ST7LITE25F2 View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
ST7LITE25F2
STMicroelectronics
'ST7LITE25F2' PDF : 166 Pages View PDF
ST7LITE20F2 ST7LITE25F2 ST7LITE29F2
Power saving modes
9.6
Auto-wakeup from HALT mode
Auto Wake Up From Halt (AWUF) mode is similar to Halt mode with the addition of a specific
internal RC oscillator for wake-up (Auto Wake Up from Halt Oscillator). Compared to
ACTIVE-HALT mode, AWUF has lower power consumption (the main clock is not kept
running, but there is no accurate realtime clock available. It is entered by executing the HALT
instruction when the AWUEN bit in the AWUCSR register has been set.
Figure 28. AWUF mode block diagram
AWU RC
oscillator
fAWU_RC
to Timer input capture
/64
divider
AWUFH
AWUFH interrupt
prescaler/1 .. 255 (ei0 source)
As soon as HALT mode is entered, and if the AWUEN bit has been set in the AWUCSR
register, the AWU RC oscillator provides a clock signal (fAWU_RC). Its frequency is divided by
a fixed divider and a programmable prescaler controlled by the AWUPR register. The output
of this prescaler provides the delay time. When the delay has elapsed the AWUF flag is set
by hardware and an interrupt wakes-up the MCU from Halt mode. At the same time the main
oscillator is immediately turned on and a 256 or 4096 cycle delay is used to stabilize it. After
this start-up delay, the CPU resumes operation by servicing the AWUF interrupt. The AWU
flag and its associated interrupt are cleared by software reading the AWUCSR register.
To compensate for any frequency dispersion of the AWU RC oscillator, it can be calibrated
by measuring the clock frequency fAWU_RC and then calculating the right prescaler value.
Measurement mode is enabled by setting the AWUM bit in the AWUCSR register in Run
mode. This connects fAWU_RC to the input capture of the 12-bit Auto-Reload timer, allowing
the fAWU_RC to be measured using the main oscillator clock as a reference timebase.
Similarities with HALT mode:
The following AWUF mode behavior is the same as normal Halt mode:
● The MCU can exit AWUF mode by means of any interrupt with exit from Halt capability
or a reset (see Section 9.4: HALT mode).
● When entering AWUF mode, the I bit in the CC register is forced to 0 to enable
interrupts. Therefore, if an interrupt is pending, the MCU wakes up immediately.
● In AWUF mode, the main oscillator is turned off causing all internal processing to be
stopped, including the operation of the on-chip peripherals. None of the peripherals are
clocked except those which get their clock supply from another clock generator (such
as an external or auxiliary oscillator like the AWU oscillator).
● The compatibility of Watchdog operation with AWUF mode is configured by the
WDGHALT option bit in the option byte. Depending on this setting, the HALT instruction
when executed while the Watchdog system is enabled, can generate a Watchdog
RESET.
Doc ID 8349 Rev 5
59/166
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