PIC18F47J53 Datasheet, PDF (359/586 Pages) Microchip Technology – 28/44-Pin, High-Performance USB Microcontrollers with nanoWatt XLP Technology

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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 in Idle mode and returns to normal operation.
This signals to the user that the Sync Break event is
over.
21.2.4.1 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 (EOC) and cause data or framing
errors. To work properly, therefore, the initial character
in the transmission must be all ‘0’s. This can be 00h
(8 bytes) for standard RS-232 devices or 000h
(12 bits) for LIN/J2602 bus.
Oscillator start-up time must also be considered,
especially in applications using oscillators with
longer start-up intervals (i.e., HS or HSPLL 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.
21.2.4.2 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 Idle mode. 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 Sleep mode.
FIGURE 21-8:
AUTO-WAKE-UP BIT (WUE) TIMINGS DURING NORMAL OPERATION
OSC1
WUE bit⁽¹⁾
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
Bit set by user
Auto-Cleared
RXx/DTx Line
RCxIF
Cleared due to user read of RCREGx
Note 1: The EUSART remains in Idle while the WUE bit is set.
FIGURE 21-9:
AUTO-WAKE-UP BIT (WUE) TIMINGS DURING SLEEP
Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4
OSC1
WUE bit⁽²⁾
Bit set by user
RXx/DTx Line
RCxIF
SLEEP Command Executed
Q1
Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4
Auto-Cleared
Sleep Ends
Note 1
Cleared due to user read of RCREGx
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 in Idle while the WUE bit is set.
 2010 Microchip Technology Inc.
Preliminary
DS39964B-page 359