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SN65HVD234D layout

Designing with the SN65HVD234D, what are recommended values for serial resistance as well as bus capacitance?  The layout in the datasheet (Figure 41) shows them but does not  recommend values.

Thanks

Angela

  • Hi Angela,

    Figure 41 shows some series resistors on the RXD and TXD lines, although in most applications these are not needed.  They can be useful for impedance matching if long PCB traces are used (in which case they are typically small values, e.g., ~20-30 Ohms) or for improving EMI by reducing the edge rates (in which case they may be up to ~1 kOhm in value).  R5 and R6 are CAN termination resistors, and these should be 60 Ohms each.  This gives 120 Ohms in total between CANH and CANL at the terminating node.  (There should be exactly two terminating nodes on a CAN bus in order to provide proper loading as well as impedance matching, so nodes that do not implement termination would not include these resistors at all.)

    The bus capacitances (C8 and C9) provide high-frequency noise filtering but also slow down the CAN signaling, so there is a trade-off to evaluate when picking these values.  A typical target would be around 100 pF on each line.  This could increase if more filtering is needed or would decrease if the application requires very high data rates or large numbers of nodes on the bus.  The capacitor that is used as part of the termination network (C7) provides filtering for the common mode component of the CAN signal without degrading the differential component, and so it can be a relatively large value.  A typical value would be 4.7 nF.

    I hope this helps - please let me know if you have any other questions.

    Max

  • Hi Max, 

    Thanks for your answer, it is very helpful.  Just a couple other questions:  For the split termination resistors R5, R6 is it critical they are 60 ohm?  Will 60.4 ohm work?  Are R3, R4 typically the same value as R1, R2?  Are C1, C4 typically 100pF as well or another value?

    Thanks,

    Angela

  • Angela,

    R5 and R6 don't need to be exactly 60 Ohms; if they are a few percent off that won't make much difference.

    It isn't very clear from Figure 41, but R3 and R4 are typically used as pull-up/pull-down resistors that are used to set logic states on the RS and EN pins when those pins are not driven. They should be in the 1 kOhm to 10 kOhm range. For example, you could tie the EN pin to the output of an MCU but also pull it up through a resistor to VCC so that the transceiver is enabled unless the MCU drives the pin low.

    A 100 pF capacitor would work on C4. For C1, you would choose the value in conjunction with R1 so that the time constant of the RC low-pass filter they create is much smaller than the time required for one bit of data. This filtering is optional, so many applications would use 0 Ohms for R1 and leave C1 open.

    Max
  • Max,
    I'm a little confused. If I use 1k resistor, it takes .000001 for 1 bit of data, and I choose a time constant of .0001s/2pi then the capacitor comes out as 0.157uF. The frequency of the filter is f=1/(2pi*10^3*1.57*10^-7)=~1k. Don't I need the filter frequency to be greater than the data frequency?
  • Angela,

    Yes, you need the corner frequency of the filter to be much higher than the frequency of the data (or, equivalently, the time constant of the filter to be much smaller than the period/bit duration of the data). For example, if you were operating at 100 kbps, each bit would be 10 us long. In that case, you might consider a filter with a time constant in the 200 ns range. This would mean a corner frequency at ~800 kHz. With a 1 kOhm series resistance, the capacitor value would then be 200 pF.

    Max