AT89C51

4-32

When the AT89C51 is executing code from external pro-

gram memory, PSEN is activated twice each machine

cycle, except that two PSEN activations are skipped during

each access to external data memory.

External Access Enable. EA must be strapped to GND in

order to enable the device to fetch code from external pro-

gram memory locations starting at 0000H up to FFFFH.

Note, however, that if lock bit 1 is programmed, EA will be

internally latched on reset.

tions.

This pin also receives the 12-volt programming enable volt-

XTAL1

Input to the inverting oscillator amplifier and input to the

internal clock operating circuit.

XTAL2

Output from the inverting oscillator amplifier.

Oscillator Characteristics

XTAL1 and XTAL2 are the input and output, respectively,

of an inverting amplifier which can be configured for use as

an on-chip oscillator, as shown in Figure 1. Either a quartz

crystal or ceramic resonator may be used. To drive the

device from an external clock source, XTAL2 should be left

unconnected while XTAL1 is driven as shown in Figure 2.

There are no requirements on the duty cycle of the external

clock signal, since the input to the internal clocking circuitry

is through a divide-by-two flip-flop, but minimum and maxi-

mum voltage high and low time specifications must be

observed.

Idle Mode

In idle mode, the CPU puts itself to sleep while all the on-

chip peripherals remain active. The mode is invoked by

software. The content of the on-chip RAM and all the spe-

cial functions registers remain unchanged during this

mode. The idle mode can be terminated by any enabled

interrupt or by a hardware reset.

It should be noted that when idle is terminated by a hard

ware reset, the device normally resumes program execu-

tion, from where it left off, up to two machine cycles before

the internal reset algorithm takes control. On-chip hardware

inhibits access to internal RAM in this event, but access to

the port pins is not inhibited. To eliminate the possibility of

an unexpected write to a port pin when Idle is terminated by

reset, the instruction following the one that invokes Idle

should not be one that writes to a port pin or to external

memory.

Figure 1. Oscillator Connections

Note:

C1, C2 = 30 pF

± 10 pF for Crystals

= 40 pF

± 10 pF for Ceramic Resonators

Figure 2. External Clock Drive Configuration

C2

XTAL2

GND

XTAL1

C1

Status of External Pins During Idle and Power Down Modes

Mode

Program Memory

ALE

PSEN

PORT0

PORT1

PORT2

PORT3

Idle

Internal

1

1

Data

Data

Data

Data

Idle

External

1

1

Float

Data

Address

Data

Power Down

Internal

0

0

Data

Data

Data

Data

Power Down

External

0

0

Float

Data

Data

Data