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PIC18F8722 FAMILY
DS39646C-page 262 © 2008 Microchip Technology Inc.
20.2.4 AUTO-WAKE-UP ON SYNC BREAK
CHARACTER
During Sleep mode, all clocks to the EUSART are
suspended. Because of this, the Baud Rate Generator is
inactive and a proper byte reception cannot be
performed. The auto-wake-up feature allows the
controller to wake-up due to activity on the RXx/DTx line,
while the EUSART is operating in Asynchronous mode.
The auto-wake-up feature is enabled by setting the
WUE bit (BAUDCONx<1>). Once set, the typical receive
sequence on RXx/DTx is disabled and the EUSART
remains in an Idle state, monitoring for a wake-up event
independent of the CPU mode. A wake-up event
consists of a high-to-low transition on the RXx/DTx line.
(This coincides with the start of a Sync Break or a
Wake-up Signal character for the LIN protocol.)
Following a wake-up event, the module generates an
RCxIF interrupt. The interrupt is generated synchro-
nously to the Q clocks in normal operating modes
(Figure 20-8) and asynchronously, if the device is in
Sleep mode (Figure 20-9). The interrupt condition is
cleared by reading the RCREGx register.
The WUE bit is automatically cleared once a low-to-
high transition is observed on the RXx line following the
wake-up event. At this point, the EUSART module is
inactive and returns to normal operation. This signals to
the user that the Sync Break event is over.
20.2.4.1 Special Considerations Using
Auto-Wake-up
Since auto-wake-up functions by sensing rising edge
transitions on RXx/DTx, information with any state
changes before the Stop bit may signal a false end-of-
character and cause data or framing errors. To work
properly, therefore, the initial character in the transmis-
sion must be all ‘0’s. This can be 00h (8 bytes) for
standard RS-232 devices or 000h (12 bits) for LIN bus.
Oscillator start-up time must also be considered,
especially in applications using oscillators with longer
start-up intervals (i.e., XT or HS mode). The Sync
Break (or Wake-up Signal) character must be of
sufficient length and be followed by a sufficient interval
to allow enough time for the selected oscillator to start
and provide proper initialization of the EUSART.
20.2.4.2 Special Considerations Using
the WUE Bit
The timing of WUE and RCxIF events may cause
some confusion when it comes to determining the
validity of received data. As noted, setting the WUE bit
places the EUSART in an inactive state. The wake-up
event causes a receive interrupt by setting the RCxIF
bit. The WUE bit is cleared after this when a rising
edge is seen on RXx/DTx. The interrupt condition is
then cleared by reading the RCREGx register.
Ordinarily, the data in RCREGx will be dummy data
and should be discarded.
The fact that the WUE bit has been cleared (or is still
set) and the RCxIF flag is set should not be used as an
indicator of the integrity of the data in RCREGx. Users
should consider implementing a parallel method in
firmware to verify received data integrity.
To assure that no actual data is lost, check the RCIDL
bit to verify that a receive operation is not in process. If
a receive operation is not occurring, the WUE bit may
then be set just prior to entering the Sleep mode.
FIGURE 20-8: AUTO-WAKE-UP BIT (WUE) TIMINGS DURING NORMAL OPERATION
FIGURE 20-9: AUTO-WAKE-UP BIT (WUE) TIMINGS DURING SLEEP
Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 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
(1)
RXx/DTx Line
RCxIF
Note 1:The EUSART remains inactive while the WUE bit is set.
Bit set by user
Cleared due to user read of RCREGx
Auto-Cleared
Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 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
(2)
RXx/DTx Line
RCxIF
Cleared due to user read of RCREGx
Sleep Command Executed
Note 1: If the wake-up event requires long oscillator warm-up time, the auto-clear of the WUE bit can occur before the oscillator is ready. This
sequence should not depend on the presence of Q clocks.
2: The EUSART remains inactive while the WUE bit is set.
Sleep Ends
Note 1
Auto-Cleared
Bit set by user