4 clock source selection, Switchback – Maxim Integrated High-Speed Microcontroller User Manual
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High-Speed Microcontroller User’s Guide
Rev: 062210
99 of 176
system that one has occurred. If PMM and serial port switchback functions are used in a noisy
environment, the user is advised to periodically check if the device has accidentally exited PMM. A
similar problem can occur if multiprocessor communication protocols are used in conjunction with PMM.
The high-speed microcontroller family supports both the use of the SM2 flag (
.5 or
.5),
and the slave address recognition registers (
;AAh,
;B9h,
;BAh) for multiprocessor communications. The problem is that an invalid address that should be
ignored by a particular processor will still generate a switchback. As a result it is not recommended to use
a multiprocessor communication scheme in conjunction with PMM. If the system power considerations
will allow for an occasional erroneous switchback, a polling scheme can be used to place the device back
into PMM.
7.3.4 Clock Source Selection
The high-speed microcontroller family supports three different clock sources for operation. As with most
microcontrollers, the device can be clocked from an external crystal using the on-board crystal amplifier,
or a clock source can supplied by an external oscillator. In addition, some members of the high-speed
microcontroller family incorporate an on-board ring oscillator to provide a quick resumption from Stop
mode. The ring oscillator is a low power digital oscillator internal to the microcontroller. When enabled,
it provides an approximately 2MHz–4MHz clock source for device operation without external
components. The ring oscillator is not as stable as an external crystal, and software should refrain from
performing timing dependent operations, including serial port activity, while operating from the ring
oscillator.
The ring oscillator provides many advantages to the designers of microcontroller-based systems. One is
that it allows Maxim microcontrollers to perform a fast resume from Stop mode, eliminating the crystal
warmup delay when restarting the device. As an added feature, the DS87C520 and DS87C530 will also
support extended operation from the ring oscillator, not only during the crystal warmup period when
resuming from Stop. All devices in the high-speed microcontroller family must begin operation following
a power-on reset from an external clock source, either an external crystal or oscillator. Software can then
disable the crystal and run from the lower power ring oscillator. The control and status bits that support
the new and/or enhanced features are shown in