AT80C51RB2-3CSIM datasheet(58/83 Pages) ATMEL

58 / 83 page

AT80C51RD2/AT83C51Rx2

4113B–8051–03/05

Power Management

Idle Mode

An instruction that sets PCON.0 indicates that it is the last instruction to be executed

before going into Idle mode. In Idle mode, the internal clock signal is gated off to the

CPU, but not to the interrupt, Timer, and Serial Port functions. The CPU status is pre-

served in its entirety: the Stack Pointer, Program Counter, Program Status Word,

Accumulator and all other registers maintain their data during idle. The port pins hold the

logical states they had at the time Idle was activated. ALE and PSEN hold at logic high

level.

There are two ways to terminate the Idle mode. Activation of any enabled interrupt will

cause PCON.0 to be cleared by hardware, terminating the Idle mode. The interrupt will

be serviced, and following RETI the next instruction to be executed will be the one fol-

lowing the instruction that put the device into idle.

The flag bits GF0 and GF1 can be used to give an indication if an interrupt occurred dur-

ing normal operation or during idle. For example, an instruction that activates idle can

also set one or both flag bits. When idle is terminated by an interrupt, the interrupt ser-

vice routine can examine the flag bits.

The other way of terminating the Idle mode is with a hardware reset. Since the clock

oscillator is still running, the hardware reset needs to be held active for only two

machine cycles (24 oscillator periods) to complete the reset.

Power-down Mode

To save maximum power, a power-down mode can be invoked by software (refer to

Table 30, PCON register).

In power-down mode, the oscillator is stopped and the instruction that invoked power-

down mode is the last instruction executed. The internal RAM and SFRs retain their

value until the power-down mode is terminated. V

CC can be lowered to save further

power. Either a hardware reset or an external interrupt can cause an exit from power-

down. To properly terminate power-down, the reset or external interrupt should not be

executed before V

CC is restored to its normal operating level and must be held active

long enough for the oscillator to restart and stabilize.

Only external interrupts INT0, INT1 and Keyboard Interrupts are useful to exit from

power-down. Thus, the interrupt must be enabled and configured as level - or edge -

sensitive interrupt input. When Keyboard Interrupt occurs after a power-down mode,

1024 clocks are necessary to exit to power-down mode and enter in operating mode.

Holding the pin low restarts the oscillator but bringing the pin high completes the exit as

detailed in Figure 22. When both interrupts are enabled, the oscillator restarts as soon

as one of the two inputs is held low and power-down exit will be completed when the first

input is released. In this case, the higher priority interrupt service routine is executed.

Once the interrupt is serviced, the next instruction to be executed after RETI will be the

one following the instruction that put T8xc51Rx2 into power-down mode.

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