PIC16C925T-SSLASHCL datasheet(113/182 Pages) MICROCHIP

113 / 182 page

 2001 Microchip Technology Inc.

Preliminary

DS39544A-page 111

PIC16C925/926

12.8

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 hi-impedance).

For lowest current consumption in this mode, place all

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

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

the A/D, disable external clocks. Pull all I/O pins that

are hi-impedance inputs, high or low externally, to avoid

switching currents caused by floating inputs. The

T0CKI input should also be at VDD or 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).

12.8.1

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 RB0/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

execution 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:

TMR1 interrupt. Timer1 must be operating as an

asynchronous counter.

SSP (START/STOP) bit detect interrupt.

SSP transmit or

receive

Slave

mode

(SPI/I2C).

CCP Capture mode interrupt.

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

Special event trigger (Timer1 in Asynchronous

mode using an external clock).

LCD module.

Other peripherals can not generate interrupts since

during SLEEP, no on-chip Q clocks are present.

When the SLEEP instruction is being executed, the next

instruction (PC + 1) is pre-fetched. 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.

12.8.2

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 com-

plete 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 execu-

tion of a SLEEP instruction, the device will imme-

diately 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 instruc-

tion should be executed before a SLEEP instruction.

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