Maxim Integrated Ultra-High-Speed Flash Microcontroller - Manual (Page 104)

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Ultra-High-Speed Flash
Microcontroller User’s Guide
Clock Source Selection
The ultra-high-speed flash microcontroller family supports three 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 be supplied by an external oscil-
lator. In addition, some members of the 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 10MHz
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 micro-
controllers to perform a fast resume from stop mode, eliminating the crystal warmup delay when restarting the device. The microcon-
troller must begin operation following a power-on reset from an external clock source, either an external crystal or oscillator. The con-
trol and status bits which support the new and/or enhanced features are shown in Table 7-5.
Ring Oscillator Resume From Stop
To achieve the minimum power consumption during periods of processor inactivity, software can place the device into stop mode. Such
systems typically resume operation using an external interrupt, perform some activity, and then return to stop mode. Traditional designs
that rely upon an external crystal as the clock source must incur the startup delay of the crystal when resuming from stop mode. This
is a waste of time and power, as no work can be performed until the crystal has stabilized.
Although the ring oscillator provides an approximately 10MHz clock source for device operation, it is not as stable as an external crys-
tal. As a result, high-accuracy 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 (1, 2, 3). Serial ports operating in mode 0 are unaf-
fected by the stability of the clock source as a separate synchronizing clock is generated.
If the ring oscillator select bit RGSL, (EXIF.1) is set, the device resumes operation immediately using the internal ring oscillator as the
clock source. The device continues to run from the ring oscillator until the crystal warmup period of 65,536 clock cycles (measured
from the external source) has completed. At this time, the device switches to clock source active before it enters stop mode and con-
tinues operation. This allows software execution to begin immediately upon resuming from stop mode. The current clock source is indi-
cated by the ring oscillator mode bit, RGMD (EXIF.2). In stop mode, enabled interrupts become true edge triggered interrupts, com-
pared 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.
BIT NAME LOCATION FUNCTION RESET WRITE ACCESS
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.
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,
the device will switch to the crystal.
— Unrestricted
Table 7-5. Clock Control and Status Bit Summary
Maxim Integrated