PIC18F97J60 Datasheet, PDF (318/474 Pages) Microchip Technology – 64/80/100-Pin, High-Performance, 1 Mbit Flash Microcontrollers with Ethernet

PIC18F97J60 FAMILY
20.2.4 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 con-
sists 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
EUSARTx module is in Idle mode 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 trans-
mission 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 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.
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
EUSARTx 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 the Sleep mode.
DS39762A-page 316
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© 2006 Microchip Technology Inc.