ELANSC400-66AC 参数 Datasheet PDF下载

CLOCKING

Clock Generation

The ÉlanSC400 and ÉlanSC410 microcontrollers re-

quire only one 32.768-kHz crystal to generate all the

other clock frequencies required by the system. The

output of the on-chip crystal oscillator circuit is used to

generate the various frequencies by utilizing four

Phase-Locked Loop (PLL) circuits. The PLL clock dis-

tribution scheme is shown in Figure 4. Table 26 shows

all the PLL output frequencies and their usage. (Note

that these four PLL circuits are in addition to the inter-

nal CPU PLL and do not replace it.)

The crystal oscillator needs two pins, but it does not re-

quire any external components except the crystal; the

load capacitors and the feedback resistor are inte-

grated on-chip.

The four PLLs are called Intermediate PLL, Low-Speed

PLL, High-Speed PLL, and Graphics Dot Clock PLL.

Each of the integrated phase-locked loops has a dedi-

cated pin to support the required external loop filter.

These pins are: LF_INT (Intermediate PLL), LF_LS

(Low-Speed PLL), LF_HS (High-Speed PLL), and

LF_VID (Graphics Dot Clock PLL). (The LF_VID pin is

not supported on the ÉlanSC410 microcontroller.) Two

capacitors and one resistor are required to implement

each loop filter.

32.768-kHz

Crystal

Oscillator

32.768 kHz

Real-Time

Clock

PMU

Intermediate

PLL

1.47456 MHz

DRAM

Controller

36.864 MHz

Divisors

UART

Timer

Graphics

Controller

DRAM

Controller

Divisors

ISA Bus

DMA

Controller

CPU and

VL-Bus

Controller

ROM/Flash

Interface

PC Card

Controller

Low-Speed

PLL

Graphics

Dot Clock

PLL

High-Speed

PLL

66.3552 MHz

20.736–

36.864 MHz

Notes:

On the ÉlanSC400 microcontroller, the graphics controller’s DRAM interface is clocked by the 66-MHz DRAM clock.

Both the ROM/Flash memory interface and the PC Card controller are clocked from the CPU clock. They also have the option

to be run from the slow system clock.

Neither the graphics controller nor the PC Card controller are supported on the ÉlanSC410 microcontroller.

Figure 4. Clock Generation Block Diagram

76

Élan™SC400 and ÉlanSC410 Microcontrollers Data Sheet