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 twelve ‘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 which can be Extract Locked Program of Microcomputer dsPIC30F5016. Note that the value of data written to TXREG 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 TXREG 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 transmission. See Figure 18-10 for the timing of the Break character 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.
Configure the EUSART for the desired mode.
Set the TXEN and SENDB bits to set up the Break character.
Load the TXREG with a dummy character to initiate transmission (the value is ignored).
Write ‘55h’ to TXREG to load the Sync character into the transmit FIFO buffer
After the Break has been sent, the SENDB bit is reset by hardware. The Sync character now transmits in the preconfigured mode. When the TXREG becomes empty, as indicated by the TXIF, the next data byte can be written to TXREG.
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 when Extract Locked Program of Microcomputer dsPIC30F5016. 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 18.2.4 “Auto-Wake-up on Sync Break Character”. By enabling this feature, the EUSART 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 once the TXIF interrupt is observed.
