AT89LP51 datasheet(29/113 Pages) ATMEL

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3709C–MICRO–5/11

AT89LP51/52 - Preliminary

System Clock

The system clock is generated directly from one of three selectable clock sources. The three

sources are the on-chip crystal oscillator, external clock source, and internal RC oscillator. A dia-

gram of the clock subsystem is shown in Figure 6-1. The on-chip crystal oscillator may also be

configured for low or high power operation. The clock source is selected by the Clock Source

User Fuses as shown in Table 6-1. See “User Configuration Fuses” on page 86. By default, in

Fast mode no internal clock division is used to generate the CPU clock from the system clock. In

Compatibility mode the default is to divide the oscillator output by two. The system clock divider

may be used to prescale the system clock with other values. The choice of clock source also

affects the start-up time after a POR, BOD or Power-down event (See “Reset” on page 32 or

“Power-down Mode” on page 35)

Figure 6-1.

Clock Subsystem Diagram

6.1

Crystal Oscillator

When enabled, the internal inverting oscillator amplifier is connected between XTAL1 and

XTAL2 for connection to an external quartz crystal or ceramic resonator. The oscillator may

operate in either high-power or low-power mode. Low-speed mode is intended for crystals of 12

MHz or less and consumes less power than the higher speed mode. The configuration as shown

in Figure 6-2 applies for both high and low power oscillators. Note that in some cases, external

capacitors C1 and C2 may NOT be required due to the on-chip capacitance of the XTAL1 and

XTAL2 inputs (approximately 10 pF each). When using the crystal oscillator, P4.6 and P4.7 will

have their inputs and outputs disabled. Also, XTAL2 in crystal oscillator mode should not be

used to directly drive a board-level clock without a buffer.

Table 6-1.

Clock Source Settings

Clock Source

Fuse 1

Clock Source

Fuse 0

Selected Clock Source

High Power Crystal Oscillator (f > 12 MHz)

Low Power Crystal Oscillator (f

≤12 MHz)

External Clock on XTAL1

Internal 1.8432 MHz Auxiliary Oscillator

XTAL1

XTAL2

SYSTEM CLOCK

(CLKSYS)

INTERNAL

1.8432MHz

OSC

5-BIT

CLOCK

DIVIDER

CLKIRC

CLKEXT

CLKXTAL

12345

CDV2-0

CLOCK FUSES

4-BIT

PRESCALER

TPS3-0

Timer 0

Timer 1

Timer 2

Watchdog

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