Microchip Technology MA160014 Data Sheet
2010-2012 Microchip Technology Inc.
DS41412F-page 285
PIC18(L)F2X/4XK22
16.4.2
AUTO-BAUD OVERFLOW
During the course of automatic baud detection, the
ABDOVF bit of the BAUDCONx register will be set if the
baud rate counter overflows before the fifth rising edge
is detected on the RX pin. The ABDOVF bit indicates
that the counter has exceeded the maximum count that
can fit in the 16 bits of the SPBRGHx:SPBRGx register
pair. After the ABDOVF has been set, the counter con-
tinues to count until the fifth rising edge is detected on
the RXx/DTx pin. Upon detecting the fifth RXx/DTx
edge, the hardware will set the RCxIF interrupt flag and
clear the ABDEN bit of the BAUDCONx register. The
RCxIF flag can be subsequently cleared by reading the
RCREGx. The ABDOVF flag can be cleared by soft-
ware directly.
ABDOVF bit of the BAUDCONx register will be set if the
baud rate counter overflows before the fifth rising edge
is detected on the RX pin. The ABDOVF bit indicates
that the counter has exceeded the maximum count that
can fit in the 16 bits of the SPBRGHx:SPBRGx register
pair. After the ABDOVF has been set, the counter con-
tinues to count until the fifth rising edge is detected on
the RXx/DTx pin. Upon detecting the fifth RXx/DTx
edge, the hardware will set the RCxIF interrupt flag and
clear the ABDEN bit of the BAUDCONx register. The
RCxIF flag can be subsequently cleared by reading the
RCREGx. The ABDOVF flag can be cleared by soft-
ware directly.
To terminate the auto-baud process before the RCxIF
flag is set, clear the ABDEN bit then clear the ABDOVF
bit. The ABDOVF bit will remain set if the ABDEN bit is
not cleared first.
flag is set, clear the ABDEN bit then clear the ABDOVF
bit. The ABDOVF bit will remain set if the ABDEN bit is
not cleared first.
16.4.3
AUTO-WAKE-UP ON BREAK
During Sleep mode, all clocks to the EUSART are
suspended. Because of this, the Baud Rate Generator
is inactive and a proper character reception cannot be
performed. The Auto-Wake-up feature allows the
controller to wake-up due to activity on the RXx/DTx
line. This feature is available only in Asynchronous
mode.
suspended. Because of this, the Baud Rate Generator
is inactive and a proper character reception cannot be
performed. The Auto-Wake-up feature allows the
controller to wake-up due to activity on the RXx/DTx
line. This feature is available only in Asynchronous
mode.
The Auto-Wake-up feature is enabled by setting the
WUE bit of the BAUDCONx register. Once set, the
normal 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.)
WUE bit of the BAUDCONx register. Once set, the
normal 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.)
The EUSART module generates an RCxIF interrupt
coincident with the wake-up event. The interrupt is
generated synchronously to the Q clocks in normal CPU
operating modes (
coincident with the wake-up event. The interrupt is
generated synchronously to the Q clocks in normal CPU
operating modes (
), and asynchronously if
the device is in Sleep mode (
). The interrupt
condition is cleared by reading the RCREGx register.
The WUE bit is automatically cleared by the low-to-high
transition on the RXx line at the end of the Break. This
signals to the user that the Break event is over. At this
point, the EUSART module is in Idle mode waiting to
receive the next character.
transition on the RXx line at the end of the Break. This
signals to the user that the Break event is over. At this
point, the EUSART module is in Idle mode waiting to
receive the next character.
16.4.3.1
Special Considerations
Break Character
To avoid character errors or character fragments during
a wake-up event, the wake-up character must be all
zeros.
a wake-up event, the wake-up character must be all
zeros.
When the wake-up is enabled the function works
independent of the low time on the data stream. If the
WUE bit is set and a valid non-zero character is
received, the low time from the Start bit to the first rising
edge will be interpreted as the wake-up event. The
remaining bits in the character will be received as a
fragmented character and subsequent characters can
result in framing or overrun errors.
independent of the low time on the data stream. If the
WUE bit is set and a valid non-zero character is
received, the low time from the Start bit to the first rising
edge will be interpreted as the wake-up event. The
remaining bits in the character will be received as a
fragmented character and subsequent characters can
result in framing or overrun errors.
Therefore, the initial character in the transmission must
be all ‘0’s. This must be 10 or more bit times, 13-bit
times recommended for LIN bus, or any number of bit
times for standard RS-232 devices.
be all ‘0’s. This must be 10 or more bit times, 13-bit
times recommended for LIN bus, or any number of bit
times for standard RS-232 devices.
Oscillator Start-up Time
Oscillator start-up time must be considered, especially
in applications using oscillators with longer start-up
intervals (i.e., LP, XT or HS/PLL 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.
in applications using oscillators with longer start-up
intervals (i.e., LP, XT or HS/PLL 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.
WUE Bit
The wake-up event causes a receive interrupt by
setting the RCxIF bit. The WUE bit is cleared by
hardware by a rising edge on RXx/DTx. The interrupt
condition is then cleared by software by reading the
RCREGx register and discarding its contents.
setting the RCxIF bit. The WUE bit is cleared by
hardware by a rising edge on RXx/DTx. The interrupt
condition is then cleared by software by reading the
RCREGx register and discarding its contents.
To ensure that no actual data is lost, check the RCIDL
bit to verify that a receive operation is not in process
before setting the WUE bit. If a receive operation is not
occurring, the WUE bit may then be set just prior to
entering the Sleep mode.
bit to verify that a receive operation is not in process
before setting the WUE bit. If a receive operation is not
occurring, the WUE bit may then be set just prior to
entering the Sleep mode.