W79E632A Datasheet, PDF (32/79 Pages) Winbond – 8-BIT MICROCONTROLLER

W79E632A/W79L632A
8. POWER MANAGEMENT
The W79E(L)632 has several features that help the user to control the power consumption of the device.
The power saving features are basically the POWER DOWN mode and the IDLE mode of operation.
Idle Mode
The user can put the device into idle mode by writing 1 to the bit PCON.0. The instruction that sets the
idle bit is the last instruction that will be executed before the device goes into Idle Mode. In the Idle
mode, the clock to the CPU is halted, but not to the Interrupt, Timer, Watchdog timer and Serial port
blocks. This forces the CPU state to be frozen; the Program counter, the Stack Pointer, the Program
Status Word, the Accumulator and the other registers hold their contents. The ALE and PSEN pins are
held high during the Idle state. The port pins hold the logical states they had at the time Idle was
activated. The Idle mode can be terminated in two ways. Since the interrupt controller is still active, the
activation of any enabled interrupt can wake up the processor. This will automatically clear the Idle bit,
terminate the Idle mode, and the Interrupt Service Routine(ISR) will be executed. After the ISR,
execution of the program will continue from the instruction which put the device into Idle mode.
The Idle mode can also be exited by activating the reset. The device can be put into reset either by
applying a high on the external RST pin, a Power on reset condition or a Watchdog timer reset. The
external reset pin has to be held high for at least two machine cycles i.e. 8 clock periods to be
recognized as a valid reset. In the reset condition the program counter is reset to 0000h and all the SFRs
are set to the reset condition. Since the clock is already running there is no delay and execution starts
immediately. In the Idle mode, the Watchdog timer continues to run, and if enabled, a time-out will cause
a watchdog timer interrupt which will wake up the device. The software must reset the Watchdog timer in
order to preempt the reset which will occur after 512 clock periods of the time-out. When the
W79E(L)632 is exiting from an Idle mode with a reset, the instruction following the one which put the
device into Idle mode is not executed. So there is no danger of unexpected writes.
Power Down Mode
The device can be put into Power Down mode by writing 1 to bit PCON.1. The instruction that does this
will be the last instruction to be executed before the device goes into Power Down mode. In the Power
Down mode, all the clocks are stopped and the device comes to a halt. All activity is completely stopped
and the power consumption is reduced to the lowest possible value. In this state the ALE and PSEN
pins are pulled low. The port pins output the values held by their respective SFRs.
The W79E(L)632 will exit the Power Down mode with a reset or by an external interrupt pin enabled as
level detect. An external reset can be used to exit the Power down state. The high on RST pin terminates
the Power Down mode, and restarts the clock. The program execution will restart from 0000h. In the
Power down mode, the clock is stopped, so the Watchdog timer cannot be used to provide the reset to
exit Power down mode.
The W79E(L)632 can be woken from the Power Down mode by forcing an external interrupt pin
activated, provided the corresponding interrupt is enabled, while the global enable(EA) bit is set and the
external input has been set to a level detect mode. If these conditions are met, then the low level on the
external pin re-starts the oscillator. Then device executes the interrupt service routine for the
corresponding external interrupt. After the interrupt service routine is completed, the program execution
returns to the instruction after the one which put the device into Power Down mode and continues from
there.
Table 5. Status of external pins during Idle and Power Down
MODE
PROGRAM ^ALE
PSEN
PORT0
PORT1
PORT2 PORT3
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