PIC16F688
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.
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.
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.
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.
When entering Sleep with auto-wake-up enabled, the
following procedure should be used.
1. Clear all interrupt flags including RCIF.
2. Check RCIDL to ensure no receive is currently
in progress.
3. No characters are being received so the WUE
bit can be set.
4. Sleep.
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
FIGURE 10-7:
AUTO-WAKE-UP BIT (WUE) TIMINGS DURING NORMAL OPERATION
Q1 Q2 Q3 Q4 Q1 Q2 Q3Q4 Q1Q2 Q3 Q4 Q1Q2 Q3 Q4 Q1Q2 Q3 Q4 Q1Q2 Q3 Q4 Q1Q2 Q3 Q4 Q1Q2 Q3Q4 Q1 Q2 Q3 Q4 Q1Q2 Q3Q4
OSC1
Auto Cleared
Bit Set by User
WUE bit
RX/DT Line
RCIF
Cleared due to User Read of RCREG
Note: The USART remains in IDLE while the WUE bit is set.
FIGURE 10-8:
AUTO-WAKE-UP BIT (WUE) TIMINGS DURING SLEEP
Q4
Q1Q2Q3Q4 Q1Q2Q3Q4 Q1Q2Q3
Bit Set by User
Q1
Q2Q3Q4Q1Q2Q3Q4Q1Q2Q3Q4Q1Q2Q3Q4 Q1Q2Q3Q4
Auto Cleared
OSC1
WUE bit
RX/DT Line
RCIF
Cleared due to User Read of RCREG
Sleep Ends
Sleep Command Executed
2004 Microchip Technology Inc.
Preliminary
DS41203B-page 91
MICROCHIP [ MICROCHIP ]
