PIC16F876ATISLASHML datasheet(158/234 Pages) MICROCHIP

158 / 234 page

PIC16F87XA

DS39582B-page 156

 2003 Microchip Technology Inc.

14.14 Power-down Mode (Sleep)

Power-down mode is entered by executing a SLEEP

instruction.

If enabled, the Watchdog Timer will be cleared but

keeps running, the PD bit (Status<3>) is cleared, the

TO (Status<4>) bit is set and the oscillator driver is

turned off. The I/O ports maintain the status they had

before the SLEEP instruction was executed (driving

high, low or high-impedance).

For lowest current consumption in this mode, place all

I/O pins at either VDD or VSS, ensure no external

circuitry is drawing current from the I/O pin, power-

down the A/D and disable external clocks. Pull all I/O

pins that are high-impedance inputs, high or low

externally, to avoid switching currents caused by

floating inputs. The T0CKI input should also be at VDD

VSS

for

lowest

current

consumption.

The

contribution from on-chip pull-ups on PORTB should

also be considered.

The MCLR pin must be at a logic high level (VIHMC).

14.14.1

WAKE-UP FROM SLEEP

The device can wake-up from Sleep through one of the

following events:

External Reset input on MCLR pin.

Watchdog Timer wake-up (if WDT was enabled).

Interrupt from INT pin, RB port change or

peripheral interrupt.

External MCLR Reset will cause a device Reset. All other

events are considered a continuation of program execu-

tion and cause a “wake-up”. The TO and PD bits in the

Status register can be used to determine the cause of

device Reset. The PD bit, which is set on power-up, is

cleared when Sleep is invoked. The TO bit is cleared if a

WDT time-out occurred and caused wake-up.

The following peripheral interrupts can wake the device

from Sleep:

PSP read or write (PIC16F874/877 only).

TMR1 interrupt. Timer1 must be operating as an

asynchronous counter.

CCP Capture mode interrupt.

Special event trigger (Timer1 in Asynchronous

mode using an external clock).

SSP (Start/Stop) bit detect interrupt.

SSP transmit or receive in Slave mode (SPI/I2C).

USART RX or TX (Synchronous Slave mode).

A/D conversion (when A/D clock source is RC).

EEPROM write operation completion.

10. Comparator output changes state.

Other peripherals cannot generate interrupts since

during Sleep, no on-chip clocks are present.

When the SLEEP instruction is being executed, the next

instruction (PC + 1) is prefetched. For the device to

wake-up through an interrupt event, the corresponding

interrupt enable bit must be set (enabled). Wake-up is

regardless of the state of the GIE bit. If the GIE bit is

clear (disabled), the device continues execution at the

instruction after the SLEEP instruction. If the GIE bit is

set (enabled), the device executes the instruction after

the SLEEP instruction and then branches to the inter-

rupt address (0004h). In cases where the execution of

the instruction following SLEEP is not desirable, the

user should have a NOP after the SLEEP instruction.

14.14.2

WAKE-UP USING INTERRUPTS

When global interrupts are disabled (GIE cleared) and

any interrupt source has both its interrupt enable bit

and interrupt flag bit set, one of the following will occur:

• If the interrupt occurs before the execution of a

SLEEP

instruction, the SLEEP instruction will

complete as a NOP. Therefore, the WDT and WDT

postscaler will not be cleared, the TO bit will not

be set and PD bits will not be cleared.

• If the interrupt occurs during or after the

execution of a SLEEP instruction, the device will

immediately wake-up from Sleep. The SLEEP

instruction will be completely executed before the

wake-up. Therefore, the WDT and WDT

postscaler will be cleared, the TO bit will be set

and the PD bit will be cleared.

Even if the flag bits were checked before executing a

SLEEP

instruction, it may be possible for flag bits to

become set before the SLEEP instruction completes. To

determine whether a SLEEP instruction executed, test

the PD bit. If the PD bit is set, the SLEEP instruction

was executed as a NOP.

To ensure that the WDT is cleared, a CLRWDT

instruction should be executed before a SLEEP

instruction.

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