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MCP2510-I Datasheet(PDF) 7 Page - Microchip Technology |
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MCP2510-I Datasheet(HTML) 7 Page - Microchip Technology |
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7 / 80 page  © 2007 Microchip Technology Inc. DS21291F-page 7 MCP2510 2.0 CAN MESSAGE FRAMES The MCP2510 supports Standard Data Frames, Extended Data Frames, and Remote Frames (Stan- dard and Extended) as defined in the CAN 2.0B speci- fication. 2.1 Standard Data Frame The CAN Standard Data Frame is shown in Figure 2-1. In common with all other frames, the frame begins with a Start Of Frame (SOF) bit, which is of the dominant state, which allows hard synchronization of all nodes. The SOF is followed by the arbitration field, consisting of 12 bits; the 11-bit ldentifier and the Remote Trans- mission Request (RTR) bit. The RTR bit is used to dis- tinguish a data frame (RTR bit dominant) from a remote frame (RTR bit recessive). Following the arbitration field is the control field, con- sisting of six bits. The first bit of this field is the Identifier Extension (IDE) bit which must be dominant to specify a standard frame. The following bit, Reserved Bit Zero (RB0), is reserved and is defined to be a dominant bit by the can protocol. the remaining four bits of the con- trol field are the Data Length Code (DLC) which speci- fies the number of bytes of data contained in the message. After the control field is the data field, which contains any data bytes that are being sent, and is of the length defined by the DLC above (0-8 bytes). The Cyclic Redundancy Check (CRC) Field follows the data field and is used to detect transmission errors. The CRC Field consists of a 15-bit CRC sequence, followed by the recessive CRC Delimiter bit. The final field is the two-bit acknowledge field. During the ACK Slot bit, the transmitting node sends out a recessive bit. Any node that has received an error free frame acknowledges the correct reception of the frame by sending back a dominant bit (regardless of whether the node is configured to accept that specific message or not). The recessive acknowledge delimiter com- pletes the acknowledge field and may not be overwrit- ten by a dominant bit. 2.2 Extended Data Frame In the Extended CAN Data Frame, the SOF bit is fol- lowed by the arbitration field which consists of 32 bits, as shown in Figure 2-2. The first 11 bits are the most significant bits (Base-lD) of the 29-bit identifier. These 11 bits are followed by the Substitute Remote Request (SRR) bit which is defined to be recessive. The SRR bit is followed by the lDE bit which is recessive to denote an extended CAN frame. It should be noted that if arbitration remains unresolved after transmission of the first 11 bits of the identifier, and one of the nodes involved in the arbitration is sending a standard CAN frame (11-bit identifier), then the stan- dard CAN frame will win arbitration due to the assertion of a dominant lDE bit. Also, the SRR bit in an extended CAN frame must be recessive to allow the assertion of a dominant RTR bit by a node that is sending a stan- dard CAN remote frame. The SRR and lDE bits are followed by the remaining 18 bits of the identifier (Extended lD) and the remote trans- mission request bit. To enable standard and extended frames to be sent across a shared network, it is necessary to split the 29- bit extended message identifier into 11-bit (most signif- icant) and 18-bit (least significant) sections. This split ensures that the lDE bit can remain at the same bit position in both standard and extended frames. Following the arbitration field is the six-bit control field. the first two bits of this field are reserved and must be dominant. the remaining four bits of the control field are the Data Length Code (DLC) which specifies the num- ber of data bytes contained in the message. The remaining portion of the frame (data field, CRC field, acknowledge field, end of frame and lntermission) is constructed in the same way as for a standard data frame (see Section 2.1). 2.3 Remote Frame Normally, data transmission is performed on an auton- omous basis by the data source node (e.g. a sensor sending out a data frame). It is possible, however, for a destination node to request data from the source. To accomplish this, the destination node sends a remote frame with an identifier that matches the identifier of the required data frame. The appropriate data source node will then send a data frame in response to the remote frame request. There are two differences between a remote frame (shown in Figure 2-3) and a data frame. First, the RTR bit is at the recessive state, and second, there is no data field. In the event of a data frame and a remote frame with the same identifier being transmitted at the same time, the data frame wins arbitration due to the dominant RTR bit following the identifier. In this way, the node that transmitted the remote frame receives the desired data immediately. 2.4 Error Frame An Error Frame is generated by any node that detects a bus error. An error frame, shown in Figure 2-4, con- sists of two fields, an error flag field followed by an error delimiter field. There are two types of error flag fields. Which type of error flag field is sent depends upon the error status of the node that detects and generates the error flag field. If an error-active node detects a bus error then the node interrupts transmission of the current message by generating an active error flag. The active error flag is composed of six consecutive dominant bits. This bit |
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