DS80C320-MCG Datasheet, PDF (102/175 Pages) Dallas Semiconductor – High-Speed Microcontroller User Guide

High-Speed Microcontroller User’s Guide
CLOCK CONTROL AND STATUS BIT SUMMARY Table 7-5
BIT LOCATION
NAME
FUNCTION
RESET WRITE ACCESS
XT/ RG
EXIF.3 Crystal/Ring Clock Source Select. This bit is not present on 1 0 anytime;
the 80C320.
1 when XTUP=1
1=Select crystal or external clock as clock source,
& XTOFF=0
0=Select ring oscillator as clock source
RGMD
EXIF.2
Ring Oscillator Mode Status.
1=Ring oscillator is current clock source,
0=Crystal or external clock is current clock source.
0 None
RGSL
EXIF.1 Ring Oscillator Select, Stop Mode.
Unrestricted
1=Ring oscillator will be the clock source when resuming
from Stop mode,
0=Crystal or external clock will be the clock
source when resuming from Stop mode
Note: Upon completion of crystal warm up period,
DS80C320 devices will switch to crystal.
DS87C520 and DS87C530 devices will switch to clock
source designated by XT/RG bit
XTOFF
PMR.3 Crystal Oscillator Disable. Disables crystal operation
0 0 anytime;
after ring mode has been selected. This bit is not present on
the 80C320.
1 when XT/ RG =0
1=Crystal amplifier is disabled.
0=Crystal amplifier is enabled. Check XTUP for status.
XTUP STATUS.4 Crystal Oscillator Warm Up Status. This bit is
1 None
not present on the 80C320.
1=Oscillator warm up complete.
0=Oscillator warm up still in progress, crystal not available.
USING THE RING OSCILLATOR
The ring oscillator is an internal 2-4 MHz clock source used to quickly exit Stop mode and resume
operation without waiting for an external clock source to stabilize. Some devices feature the additional
capability of using the ring oscillator as the primary clock source during normal operation, once the
device has performed an initial power-on reset using an external clock source. Because the ring oscillator
lacks the stability of a piezoelectric-generated clock source, high-precision timing operations should be
avoided while running from the ring oscillator. This includes using the timers for pulse measurement,
and the use of the serial ports in asynchronous modes. Serial ports operating in mode 0 are unaffected by
the stability of the clock source because this mode utilizes a synchronizing clock.
If the Ring Oscillator Select bit, RGSL (EXIF.1) is set, the device will resume operation immediately
using the internal ring oscillator as the clock source. The device will continue to run from the ring
oscillator until the crystal warm-up period of 65,536 clock cycles (measured from the external source) has
completed. At this time the device will switch to the clock source active before it entered Stop mode and
continue operation. This allows software execution to begin immediately upon resuming from Stop
mode. The current clock source is indicated by the Ring Oscillator Mode bit, RGMD (EXIF.2). In Stop
mode, enabled interrupts become true edge triggered interrupts, compared with the sampled edge
detection used during normal operation. This means that external interrupts are more sensitive to noise in
Stop mode than during normal operation. Applications should be carefully designed to ensure that noise
will not cause an erroneous exit from Stop mode.
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