Книга: Advanced PIC Microcontroller Projects in C
9.1.2 Arbitration Field
9.1.2 Arbitration Field
Arbitration is used to resolve bus conflicts that occur when several devices at once start sending messages on the bus. The arbitration field indicates the priority of a frame, and it is different in the standard and extended formats. In the standard format there are 11 bits, and up to 2032 IDs can be set. The extended format ID consists of 11 base IDs plus 18 extended IDs. Up to 2032?218 discrete IDs can be set.
During the arbitration phase, each transmitting device transmits its identifier and compares it with the level on the bus. If the levels are equal, the device continues to transmit. If the device detects a dominant level on the bus while it is trying to transmit a recessive level, it quits transmitting and becomes a receiving device. After arbitration only one transmitter is left on the bus, and this transmitter continues to send its control field, data field, and other data.
The process of arbitration is illustrated in Figure 9.9 by an example consisting of three nodes having identifiers:
Node 1: 11100110011 Node 2: 11100111111 Node 3: 11100110001
Figure 9.9: Example CAN bus arbitration
Assuming the recessive level corresponds to 1 and the dominant level to 0, the arbitration is performed as follows:
• All the nodes start transmitting simultaneously, first sending SOF bits.
• Then they send their identifier bits. The 8th bit of Node 2 is in the recessive state, while the corresponding bits of Nodes 1 and 3 are in the dominant state. Therefore Node 2 stops transmitting and returns to receive mode. The receiving phase is indicated by a gray field.
• The 10th bit of Node 1 is in the recessive state, while the same bit of Node 3 is in dominant state. Thus Node 1 stops transmitting and returns to receive mode.
• The bus is now left to Node 3, which can send its control and data fields freely.
Notably, the devices on the bus have no addresses. Instead, all the devices pick up all the data on the bus, and every node must filter out the messages it does not want.
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