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1921C–MICRO–3/05

AT89C55WD

16. Oscillator Characteristics

XTAL1 and XTAL2 are the input and output, respectively, of an inverting amplifier that can be

configured for use as an on-chip oscillator, as shown in Figure 18-1. 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 18-2. There are no

requirements on the duty cycle of the external clock signal, since the input to the internal clock-

ing circuitry is through a divide-by-two flip-flop, but minimum and maximum voltage high and low

time specifications must be observed.

17. 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 special functions regis-

ters remain unchanged during this mode. The idle mode can be terminated by any enabled

interrupt or by a hardware reset.

Note that when idle mode is terminated by a hardware reset, the device normally resumes pro-

gram execution 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 mode is terminated by a reset, the instruction following the one that invokes

idle mode should not write to a port pin or to external memory.

18. Power-down Mode

In the Power-down mode, the oscillator is stopped, and the instruction that invokes Power-down

is the last instruction executed. The on-chip RAM and Special Function Registers retain their

values until the Power-down mode is terminated. Exit from Power-down can be initiated either

by a hardware reset or by an enabled external interrupt. Reset redefines the SFRs but does not

change the on-chip RAM. The reset should not be activated before V

operating level and must be held active long enough to allow the oscillator to restart and

stabilize.