
wait mode, power consumption can be reduced by disabling the PWU module (PWUON = 0) before
entering wait mode.
MC68HC908QC16 • MC68HC908QC8 • MC68HC908QC4 Data Sheet, Rev. 3
118
Freescale Semiconductor