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RXB0DLC Datasheet(PDF) 8 Page - Microchip Technology |
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RXB0DLC Datasheet(HTML) 8 Page - Microchip Technology |
8 / 92 page MCP2515 DS21801G-page 8 2003-2012 Microchip Technology Inc. 2.4.1 ACTIVE ERRORS If an error-active node detects a bus error, 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 sequence actively violates the bit-stuffing rule. All other stations recognize the resulting bit-stuffing error and, in turn, generate error frames themselves, called error echo flags. The error flag field, therefore, consists of between six and twelve consecutive dominant bits (generated by one or more nodes). The error delimiter field (eight recessive bits) completes the error frame. Upon completion of the error frame, bus activity returns to normal and the interrupted node attempts to resend the aborted message. 2.4.2 PASSIVE ERRORS If an error-passive node detects a bus error, the node transmits an error-passive flag followed by the error delimiter field. The error-passive flag consists of six consecutive recessive bits. The error frame for an error- passive node consists of 14 recessive bits. From this, it follows that unless the bus error is detected by an error- active node or the transmitting node, the message will continue transmission because the error-passive flag does not interfere with the bus. If the transmitting node generates an error-passive flag, it will cause other nodes to generate error frames due to the resulting bit-stuffing violation. After transmission of an error frame, an error-passive node must wait for six consecutive recessive bits on the bus before attempting to rejoin bus communications. The error delimiter consists of eight recessive bits, and allows the bus nodes to restart bus communications cleanly after an error has occurred. 2.5 Overload Frame An overload frame, shown in Figure 2-5, has the same format as an active-error frame. An overload frame, however, can only be generated during an interframe space. In this way, an overload frame can be differentiated from an error frame (an error frame is sent during the transmission of a message). The overload frame consists of two fields: an overload flag followed by an overload delimiter. The overload flag consists of six dominant bits followed by overload flags generated by other nodes (and, as for an active error flag, giving a maximum of twelve dominant bits). The overload delimiter consists of eight recessive bits. An overload frame can be generated by a node as a result of two conditions: 1. The node detects a dominant bit during the interframe space, an illegal condition. Exception: The dominant bit is detected during the third bit of IFS. In this case, the receivers will interpret this as a SOF. 2. Due to internal conditions, the node is not yet able to begin reception of the next message. A node may generate a maximum of two sequential overload frames to delay the start of the next message. 2.6 Interframe Space The interframe space separates a preceding frame (of any type) from a subsequent data or remote frame. The interframe space is composed of at least three recessive bits called the Intermission. This allows nodes time for internal processing before the start of the next message frame. After the intermission, the bus line remains in the recessive state (bus idle) until the next transmission starts. Note: Error echo flags typically occur when a localized disturbance causes one or more (but not all) nodes to send an error flag. The remaining nodes generate error flags in response (echo) to the original error flag. Note: Case 2 should never occur with the MCP2515 due to very short internal delays. |
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