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The CCS C Compiler has added built-in Local Interconnect Network (LIN) bus support to the #use rs232() library. LIN bus is an inexpensive serial communication protocol used primarily in the automotive industry to complement the existing CAN Bus network. The LIN bus network contains one master node and up to 15 slave nodes for a total of 16 nodes, and supports bit rates up to 20 kbit/s.
The LIN bus protocol's message frame consist of two parts, the header and the response. The header is always sent by the master node, meaning all communication is initiated by the master node. After the header is sent only one node sends the response. The header consists of three main fields, the break field, the sync field and the identifier field. The break field is used to get the attention of all LIN slave nodes on the network. The sync field is the hexadecimal value 0x55 used by the slave nodes to determine the current bit time of the bus. Finally the identifier field is used to determine which node will respond during the response part of the frame. The response consist of two fields, the data field and the checksum field. The data field contains 0 to 8 bytes and the checksum field is one byte. Depending on the LIN bus protocol specification being used the checksum is either the checksum of the data field bytes, or the identifier field and the data field bytes.
The following options have been added to the CCS C Compiler's #use rs232() library to enable LIN bus master or slave protocol support, LIN=MASTER and LIN=SLAVE. Additionally the options LIN_CHECKSUM=LEGACY or LIN_CHECKSUM=ENHANCED can be used to select which checksum type is used by default by the functions. Legacy checksum only uses the data field when calculating the checksum, and enhanced uses both the identifier field and the data field when calculating the checksum.
Whether a device can use the library's built-in LIN bus protocol support depends on the mode being used and configuration. When setup as a LIN bus master both software and hardware configurations are support, the only limitation is that when using the hardware UART peripheral the device is required to have an advanced UART peripheral or an UART peripheral with built-in protocol support. When setup as a LIN bus slave it's only supported when using the hardware UART peripheral on devices that have an advanced UART peripheral or an UART peripheral with built-in protocol support. For devices that have a hardware UART peripheral, most devices have a UART peripheral that will work with the #use rs232() library's built-in LIN bus protocol.
When built for a LIN bus master the following functions are added by the #use rs232() library: linbus_header(), linbus_rx_response(), linbus_tx_response() and linbus_checksum_type(). The linbus_header() function is used to by the master to send the header part of the LIN bus message frame, the parity bits of the identifier field is automatically calculated by the function. The linbus_rx_response() function is used by the master to receive the response from one of the slave nodes during the response part of the LIN bus message frame. The linbus_tx_response() function is used by the master to transmit the response during the response part of the LIN bus message frame, the master node only sends the response when the identifier it sent indicates that it should transmit the response. Finally the linbus_checksum_type() function can be used to change how the linbus_tx_response() function calculates the checksum that it sends when used to send the response part of the LIN bus message frame.
When built for a LIN bus slave the following functions are added by the #use rs232() library: linbus_header_hit(), linbus_header_get(), linbus_rx_response(), linbus_tx_response() and linbus_checksum_type(). The linbus_header_hit() function is used to determine if the header as been received. For devices that have a UART with build-in protocol support this function returns TRUE after the entire header as been received, and for devices with an advanced UART it returns TRUE after the first 8 bits of the break byte is received. The linbus_rx_header() function is used to retrieve the identifier field of the received header, the parity bits masked off. Additionally for devices with an advanced UART peripheral this functions also sets up the UART peripheral to receive the sync field. The linbus_rx_response() function is used by the slave to receive the response from another node during the response part of the LIN bus message frame. For devices with a UART peripheral with built-in protocol support this function only needs to be called if the received identifier indicates that the message is to be received by that node. For devices with an advanced UART this function should be called for all messages were the received identifier doesn't indicates that the node should transmit the response. The linbus_tx_reponse() function is used by the slave to transmit the response during the response part of the LIN bus message frame. This function should only be called when the received identifier indicates that the node should transmit the response. Finally the linbus_checksum_type() function can be used to change how the linbus_tx_response() function calculates the checksum that it sends when used to send the response part of the LIN bus message frame.
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