DS80C320 Datasheet, PDF (17/42 Pages) Dallas Semiconductor – High-Speed/Low-Power Micro

DS80C320/DS80C323
The second feature allows an additional power saving option. This is the ability to start instantly when
exiting Stop mode. It is accomplished using an internal ring oscillator that can be used when exiting Stop
mode in response to an interrupt. The benefit of the ring oscillator is as follows.
Using Stop mode turns off the crystal oscillator and all internal clocks to save power. This requires that
the oscillator be restarted when exiting Stop mode. Actual start-up time is crystal dependent, but is
normally at least 4 ms. A common recommendation is 10 ms. In an application that will wake-up,
perform a short operation, then return to sleep, the crystal start- up can be longer than the real transaction.
However, the ring oscillator will start instantly. The user can perform a simple operation and return to
sleep before the crystal has even stabilized. If the ring is used to start and the processor remains running,
hardware will automatically switch to the crystal once a power-on reset interval (65536 clocks) has
expired. This value is used to guarantee stability even though power is not being cycled.
If the user returns to Stop mode prior to switching of crystal, then all clocks will be turned off again. The
ring oscillator runs at approximately 3 MHz (1.5 MHz at 3V) but will not be a precision value. No real-
time precision operations (including serial communication) should be conducted during this ring period.
Figure 7 shows how the operation would compare when using the ring, and when starting up normally.
The default state is to come out of Stop mode without using the ring oscillator.
This function is controlled using the RGSL - Ring Select bit at EXIF.1 (EXIF - 91h). When EXIF.1 is set,
the ring oscillator will be used to come out of Stop mode quickly. As mentioned above, the processor will
automatically switch from the ring (if enabled) to the crystal after a delay of 65536 crystal clocks. For a
3.57 MHz crystal, this is approximately 18 ms. The processor sets a flag called RGMD - Ring Mode to
tell software that the ring is being used. This bit at EXIF.2 will be a logic 1 when the ring is in use. No
serial communication or precision timing should be attempted while this bit is set, since the operating
frequency is not precise.
RING OSCILLATOR START -UP Figure 4
Diagram assumes that the operation following Stop requires less than 18 ms complete.
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