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MCP2510-ISO Datasheet(PDF) 35 Page - Microchip Technology |
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MCP2510-ISO Datasheet(HTML) 35 Page - Microchip Technology |
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35 / 80 page  © 2007 Microchip Technology Inc. DS21291F-page 35 MCP2510 5.0 BIT TIMING All nodes on a given CAN bus must have the same nominal bit rate. The CAN protocol uses Non Return to Zero (NRZ) coding which does not encode a clock within the data stream. Therefore, the receive clock must be recovered by the receiving nodes and syn- chronized to the transmitters clock. As oscillators and transmission time may vary from node to node, the receiver must have some type of Phase Lock Loop (PLL) synchronized to data transmis- sion edges to synchronize and maintain the receiver clock. Since the data is NRZ coded, it is necessary to include bit stuffing to ensure that an edge occurs at least every six bit times, to maintain the Digital Phase Lock Loop (DPLL) synchronization. The bit timing of the MCP2510 is implemented using a DPLL that is configured to synchronize to the incoming data, and provide the nominal timing for the transmitted data. The DPLL breaks each bit time into multiple seg- ments made up of minimal periods of time called the time quanta (TQ). Bus timing functions executed within the bit time frame, such as synchronization to the local oscillator, network transmission delay compensation, and sample point positioning, are defined by the programmable bit timing logic of the DPLL. All devices on the CAN bus must use the same bit rate. However, all devices are not required to have the same master oscillator clock frequency. For the different clock frequencies of the individual devices, the bit rate has to be adjusted by appropriately setting the baud rate prescaler and number of time quanta in each seg- ment. The nominal bit rate is the number of bits transmitted per second assuming an ideal transmitter with an ideal oscillator, in the absence of resynchronization. The nominal bit rate is defined to be a maximum of 1 Mb/s. Nominal Bit Time is defined as: TBIT = 1 / NOMlNAL BlT RATE The nominal bit time can be thought of as being divided into separate non-overlapping time segments. These segments are shown in Figure 5-1. - Synchronization Segment (Sync_Seg) - Propagation Time Segment (Prop_Seg) - Phase Buffer Segment 1 (Phase_Seg1) - Phase Buffer Segment 2 [Phase_Seg2) Nominal Bit Time = TQ * (Sync_Seg + Prop_Seg + Phase_Seg1 + Phase_Seg2) The time segments and also the nominal bit time are made up of integer units of time called time quanta or TQ (see Figure 5-1). By definition, the nominal bit time is programmable from a minimum of 8 TQ to a maxi- mum of 25 TQ. Also, by definition the minimum nominal bit time is 1 µs, corresponding to a maximum 1 Mb/s rate. FIGURE 5-1: BIT TIME PARTITIONING Input Signal Sync Prop Segment Phase Segment 1 Phase Segment 2 Sample Point TQ |
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Similar Description - MCP2510-ISO |
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