AT89LP51RB2 datasheet(43/254 Pages) ATMEL

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3722A–MICRO–10/11

AT89LP51RB2/RC2/IC2 Preliminary

6.2

External Clock Source A

The external clock option disables the oscillator amplifier and allows XTAL1A to be driven

directly by an external clock source as shown in Figure 6-4. XTAL2A may be left unconnected,

used as general purpose I/O P4.7, or configured to output a divided version of the system clock.

Figure 6-4.

External Clock A Drive Configuration

6.3

Internal RC Oscillator

The AT89LP51RB2/RC2/IC2 has an Internal RC oscillator tuned to 8.0 MHz ±2.5%. When

enabled as clock source A, XTAL1A and XTAL2A may be used as P4.6 and P4.7 respectively.

For AT89LP51IC2 the internal oscillator can also be selected for clock source B, freeing up

XTAL1B and XTAL2B to act as P1.0 and P4.2 respectively. The frequency of the oscillator may

be adjusted within limits by changing the RC Calibration Byte stored at byte 384 of the User Sig-

nature Array. This location may be updated using the IAP interface or by an external device

programmer (User Signature location 0180H). See Section 24.1.2 “Atmel Signature Array” on

page 188. A copy of the factory calibration byte is stored at byte 8 of the Atmel Signature Array

(0008H in SIG space).

6.4

Crystal Oscillator B (AT89LP51IC2)

AT89LP51IC2 includes a second crystal oscillator for low-frequency (~32 KHz) operation. When

enabled, internal inverting oscillator amplifier B is connected between XTAL1B and XTAL2B for

connection to an external quartz crystal or ceramic resonator as shown in Figure 6-5. Note that

in some cases, external capacitors C1 and C2 may be reduced due to the on-chip capacitance

of the XTAL1B and XTAL2B inputs (approximately 10 pF each). An on-chip series resistance is

included between the amplifier and the XTAL2B pad to limit the drive level. In most cases an

external series resistor is not required. When using the crystal oscillator, P1.0 and P4.2 will have

their inputs and outputs disabled. Also, XTAL2B in crystal oscillator mode should not be used to

directly drive a board-level clock without a buffer.

Please note that the low-frequency oscillator may have a very long settling time. The system

must ensure that the oscillator has sufficient time to stabilize before the device is allowed to

operate from this clock source.

XTAL2A (P4.7)

XTAL1A (P4.6)

GND

NC GPIO

EXTERNAL

OSCILLATOR

SIGNAL

AT89LP51RB2 Datasheet(PDF) 43 Page - ATMEL Corporation