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COP884BC Datasheet(PDF) 31 Page - National Semiconductor (TI) |
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COP884BC Datasheet(HTML) 31 Page - National Semiconductor (TI) |
31 / 57 page Frame Formats (Continued) START OF FRAME (SOF) The Start of Frame indicates the beginning of data and re- mote frames. It consists of a single “dominant” bit. A node is only allowed to start transmission when the bus is idle. All nodes have to synchronize to the leading edge (first edge af- ter the bus was idle) caused by SOF of the node which starts transmission first. ARBITRATION FIELD The arbitration field is composed of the identifier field and the RTR (Remote Transmission Request) bit. The value of the RTR bit is “dominant” in a data frame and “recessive” in a re- mote frame. CONTROL FIELD The control field consists of six bits. It starts with two bits re- served for future expansion followed by the four-bit Data Length Code. Receivers must accept all possible combina- tions of the two reserved bits. Until the function of these re- served bits is defined, the transmitter only sends “0” (domi- nant) bits. The first reserved bit (IDE) is actually defined to indicate an extended frame with 29 Identifier bits if set to “1”. CAN chips must tolerate extended frames, even if they can only understand standard frames, to prevent the destruction of an extended frames on an existing network. The Data Length Code indicates the number of bytes in the data field. This Data Length Code consists of four bits. The data field can be of length zero. The permissible number of data bytes for a data frame ranges from 0 to 8. DATA FIELD The Data field consists of the data to be transferred within a data frame. It can contain 0 to 8 bytes and each byte con- tains 8 bits. A remote frame has no data field. CRC FIELD The CRC field consists of the CRC sequence followed by the CRC delimiter. The CRC sequence is derived by the trans- mitter from the modulo 2 division of the preceding bit fields, starting with the SOF up to the end of the data field, exclud- ing stuff-bits, by the generator polynomial: χ15 + χ14 + χ10 + χ8 + χ7 + χ4 + χ3 +1 The remainder of this division is the CRC sequence transmit- ted over the bus. On the receiver side the module divides all bit fields up to the CRC delimiter, excluding stuff-bits, and checks if the result is zero. This will then be interpreted as a valid CRC. After the CRC sequence a single “recessive” bit is transmitted as the CRC delimiter. ACK FIELD The ACK field is two bits long and contains the ACK slot and the ACK delimiter. The ACK slot is filled with a “recessive” bit by the transmitter. This bit is overwritten with a “dominant” bit by every receiver that has received a correct CRC se- quence. The second bit of the ACK field is a “recessive” bit called the acknowledge delimiter. As a consequence the ac- knowledge flag of a valid frame is surrounded by two “reces- sive” bits, the CRC-delimiter and the ACK delimiter. EOF FIELD The End of Frame Field closes a data and a remote frame. It consists of seven “recessive” bits. INTERFRAME SPACE Data and remote frames are separate from every preceding frame (data, remote, error and overload frames) by the inter- frame space see Figure 25 and Figure 26 for details. Error and overload frames are not preceded by an interframe space. They can be transmitted as soon as the condition oc- curs. The interframe space consists of a minimum of three bit fields depending on the error state of the node. These bit fields are coded as follows: The intermission has the fixed form of three “recessive” bits. While this bit field is active, no node is allowed to start a transmission of a data or a remote frame. The only action to be taken is signaling an overload condition. This means that an error in this bit field would be interpreted as an overload condition. Suspend transmission has to be inserted by error- passive nodes that were transmitter for the last message. This bit field has the form of eight “recessive” bits. However, it may be overwritten by a “dominant” start-bit from another non error passive node which starts transmission. The bus idle field consists of “recessive” bits. Its length is not speci- fied and depends on the bus load. ERROR FRAME The Error Frame consists of two bit fields: the error flag and the error delimiter. The error field is built up from the various error flags of the different nodes. Therefore, its length may vary from a minimum of six bits up to a maximum of twelve bits depending on when a module detects the error. When- ever a bit error, stuff error, form error, or acknowledgment er- ror is detected by a node, this node starts transmission of the error flag at the next bit. If a CRC error is detected, transmis- sion of the error flag starts at the bit following the acknowl- edge delimiter, unless an error flag for a previous error con- dition has already been started. Figure 27 shows how a local fault at one module (module 2) leads to a 12-bit error frame on the bus. The bus level may either be “dominant” for an error-active node or “recessive” for an error-passive node. An error ac- DS012067-59 FIGURE 24. CAN Frame Format www.national.com 31 |
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