PIC18F6X2X Datasheet, PDF (230/386 Pages) Microchip Technology – 64/80-Pin High Performance, 64-Kbyte Enhanced FLASH Microcontrollers with A/D

PIC18F6X2X/8X2X
19.2.5 BREAK CHARACTER SEQUENCE
The enhanced USART module has the capability of
sending the special Break character sequences that
are required by the LIN bus standard. The Break char-
acter transmit consists of a START bit, followed by
twelve ‘0’ bits and a STOP bit. The frame Break char-
acter is sent whenever the SENDB and TXEN bits
(TXSTAx<3> and TXSTAx<5>) are set while the Trans-
mit Shift register is loaded with data. Note that the value
of data written to TXREGx will be ignored and all ‘0’s
will be transmitted.
The SENDB bit is automatically reset by hardware after
the corresponding STOP bit is sent. This allows the
user to preload the transmit FIFO with the next transmit
byte following the Break character (typically, the Sync
character in the LIN specification).
Note that the data value written to the TXREGx for the
Break character is ignored. The write simply serves the
purpose of initiating the proper sequence.
The TRMT bit indicates when the transmit operation is
active or IDLE, just as it does during normal transmis-
sion. See Figure 19-9 for the timing of the Break
character sequence.
19.2.5.1 Break and Sync Transmit Sequence
The following sequence will send a message frame
header made up of a Break, followed by an auto baud
Sync byte. This sequence is typical of a LIN bus
master.
1. Configure the USART for the desired mode.
2. Set the TXEN and SENDB bits to set up the
Break character.
3. Load the TXREGx with a dummy character to
initiate transmission (the value is ignored).
4. Write ‘55h’ to TXREGx to load the Sync
character into the transmit FIFO buffer.
5. After the Break has been sent, the SENDB bit is
reset by hardware. The Sync character now
transmits in the preconfigured mode.
When the TXREGx becomes empty, as indicated by
the TXIF, the next data byte can be written to TXREGx.
19.2.6 RECEIVING A BREAK CHARACTER
The Enhanced USART module can receive a Break
character in two ways.
The first method forces configuration of the baud rate
at a frequency of 9/13 the typical speed. This allows for
the STOP bit transition to be at the correct sampling
location (13 bits for Break versus START bit and 8 data
bits for typical data).
The second method uses the auto wake-up feature
described in Section 19.2.4. By enabling this feature,
the USART will sample the next two transitions on RX/
DT, cause an RCIF interrupt, and receive the next data
byte followed by another interrupt.
Note that following a Break character, the user will typ-
ically want to enable the auto baud rate detect feature.
For both methods, the user can set the ABD bit once
the TXIF interrupt is observed.
FIGURE 19-9:
Write to TXREGx
BRG Output
(Shift Clock)
TX (pin)
TXIF bit
(Transmit Buffer
Reg. Empty Flag)
TRMT bit
(Transmit Shift
Reg. Empty Flag)
SENDB
(Transmit Shift
Reg. Empty Flag)
SEND BREAK CHARACTER SEQUENCE
Dummy Write
START Bit
Bit 0
Bit 1
Break
SENDB sampled here
Bit 11
STOP Bit
Auto Cleared
DS39612A-page 228
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 2003 Microchip Technology Inc.