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TXB1CTRL Datasheet(PDF) 6 Page - Microchip Technology |
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TXB1CTRL Datasheet(HTML) 6 Page - Microchip Technology |
6 / 92 page MCP2515 DS21801G-page 6 2003-2012 Microchip Technology Inc. 1.5 CAN Protocol Engine The CAN protocol engine combines several functional blocks, shown in Figure 1-4 and described below. 1.5.1 PROTOCOL FINITE STATE MACHINE The heart of the engine is the Finite State Machine (FSM). The FSM is a sequencer that controls the sequential data stream between the TX/RX shift register, the CRC register and the bus line. The FSM also controls the Error Management Logic (EML) and the parallel data stream between the TX/RX shift registers and the buffers. The FSM ensures that the processes of reception, arbitration, transmission and error-signaling are performed according to the CAN protocol. The automatic retransmission of messages on the bus line is also handled by the FSM. 1.5.2 CYCLIC REDUNDANCY CHECK The Cyclic Redundancy Check register generates the Cyclic Redundancy Check (CRC) code, which is transmitted after either the Control Field (for messages with 0 data bytes) or the Data Field and is used to check the CRC field of incoming messages. 1.5.3 ERROR MANAGEMENT LOGIC The Error Management Logic (EML) is responsible for the fault confinement of the CAN device. Its two counters, the Receive Error Counter (REC) and the Transmit Error Counter (TEC), are incremented and decremented by commands from the bit stream processor. Based on the values of the error counters, the CAN controller is set into the states error-active, error-passive or bus-off. 1.5.4 BIT TIMING LOGIC The Bit Timing Logic (BTL) monitors the bus line input and handles the bus-related bit timing according to the CAN protocol. The BTL synchronizes on a recessive- to-dominant bus transition at Start-of-Frame (hard synchronization) and on any further recessive-to- dominant bus line transition if the CAN controller itself does not transmit a dominant bit (resynchronization). The BTL also provides programmable time segments to compensate for the propagation delay time, phase shifts and to define the position of the sample point within the bit time. The programming of the BTL depends on the baud rate and external physical delay times. FIGURE 1-4: CAN PROTOCOL ENGINE BLOCK DIAGRAM Bit Timing Logic CRC<14:0> Comparator Receive<7:0> Transmit<7:0> Sample<2:0> Majority Decision StuffReg<5:0> Comparator Transmit Logic Receive Error Counter Transmit Error Counter Protocol FSM RX SAM BusMon Rec/Trm Addr. RecData<7:0> TrmData<7:0> Shift<14:0> (Transmit<5:0>, Receive<7:0>) TX REC TEC ErrPas BusOff Interface to Standard Buffer SOF |
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