PIC18F45J10 Datasheet, PDF (203/358 Pages) Microchip Technology – 28/40/44-Pin High-Performance RISC Microcontrollers with nanoWatt Technology

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16.2.4.1 Special Considerations Using
Auto-Wake-up
Since auto-wake-up functions by sensing rising edge
transitions on RX/DT, 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., 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.
16.2.4.2 Special Considerations Using
the WUE Bit
The timing of WUE and RCIF 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 RCIF bit. The WUE bit is
cleared after this when a rising edge is seen on RX/DT.
The interrupt condition is then cleared by reading the
RCREG register. Ordinarily, the data in RCREG will be
dummy data and should be discarded.
The fact that the WUE bit has been cleared (or is still
set) and the RCIF flag is set should not be used as an
indicator of the integrity of the data in RCREG. 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 16-8:
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
RX/DT Line
RCIF
Cleared due to user read of RCREG
Note 1: The EUSART remains in Idle while the WUE bit is set.
FIGURE 16-9:
AUTO-WAKE-UP BIT (WUE) TIMINGS DURING SLEEP
OSC1
WUE bit⁽²⁾
Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4
Bit set by user
RX/DT Line
RCIF
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 RCREG
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.
© 2007 Microchip Technology Inc.
Preliminary
DS39682C-page 201