Specifications

3706C–MICRO–2/11

AT89LP3240/6440

6. 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. The

on-chip crystal oscillator may also be configured for low or high speed operation. The clock

source is selected by the Clock Source User Fuses as shown in Table 6-1. S

ee “User Configura-

tion Fuses” on page 164. By default, no internal clock division is used to generate the CPU clock

from the system clock. However, the system clock divider may be used to prescale the system

clock. The choice of clock source also affects the start-up time after a POR, BOD or Power-

down event (See “Reset” on page 33 or “Power-down Mode” on page 37)

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-speed or low-speed mode. Low-speed mode is intended for 32.768 kHz

watch crystals and consumes less power than high-speed mode. The configuration as shown in

Figure 6-1 applies for both high and low speed oscillators

. Note that the internal structure of the

device adds about 10 pF of capacitance to both XTAL1 and XTAL2, so that in some cases less

external capacitance may be required. The total capacitance on XTAL1 or XTAL2, including the

external load capacitor plus internal device load, board trace and crystal loadings, shou

ld not

exceed 20 pF. An optional resistor R1 can be connected to XTAL1 in place of C1 for improved

startup performance with higher speed crystals. When using the crystal oscillator, P4.0 and P4.1

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.

Figure 6-1. Crystal Oscillator Connections

Note: 1. C1/C2 = 5–15 pF for Crystals

= 5–15 pF for Ceramic Resonators

R1 = 4–5 MΩ

Table 6-1. Clock Source Settings

Clock Source

Fuse 1

Clock Source

Fuse 0 Selected Clock Source

0 0 High Speed Crystal Oscillator (f > 500 kHz)

01Low Speed Crystal Oscillator (f ≤ 100 kHz)

10External Clock on XTAL1

1 1 Internal 8 MHz RC Oscillator

~10 pF

Optional