AT89C5131-RDTIL datasheet(19/176 Pages) ATMEL

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AT89C5131

4136B–USB–09/03

Clock Controller

Introduction

The AT89C5131 clock controller is based on an on-chip oscillator feeding an on-chip

Phase Lock Loop (PLL). All the internal clocks to the peripherals and CPU core are gen-

erated by this controller.

The AT89C5131 X1 and X2 pins are the input and the output of a single-stage on-chip

inverter (see Figure 5) that can be configured with off-chip components as a Pierce

oscillator (see Figure 6). Value of capacitors and crystal characteristics are detailed in

the section “DC Characteristics”.

The X1 pin can also be used as input for an external 48 MHz clock.

The clock controller outputs three different clocks as shown in Figure 5:

•

a clock for the CPU core

•

a clock for the peripherals which is used to generate the Timers, PCA, WD, and Port

sampling clocks

•

a clock for the USB controller

These clocks are enabled or disabled depending on the power reduction mode as

detailed in Section “Power Management”, page 146.

Figure 5. Oscillator Block Diagram

Oscillator

Two clock sources are available for CPU:

•

Crystal oscillator on X1 and X2 pins: Up to 32 MHz

•

External 48 MHz clock on X1 pin

In order to optimize the power consumption, the oscillator inverter is inactive when the

PLL output is not selected for the USB device.

PCON.1

IDL

PCON.0

Peripheral

CPU Core

CKCON.0

÷ 2

Clock

EXT48

PLLCON.2

PLL

USB

Clock

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