at89c5131 ATMEL Corporation, at89c5131 Datasheet - Page 148

• Manufacturer Part Number: at89c5131
• Description: 8-bit Flash Microcontroller With Full Speed Usb Device At89c5131
• Manufacturer: ATMEL Corporation

Power Management
Idle Mode
Power-down Mode
146
AT89C5131
An instruction that sets PCON.0 indicates that it is the last instruction to be executed
before going into the Idle mode. In the 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
preserved 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 an 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
service 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.
To save maximum power, a power-down mode can be invoked by software (refer to
Table 13, 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
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
long enough for the oscillator to restart and stabilize.
Only:
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•
•
•
are useful to exit from power-down. For that, 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 oper-
ating mode.
Holding the pin low restarts the oscillator but bringing the pin high completes the exit as
detailed in Figure 69. 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 AT89C5131 into power-down mode.
external interrupt INT0,
external interrupt INT1,
Keyboard interrupt and
USB Interrupt
CC
is restored to its normal operating level and must be held active
CC
can be lowered to save further
4136C–USB–04/05