PIC16F/LF1946/47
24.3.2
AUTO-BAUD OVERFLOW
24.3.3.1
Special Considerations
During the course of automatic baud detection, the
ABDOVF bit of the BAUDxCON 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 SPxBRGH:SPxBRGL
register pair. After the ABDOVF has been set, the coun-
ter continues 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 inter-
rupt flag and clear the ABDEN bit of the BAUDxCON
register. The RCxIF flag can be subsequently cleared
by reading the RCxREG. The ABDOVF flag can be
cleared by software directly.
Break Character
To avoid character errors or character fragments during
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.
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.
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.
Oscillator Startup 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.
24.3.3
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.
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
RCxREG register and discarding its contents.
The Auto-Wake-up feature is enabled by setting the
WUE bit of the BAUDxCON 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.)
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.
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 (Figure 24-7), and asynchronously if
the device is in Sleep mode (Figure 24-8). The interrupt
condition is cleared by reading the RCxREG 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.
DS41414A-page 306
Preliminary
2010 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
