Datasheet

PIC18F97J94 FAMILY
DS30575A-page 424  2012 Microchip Technology Inc.
FIGURE 21-7: ASYNCHRONOUS RECEPTION
21.2.4 AUTO-WAKE-UP ON SYNC BREAK
CHARACTER
During Sleep mode, all clocks to the EUSARTx are
suspended. Because of this, the Baud Rate Generator
is inactive and a proper byte reception cannot be per-
formed. The auto-wake-up feature allows the controller
to wake-up due to activity on the RXx/DTx line while the
EUSARTx 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
EUSARTx 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/J2602 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 21-8) and asynchronously if the device is in
Sleep mode (Figure 21-9). 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 EUSARTx 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 bits) for standard RS-232 devices or 000h (12 bits)
for the 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 EUSARTx.
Start
bit
bit 7/8
bit 1bit 0 bit 7/8
bit 0
Stop
bit
Start
bit
Start
bit
bit 7/8
Stop
bit
RXx (pin)
Rcv Buffer Reg
Rcv Shift Reg
Read Rcv
Buffer Reg
RCREGx
RCxIF
(Interrupt Flag)
OERR bit
CREN
Word 1
RCREGx
Word 2
RCREGx
Stop
bit
Note: This timing diagram shows three words appearing on the RXx input. The RCREGx (Receive Buffer) is read after the third word,
causing the OERR (Overrun) bit to be set.