AMD Elan SC520, User Manual, Page: 103 / 444

Clock Generation and Control
Élan™SC520 Microcontroller User’s Manual 5-5
Detailed crystal specifications and further information on crystal selection can be found in
the
Élan™SC520 Microcontroller Data Sheet, order #22003.
5.3.2.1 Running the Élan™SC520 Microcontroller at 33.333 MHz
The clock that is supplied to the PCI bus (CLKPCIOUT) is exactly the same as the frequency
of the crystal. The ÉlanSC520 microcontroller simply buffers the 33-MHz crystal input and
provides it to the CLKPCIOUT pin. Since crystals have inaccuracies, it is possible that these
inaccuracies cause the period of CLKPCIOUT to become marginally less than 30 ns.
It is up to the system designer to choose the accuracy of the crystal used with the ÉlanSC520
microcontroller. The 33.000-MHz frequency provides a better guard band than the 33.333-
MHz crystal. In practice, most PCI devices can tolerate both frequencies, but it is important
to be aware of the impact of choosing the crystal on this potential violation of the PCI bus
specification. The
PCI Local Bus Specification, Revision 2.2 requires that the minimum
clock period be 30 ns.
5.3.3 Bypassing Internal Oscillators
The 32.768-kHz and the 33-MHz ÉlanSC520 microcontroller oscillators can be bypassed
by connecting an external clock to the crystal pins. See Figure 5-3 and Figure 5-4 for
suggested circuitry.
Figure 5-3 Bypassing the 32.768-kHz Oscillator
Table 5-3 Timing Error as It Translates to Clock Accuracy
Timing Error
(Parts per Million) Seconds/Month
± 10 ± 25.9
± 20 ± 51.8
± 30 ± 77.8
± 40 ± 103.7
± 50 ± 129.6
External
32.768-kHz
Oscillator
32KXTAL2
32KXTAL1
100 k
Ω
Élan™SC520
Microcontroller
2.5-V max