Книга: Advanced PIC Microcontroller Projects in C

9.1 Data Frame

The data frame is in two formats: standard (having an 11-bit ID) and extended (having a 29-bit ID). The data frame is used by the transmitting device to send data to the receiving device, and the data frame is the most important frame handled by the user. Figure 9.7 shows the data frame’s structure. A standard data frame starts with the start of frame (SOF) bit, which is followed by an 11-bit identifier and the remote transmission request (RTR) bit. The identifier and the RTR form the 12-bit arbitration field. The control field is 6 bits wide and indicates how many bytes of data are in the data field. The data field can be 0 to 8 bytes. The data field is followed by the CRC field, which checks whether or not the received bit sequence is corrupted. The ACK field is 2 bits and is used by the transmitter to receive acknowledgment of a valid frame from any receiver. The end of the message is indicated by a 7-bit end of frame (EOF) field. In an extended data frame, the arbitration field is 32 bits wide (29-bit identifier +1-bit IDE to define the message as an extended data frame +1-bit SRR which is unused +1-bit RTR) (see Figure 9.8).

Figure 9.7: Standard data frame

Figure 9.8: Extended data frame

The data frame consists of the following fields:

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