PIC16F688-ESLASHST datasheet(93/174 Pages) MICROCHIP

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 2004 Microchip Technology Inc.

Preliminary

DS41203B-page 91

PIC16F688

10.3.4

AUTO-WAKE-UP ON SYNC BREAK

CHARACTER

The auto-wake-up feature allows the controller to

wake-up due to activity on the RX/DT line, despite the

baud clock being turned off. This allows communica-

tions systems to save power by only responding to

direct requests.

Setting the WUE bit (BAUDCTL<1>) enables the

auto-wake-up feature. When the auto-wake-up feature

is enabled, the next falling edge on the RX/DT line will

trigger an RCIF interrupt. The WUE bit will automati-

cally clear after the rising RX/DT edge after triggering a

falling edge. Receiving a RCIF interrupt after setting

the WUE bit signals to the user that the wake-up event

has occurred. See Figure 10-7 and Figure 10-8 for

timing details of the auto-wake-up process.

10.3.4.1

Special Considerations Using

Auto- Wake-up

The auto-wake-up function is edge sensitive. To

prevent data errors or framing errors, the data following

the Break should be all ‘0’s until the baud clock is

stable. If the LP, XT or HS oscillators are used, the

oscillator start-up time will affect the amount of time the

application must wait before receiving valid data.

Special care should be taken when using the

two-speed start-up or the fail-safe clock monitor

because the application will start running from the

internal oscillator before the primary oscillator is ready.

Because the auto-wake-up feature uses the RCIF flag

to signify the wake-up event, the application should

discard the data read from RCREG when servicing the

RCIF flag after setting the WUE bit.

When entering Sleep with auto-wake-up enabled, the

following procedure should be used.

Clear all interrupt flags including RCIF.

Check RCIDL to ensure no receive is currently

in progress.

No characters are being received so the WUE

bit can be set.

Sleep.

FIGURE 10-7:

AUTO-WAKE-UP BIT (WUE) TIMINGS DURING NORMAL OPERATION

FIGURE 10-8:

AUTO-WAKE-UP BIT (WUE) TIMINGS DURING SLEEP

Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4

OSC1

WUE bit

RX/DT Line

RCIF

Bit Set by User

Auto Cleared

Cleared due to User Read of RCREG

Note: The USART remains in IDLE while the WUE bit is set.

Q1Q2Q3 Q4 Q1Q2 Q3 Q4 Q1Q2 Q3 Q4

Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1Q2 Q3 Q4

OSC1

WUE bit

RX/DT Line

RCIF

Bit Set by User

Auto Cleared

Cleared due to User Read of RCREG

Sleep Command Executed

Sleep Ends

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