Élan™SC520 Microcontroller User’s Manual 5-1
CHAPTER
5
CLOCK GENERATION AND CONTROL
5.1 OVERVIEW
The ÉlanSC520 microcontroller is designed to generate all of the internal and system clocks
it requires. The ÉlanSC520 microcontroller includes on-chip oscillators and PLLs, as well
as most of the required PLL loop filter components.
The ÉlanSC520 microcontroller requires two standard crystals, one for 32.768 kHz and
one for 33 MHz. All the clocks required inside the ÉlanSC520 microcontroller are generated
from these crystals. Output clock pins are provided for selected clocks, providing up to 24
mA of sink or source current.
The ÉlanSC520 microcontroller also supplies the clocks for SDRAM and the PCI host
bridge; however external clock buffering may be required in some systems.
The clocking generation and control features include:
■ RTC low-current oscillator using standard off-the-shelf 32.768-kHz crystal
■ 33-MHz oscillator using standard off-the-shelf 33-MHz crystal (33.000 or 33.333 MHz)
■ 33-MHz clock provides clocks for the integrated Am5
x
86 CPU and external PCI bus
■ Integrated 66-MHz PLL provides clocks for external SDRAM
■ Integrated PLLs for generating 1.1882-MHz PIT clock and 18.432-MHz UART clock
■ Integrated on-chip PLL loop filters for the 66-MHz and 36.864-MHz PLLs, eliminating
the need for external capacitors
■ 33.333-MHz/30.000-MHz PCI Clock Output Pin, CLKPCIOUT
■ 66-MHz SDRAM Clock Output Pin, CLKMEMOUT
■ 33-MHz and 32.768-kHz oscillators bypass option
Note: The ÉlanSC520 microcontroller supports either a 33.000-MHz or 33.333-MHz
crystal. In this document, the term “33 MHz” refers to the system clock derived from
whichever 33-MHz crystal frequency is being used in the system.
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