PIC16F688
10.3.5
BREAK CHARACTER SEQUENCE
10.3.5.1
Break and Sync Transmit Sequence
The EUSART module has the capability of sending the
special Break character sequences that are required by
the LIN bus standard. The Break character transmit
consists of a Start bit, followed by 12 ‘0’ bits and a Stop
bit. The frame Break character is sent whenever the
SENDB and TXEN bits (TXSTA<3> and TXSTA<5>)
are set, while the Transmit Shift register is loaded with
data. Note that the value of data written to TXREG will
be ignored and all ‘0’s will be transmitted.
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 setup the
Break character.
3. Load the TXREG with a dummy character to
initiate transmission (the value is ignored).
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).
4. Write ‘55h’ to TXREG 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 Pre-Configured mode.
Note that the data value written to the TXREG for the
Break character is ignored. The write simply serves the
purpose of initiating the proper sequence.
When the TXREG becomes empty, as indicated by the
TXIF, the next data byte can be written to TXREG.
The TRMT bit indicates when the transmit operation is
active or IDLE, just as it does during normal transmis-
sion. See Figure 10-9 for the timing of the Break
character sequence.
10.3.6
RECEIVING A BREAK CHARACTER
The Enhanced USART module can receive a Break
character in two ways.
The first method forces to configure 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 10.3.4 “Auto-Wake-up on
SYNC Break Character”. 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
typically want to enable the Auto Baud Rate Detect
feature. For both methods, the user can set the ABD bit
before placing the USART in its Sleep mode.
FIGURE 10-9:
SEND BREAK CHARACTER SEQUENCE
Write to TXREG
Dummy Write
BRG Output
(Shift Clock)
TX (pin)
Start Bit
Bit 0
Bit 1
Bit 11
Stop Bit
Break
TXIF bit
(Transmit Buffer
Reg. Empty Flag)
TRMT bit
(Transmit Shift
Reg. Empty Flag)
SENDB Sampled Here
Auto Cleared
SENDB
(Transmit Shift
Reg. Empty Flag)
DS41203B-page 92
Preliminary
2004 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
