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MCP2510-ISO Datasheet(PDF) 36 Page - Microchip Technology |
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MCP2510-ISO Datasheet(HTML) 36 Page - Microchip Technology |
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36 / 80 page  MCP2510 DS21291F-page 36 © 2007 Microchip Technology Inc. 5.1 Time Quanta The Time Quanta (TQ) is a fixed unit of time derived from the oscillator period. There is a programmable baud-rate prescaler, with integral values ranging from 1 to 64, in addition to a fixed divide by two for clock gen- eration. Time quanta is defined as: where Baud Rate is the binary value represented by CNF1.BRP<5:0> For some examples: If FOSC = 16 MHz, BRP<5:0> = 00h, and Nominal Bit Time = 8 TQ; then TQ= 125 nsec and Nominal Bit Rate = 1 Mb/s If FOSC = 20 MHz, BRP<5:0> = 01h, and Nominal Bit Time = 8 TQ; then TQ= 200 nsec and Nominal Bit Rate = 625 Kb/s If FOSC = 25 MHz, BRP<5:0> = 3Fh, and Nominal Bit Time = 25 TQ; then TQ = 5.12 µsec and Nominal Bit Rate = 7.8 Kb/s The frequencies of the oscillators in the different nodes must be coordinated in order to provide a system-wide specified nominal bit time. This means that all oscilla- tors must have a TOSC that is a integral divisor of TQ. It should also be noted that although the number of TQ is programmable from 4 to 25, the usable minimum is 6 TQ. Attempting to a bit time of less than 6 TQ in length is not guaranteed to operate correctly 5.2 Synchronization Segment This part of the bit time is used to synchronize the var- ious CAN nodes on the bus. The edge of the input sig- nal is expected to occur during the sync segment. The duration is 1 TQ. 5.3 Propagation Segment This part of the bit time is used to compensate for phys- ical delay times within the network. These delay times consist of the signal propagation time on the bus line and the internal delay time of the nodes. The delay is calculated as being the round trip time from transmitter to receiver (twice the signal's propagation time on the bus line), the input comparator delay, and the output driver delay. The length of the Propagation Segment can be programmed from 1 TQ to 8 TQ by setting the PRSEG2:PRSEG0 bits of the CNF2 register (Register 5-2). The total delay is calculated from the following individ- ual delays: - 2 * physical bus end to end delay; TBUS - 2 * input comparator delay; TCOMP (depends on application circuit) - 2 * output driver delay; TDRIVE (depends on application circuit) - 1 * input to output of CAN controller; TCAN (maximum defined as 1 TQ + delay ns) -TPROPOGATION = 2 * (TBUS + TCOMP + TDRIVE) + TCAN - Prop_Seg = TPROPOGATION / TQ 5.4 Phase Buffer Segments The Phase Buffer Segments are used to optimally locate the sampling point of the received bit within the nominal bit time. The sampling point occurs between phase segment 1 and phase segment 2. These seg- ments can be lengthened or shortened by the resyn- chronization process (see Section 5.7.2). Thus, the variation of the values of the phase buffer segments represent the DPLL functionality. The end of phase segment 1 determines the sampling point within a bit time. phase segment 1 is programmable from 1 TQ to 8 TQ in duration. Phase segment 2 provides delay before the next transmitted data transition and is also pro- grammable from 1 TQ to 8 TQ in duration (however due to IPT requirements the actual minimum length of phase segment 2 is 2 TQ - see Section 5.6 below), or it may be defined to be equal to the greater of phase seg- ment 1 or the Information Processing Time (IPT). (see Section 5.6). 5.5 Sample Point The Sample Point is the point of time at which the bus level is read and value of the received bit is determined. The Sampling point occurs at the end of phase seg- ment 1. If the bit timing is slow and contains many TQ, it is possible to specify multiple sampling of the bus line at the sample point. The value of the received bit is determined to be the value of the majority decision of three values. The three samples are taken at the sam- ple point, and twice before with a time of TQ/2 between each sample. 5.6 Information Processing Time The Information Processing Time (IPT) is the time seg- ment, starting at the sample point, that is reserved for calculation of the subsequent bit level. The CAN spec- ification defines this time to be less than or equal to 2 TQ. The MCP2510 defines this time to be 2 TQ. Thus, phase segment 2 must be at least 2 TQ long. T Q 2* Baud Rate + 1 ()*T OSC = |
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